discussion 2

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Discussion 2: Nature/Nurture

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Objective: This discussion is intended to get you critically thinking about a long standing debate in the field of psychology.

Instructions

To get full points for this discussion, complete your reading to this point and then:

Submit a main post of 300 words (minimum) that answers all of the following questions by Saturday noon. (Please post your word count as the last line of your post).
Post a response (of 150 words minimum) to one other student’s post between Saturday noon and the due date (Wednesday at 2:00pm). Your response should be 100 words (minimum) and respond to one other student’s post. Be sure to follow NETiquette. (Please post your word count as the last line of your post).
Main Post

(10 points) question/prompt:

The nature/nurture debate involves whether human behavior is determined by the environment, either prenatal or during a person’s life, or by a person’s genes. While acknowledging that both “nature” (heredity) and “nurture” (environment) play a role in determining human behavior, you are to take a position on one or the other.

Respond to this discussion by stating which you believe plays a more significant role in a person’s life, explain why. You are to back up your opinion with the best evidence that you can think of—from personal observation or experience, or from what you have learned about human behavior from research, reading in chapter 1, and/or video & reading in chapter 2 —to support your position.

Response

(5 points) should include:

Return to the discussion (between Saturday noon and Wednesday at 2:00pm when both your main post and response are due).

Read several students’ main post. When you see an answer that interests you, click “reply” then type your Response. Be sure to follow the length requirement (above) for your response. Replies can include contrasting the other student’s answer to your own, making an application of their answer to your experience, adding more material, or bringing up questions related to their post.

In addition, comment on the following in your response: Pages 9-12 of the text discuss the fact that human development is said to be multidimensional and multicontextual. Which dimension and/or context of development do you believe has had the biggest effect on you (i.e. cohort, culture, age, socio-economic status and/or physical domain, cognitive domain, or psychosocial domain)?

Note: Do not expect credit for brief responses such as, “good post”!

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Chapter 2: Heredity, Prenatal Development, and Birth
In this chapter, we will begin by examining some of the ways in which heredity helps to shape
the way we are. We will look at what happens genetically during conception, and describe some
known genetic and chromosomal disorders. Next, we will consider what happens during
prenatal development, including the impact of teratogens. We will also discuss the impact that
both the mother and father have on the developing fetus. Lastly, we will present the birth
process and some of the complications that can occur during delivery. Before going into these
topics, however, it is important to understand how genes and chromosomes affect development.
Learning Objectives: Heredity
•
•
•
•
•
•
Define genes
Distinguish between mitosis and meiosis, genotype and phenotype, homozygous
and heterozygous, and dominant and recessive
Describe some genetic disorders, due to a gene defect, and chromosomal disorders
Define polygenic and incomplete dominance
Describe the function of genetic counseling and why individuals may seek genetic
counseling
Define behavioral genetics, describe genotype-environment correlations and
genotype-environmental interactions, and define epigenetics
Heredity
As your recall from chapter one, nature refers to the
contribution of genetics to one’s development. The basic
building block of the nature perspective is the gene. Genes
are specific sequence of nucleotides and are recipes for
making proteins. Proteins are responsible for influencing the
structure and functions of cells. Genes are located on the
chromosomes and there are an estimated 20,500 genes for
humans, according to the Human Genome Project (NIH,
2015). See Box 2.2 at the end of this section for more details
on the Human Genome Project.
Figure
2.1
Source
Normal human cells contain 46 chromosomes (or 23 pairs; one from each parent) in the nucleus
of the cells. After conception, most cells of the body are created by a process called mitosis.
Mitosis is defined as the cell’s nucleus making an exact copy of all the chromosomes and
splitting into two new cells. However, the cells used in sexual reproduction, called the gametes
(sperm or ova), are formed in a process called meiosis. In meiosis the gamete’s chromosomes
duplicate, and then divide twice resulting in four cells containing only half the genetic material
of the original gamete. Thus, each sperm and egg possesses only 23 chromosomes and combine
to produce the normal 46. See Figure 2.2 for details on both mitosis and meiosis. Given the
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amount of genes present and the unpredictability of the meiosis process, the likelihood of having
offspring that are genetically identical (and not twins) is one in trillions (Gould & Keeton,
1997).
Of the 23 pairs of chromosomes
created at conception, 22 pairs are
similar in length. These are called
autosomes. The remaining pair, or
sex chromosomes, may differ in
length. If a child receives the
combination of XY the child will
be genetically male. If the child
receives the combination XX the
child will be genetically female.
Figure 2.2 Mitosis vs. Meiosis
Genotypes and Phenotypes
The word genotype refers to the
sum total of all the genes a person
inherits. The word phenotype
refers to the features that are
actually expressed. Look in the
mirror. What do you see, your
genotype or your phenotype? What
determines whether or not genes
are expressed? Because genes are
inherited in pairs on the
chromosomes, we may receive either the same version of a gene from our mother and father, that
is, be homozygous for that characteristic the gene influences. If we receive a different version of
the gene from each parent, that is referred to as heterozygous. In the homozygous situation we
will display that characteristic. It is in the heterozygous condition that it becomes clear that not
all genes are created equal. Some genes are dominant, meaning they express themselves in the
phenotype even when paired with a different version of the gene, while their silent partner is
called recessive. Recessive genes express themselves only when paired with a similar version
gene. Geneticists refer to different versions of a gene as alleles. Some dominant traits include
having facial dimples, curly hair, normal vision, and dark hair. Some recessive traits include red
hair, being nearsighted, and straight hair.
Most characteristics are not the result of a single gene; they are polygenic, meaning they are the
result of several genes. In addition, the dominant and recessive patterns described above are
usually not that simple either. Sometimes the dominant gene does not completely suppress the
recessive gene; this is called incomplete dominance. An example of this can be found in the
recessive gene disorder sickle cell disease. The gene that produces healthy round-shaped red
blood cells is dominant. The recessive gene causes an abnormality in the shape of red blood
cells; they take on a sickle form, which can clog the veins and deprive vital organs of oxygen and
increase the risk of stroke. To inherit the disorder a person must receive the recessive gene from
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both parents. Those who have inherited only one recessive-gene are called carriers and should
be unaffected by this recessive trait. Yet, carriers of sickle cell have some red blood cells that
take on the c-shaped sickle pattern. Under circumstances of oxygen deprivation, such as high
altitudes or physical exertion, carriers for the sickle cell gene may experience some of the
symptoms of sickle cell (Berk, 2004).
Box 2.1 Monozygotic and Dizygotic Twins
Many students are interested in twins. Monozygotic or identical twins occur when a
fertilized egg splits apart in the first two weeks of development. The result is the
creation of two separate, but genetically identical offspring. That is, they possess the
same genotype and often the same phenotype. About one-third of twins are
monozygotic twins. Sometimes, however, two eggs or ova are released and fertilized
by two separate sperm. The result is dizygotic or fraternal twins. These two
individuals share the same amount of genetic material as would any two children
from the same mother and father. In other words, they possess a different genotype
and phenotype. Older mothers are more likely to have dizygotic twins than are
younger mothers, and couples who use fertility drugs are also more likely to give
birth to dizygotic twins. Consequently, there has been an increase in the number of
fraternal twins recently (Bortolus et al., 1999).
Source: Monozygotic Twins
Source Dizygotic Twins
Genetic Disorders
Most of the known genetic disorders are dominant gene-linked; however, the vast majority of
dominant gene linked disorders are not serious or debilitating. For example, the majority of those
with Tourette’s Syndrome suffer only minor tics from time to time and can easily control their
symptoms. Huntington’s Disease is a dominant gene linked disorder that affects the nervous
system and is fatal, but does not appear until midlife. Recessive gene disorders, such as cystic
fibrosis and sickle-cell anemia, are less common, but may actually claim more lives because they
are less likely to be detected as people are unaware that they are carriers of the disease. Some
genetic disorders are sex-linked; the defective gene is found on the X-chromosome. Males have
only one X chromosome so are at greater risk for sex-linked disorders due to a recessive gene,
such as hemophilia, color-blindness, and baldness. For females to be affected by the genetic
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defects, they need to inherit the recessive gene on both X-chromosomes, but if the defective gene
is dominant, females can be equally at risk. Table 2.1 lists several genetic disorders.
Table 2.1 Genetic Disorders
Recessive Disorders (Homozygous): The individual inherits a gene change from both
parents. If the gene is inherited from just one parent, the person is a carrier and does
not have the condition.
• Sickle Cell Disease (SCD) is a condition in which the red blood cells in the body
are shaped like a sickle (like the letter C) and affect the ability of the blood to
transport oxygen. Carriers may experience some effects, but do not have the full
condition.
• Cystic Fibrosis (CF) is a condition that affects breathing and digestion due to
thick mucus building up in the body, especially the lungs and digestive system. In
CF, the mucus is thicker than normal and sticky.
• Phenylketonuria (PKU) is a metabolic disorder in which the individual cannot
metabolize phenylalanine, an amino acid. Left untreated intellectual deficits
occur. PKU is easily detected and is treated with a special diet.
• Tay Sachs Disease is caused by enzyme deficiency resulting in the accumulation
of lipids in the nerve cells of the brain. This accumulation results in progressive
damage to the cells and a decrease in cognitive and physical development. Death
typically occurs by age five.
•
Albinism is when the individual lacks melanin and possesses little to no pigment
in the skin, hair, and eyes. Vision problems can also occur.
Cases per Birth
1 in 500 Black births
1 in 36,000 Hispanic
births
1 in 3500
1 in 10,000
1 in 4000
1in 30 American
Jews is a carrier
1 in 20 French
Canadians is a carrier
Fewer than 20,000
US cases per year
Autosomal Dominant Disorders (Heterozygous): In order to have the disorder, the
individual only needs to inherit the gene change from one parent.
• Huntington’s Disease is a condition that affects the individual’s nervous system.
Nerve cells become damaged, causing various parts of the brain to deteriorate. The
disease affects movement, behavior and cognition. It is fatal, and occurs at
midlife.
• Tourette Syndrome is a tic disorder which results in uncontrollable motor and
vocal tics as well as body jerking.
• Achondroplasia is the most common form of disproportionate short stature. The
individual has abnormal bone growth resulting in short stature, disproportionately
short arms and legs, short fingers, a large head, and specific facial features.
Cases per Birth
Sex-Linked Disorders: When the X chromosome carries the mutated gene, the
disorder is referred to as an X-linked disorder. Males are more affected than females
because they possess only one X chromosome without an additional X chromosome to
counter the harmful gene.
• Fragile X Syndrome occurs when the body cannot make enough of a protein it
needs for the brain to grow and problems with learning and behavior can occur.
Fragile X syndrome is caused from an abnormality in the X chromosome, which
then breaks. If a female has fragile X, her second X chromosome usually is
healthy, but males with fragile X don’t have a second healthy X chromosome.
This is why symptoms of fragile X syndrome usually are more serious in males .
• Hemophilia occurs when there are problems in blood clotting causing both
internal and external bleeding.
• Duchenne Muscular Dystrophy is a weakening of the muscles resulting in an
inability to move, wasting away, and possible death.
Cases per Birth
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1 in 10,000
1 in 250
1 in 15,000-40,000
1 in 4000 males
1 in 8000 females
1 in 10,000 males
1 in 3500 males
Chromosomal Abnormalities
A chromosomal abnormality occurs when a child inherits too many or two few
chromosomes. The most common cause of chromosomal abnormalities is the age of the mother.
As the mother ages, the ovum is more likely to suffer abnormalities due to longer term exposure
to environmental factors. Consequently, some gametes do not divide evenly when they are
forming. Therefore, some cells have more than 46 chromosomes. In fact, it is believed that close
to half of all zygotes have an odd number of chromosomes. Most of these zygotes fail to develop
and are spontaneously aborted by the mother’s body.
One of the most common chromosomal abnormalities is on pair 21. Trisomy 21 or Down
syndrome occurs when there are three rather than two 21st chromosomes. A person with Down
syndrome typically exhibits an intellectual disability and possesses certain physical features,
such as short fingers and toes, folds of skin over the eyes, and a protruding tongue. There is as
much variation in people with Down syndrome as in most populations, and those differences
need to be recognized and appreciated. Refer to Table 2.2 on the prevalence of Down syndrome
in our home state of Illinois. Other less common chromosomal abnormalities of live-born infants
occur on chromosome 13 and chromosome 18.
Table 2.2: Illinois Prevalence Rates (2002-2014) for Down Syndrome (Trisomy 21) based
on Maternal Age
Source
When the abnormality is on 23rd pair the result is a sex-linked chromosomal abnormality. A
person might have XXY, XYY, XXX, XO. Two of the more common sex-linked chromosomal
disorders are Turner syndrome and Klinefelter syndrome. Turner syndrome occurs when part
or all of one of the X chromosomes is lost and the resulting zygote has an XO composition. This
occurs in 1 of every 2,500 live female births (Carroll, 2007) and affects the individual’s
cognitive functioning and sexual maturation. The external genitalia appear normal, but breasts
and ovaries do not develop fully and the woman does not menstruate. Turner syndrome also
results in short stature and other physical characteristics. Klinefelter syndrome (XXY) results
when an extra X chromosome is present in the cells of a male and occurs in 1 out of 650 live
male births. The Y chromosome stimulates the growth of male genitalia, but the additional X
chromosome inhibits this development. An individual with Klinefelter syndrome typically has
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small testes, some breast development, infertility, and low levels of testosterone (National
Institutes of Health, 2019). See Table 2.3 for Chromosomal Disorders descriptions.
Table 2.3 Chromosomal DisordersAutosomal Chromosome Disorders:
The individual inherits too many or two few chromosomes.
• Down Syndrome/Trisomy 21 is caused by an extra chromosome 21 and
includes a combination of birth defects. Affected individuals have some degree
of intellectual disability, characteristic facial features, often heart defects, and
other health problems. The severity varies greatly among affected individuals.
• Trisomy 13 is caused by an extra chromosome 13. Affected individuals have
multiple birth defects and generally die in the first weeks or months of life.
• Trisomy 18 is caused by an extra chromosome 18 and the affected individual
also has multiple birth defects and early death.
Sex-Linked Chromosomal Disorders: The disorder occurs on chromosome pair
#23 or the sex chromosomes.
• Turner Syndrome is caused when all or part of one of the X chromosomes is
lost before or soon after conception due to a random event. The resulting zygote
has an XO composition. Turner Syndrome affects cognitive functioning and
sexual maturation in girls. Infertility and a short stature may be noted.
• Klinefelter Syndrome is caused when an extra X chromosome is present in the
cells of a male due to a random event. The Y chromosome stimulates the growth
of male genitalia, but the additional X chromosome inhibits this development.
The male can have some breast development, infertility, and low levels of
testosterone.
Cases per Birth
1 in 691
1 in 300 births at
age 35
1 in 7,906
1 in 3,762
Cases per Birth
1 in 2500 females
1 in 650 males
Genetic Counseling: A service that assists individuals identify, test for, and explain potential
genetic conditions that could adversely affect themselves or their offspring is referred to as
genetic counseling (CDC, 2015b). The common reasons for genetic counseling include:
•
•
•
•
Family history of a genetic condition
Membership in a certain ethnic group with a higher risk of a genetic condition
Information regarding the results of genetic testing, including blood tests, amniocentesis,
or ultra sounds
Learning about the chances of having a baby with a genetic condition if the parents are
older, have had several miscarriages, have offspring with birth defects, experience
infertility, or have a medical condition
Behavioral Genetics
Behavioral Genetics is the scientific study of the interplay between the genetic and
environmental contributions to behavior. Often referred to as the nature/nurture debate, Gottlieb
(1998, 2000, 2002) suggests an analytic framework for this debate that recognizes the interplay
between the environment, behavior, and genetic expression. This bidirectional interplay suggests
that the environment can affect the expression of genes just as genetic predispositions can impact
a person’s potentials. Additionally, environmental circumstances can trigger symptoms of a
genetic disorder. For example, a person who has sickle cell anemia, a recessive gene linked
disorder, can experience a sickle cell crisis under conditions of oxygen deprivation. Someone
predisposed genetically for type-two diabetes can trigger the disease through poor diet and little
exercise.
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Research has shown how the environment and genotype interact in several ways. GenotypeEnvironment Correlations refer to the processes by which genetic factors contribute to
variations in the environment (Plomin, DeFries, Knopik, & Niederhiser, 2013). There are three
types of genotype-environment correlations:
Passive genotype-environment correlation occurs when
Figure 2.3
children passively inherit the genes and the environments their
family provides. Certain behavioral characteristics, such as
being athletically inclined, may run in families. The children
have inherited both the genes that would enable success at these
activities, and given the environmental encouragement to
engage in these actions. Figure 2.3 highlights this correlation by
demonstrating how a family passes on water skiing skills
through both genetics and environmental opportunities.
Evocative genotype-environment correlation refers to how the social environment reacts to
individuals based on their inherited characteristics. For example, whether one has a more
outgoing or shy temperament will affect how he or she is treated by others.
Active genotype-environment correlation occurs when individuals seek out environments that
support their genetic tendencies. This is also referred to as niche picking. For example, children
who are musically inclined seek out music instruction and opportunities that facilitate their
natural musical ability.
Conversely, Genotype-Environment Interactions involve genetic susceptibility to the
environment. Adoption studies provide evidence for genotype-environment interactions. For
example, the Early Growth and Development Study (Leve, Neiderhiser, Scaramella, & Reiss,
2010) followed 360 adopted children and their adopted and biological parents in a longitudinal
study. Results have shown that children whose biological parents exhibited psychopathology,
exhibited significantly fewer behavior problems when their adoptive parents used more
structured parenting than unstructured. Additionally, elevated psychopathology in adoptive
parents increased the risk for the children’s development of behavior problems, but only when
the biological parents’ psychopathology was high. Consequently, the results show how
environmental effects on behavior differ based on the genotype, especially stressful
environments on genetically at-risk children.
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Lastly, Epigenetics studies modifications in DNA that affect gene expression and are passed on
when the cells divide. Environmental factors, such as nutrition, stress, and teratogens are thought
to change gene expression by switching genes on and off. These gene changes can then be
inherited by daughter cells. This would explain why monozygotic or identical twins may
increasingly differ in gene expression with age. For example, Fraga et al. (2005) found that
when examining differences in DNA, a group of monozygotic twins were indistinguishable
during the early years. However, when the twins were older there were significant discrepancies
in their gene expression, most likely due to different experiences. These differences included
susceptibilities to disease and a range of personal characteristics.
Box 2.2 The Human Genome Project
In 1990 the Human Genome Project (HGP), an international scientific endeavor, began the task
of sequencing the 3 billion base pairs that make up the human genome. In April of 2003, more
than two years ahead of schedule, scientists gave us the genetic blueprint for building a human.
Since then, using the information from the HGP, researchers have discovered the genes
involved in over 1800 diseases. In 2005 the HGP amassed a large data base called HapMap
that catalogs the genetic variations in 11 global populations. Data on genetic variation can
improve our understanding of differential risk for disease and reactions to medical treatments,
such as drugs. Pharmacogenomic researchers have already developed tests to determine
whether a patient will respond favorably to certain drugs used in the treatment of breast cancer,
lung cancer or HIV by using information from HapMap (NIH, 2015).
Future directions for the HGP include identifying the genetic markers for all 50 major forms of
cancer (The Cancer Genome Atlas), continued use of the HapMap for creating more effective
drugs for the treatment of disease, and examining the legal, social and ethical implications of
genetic knowledge (NIH, 2015).
From the outset, the HGP made ethical issues one of their main concerns. Part of the HGP’s
budget supports research and holds workshops that address these concerns. Who owns this
information, and how the availability of genetic information may influence healthcare and its
impact on individuals, their families, and the greater community are just some of the many
questions being addressed (NIH, 2015).
Learning Objectives: Prenatal Development
•
•
•
•
•
•
•
Describe the changes that occur in the three periods of prenatal development
Describe what occurs during prenatal brain development
Define teratogens and describe the factors that influence their effects
List and describe the effects of several common teratogens
Explain maternal and paternal factors that affect the developing fetus
Explain the types of prenatal assessment
Describe both the minor and major complications of pregnancy
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Prenatal Development
Now we turn our attention to prenatal development which is divided into three periods: The
germinal period, the embryonic period, and the fetal period. The following is an overview of
some of the changes that take place during each period.
The Germinal Period
The germinal period (about 14 days in
length) lasts from conception to implantation
of the fertilized egg in the lining of the
uterus (See Figure 2.5). At ejaculation
millions of sperm are released into the
vagina, but only a few reach the egg and
typically only one fertilizes the egg. Once a
single sperm has entered the wall of the egg,
the wall becomes hard and prevents other
sperm from entering. After the sperm has
entered the egg, the tail of the sperm breaks
off and the head of the sperm, containing the
genetic information from the father, unites
with the nucleus of the egg. It is typically
fertilized in the top section of the fallopian
Sperm and Ovum at Conception
tube and continues its journey to the uterus.
As a result, a new cell is formed. This cell,
containing the combined genetic information from both parents, is referred to as a zygote.
Figure 2.4
During this time, the organism begins cell division through mitosis. After five days of mitosis
there are 100 cells, which is now called a blastocyst. The blastocyst consists of both an inner
and outer group of cells. The inner group of cells, or embryonic disk will become the embryo,
while the outer group of cells, or trophoblast, becomes the support system which nourishes the
developing organism. This stage ends when the blastocyst fully implants into the uterine wall
(U.S. National Library of Medicine, 2015a). Approximately 50-75% of blastocysts do not
implant in the uterine wall (Betts et al., 2019).
Mitosis is a fragile process and fewer than one half of all zygotes survive beyond the first two
weeks (Hall, 2004). Some of the reasons for this include the egg and sperm do not join properly,
thus their genetic material does not combine, there is too little or damaged genetic material, the
zygote does not replicate, or the blastocyst does not implant into the uterine wall. The failure rate
is higher for in vitro conceptions. Figure 2.5 illustrates the journey of the ova from its release to
its fertilization, cell duplication, and implantation into the uterine lining.
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Figure 2.5 Germinal Period
“Human Fertilization” by Ttrue12 Source
The Embryonic Period
Starting the third week the blastocyst has
implanted in the uterine wall. Upon
implantation this multi-cellular organism is
called an embryo. Now blood vessels grow
forming the placenta. The placenta is a
structure connected to the uterus that
provides nourishment and oxygen from the
mother to the developing embryo via the
umbilical cord. During this period, cells
continue to differentiate. Growth during
prenatal development occurs in two major
directions: from head to tail called
cephalocaudal development and from the
midline outward referred to as
Photo by Lunar Caustic
proximodistal development. This means
that those structures nearest the head develop before those nearest the feet and those structures
nearest the torso develop before those away from the center of the body (such as hands and
fingers). The head develops in the fourth week and the precursor to the heart begins to pulse. In
the early stages of the embryonic period, gills and a tail are apparent. However, by the end of this
stage they disappear and the organism takes on a more human appearance. Some organisms fail
during the embryonic period, usually due to gross chromosomal abnormalities. As in the case of
Figure 2.6 The Embryo
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the germinal period, often the mother does not yet know that she is pregnant. It is during this
stage that the major structures of the body are taking form making the embryonic period the time
when the organism is most vulnerable to the greatest amount of damage if exposed to harmful
substances. Potential mothers are not often aware of the risks they introduce to the developing
embryo during this time. The embryo is approximately 1 inch in length and weighs about 8
grams at the end of eight weeks (Betts et al., 2019). The embryo can move and respond to touch
at this time.
The Fetal Period
From the ninth week until birth, the organism is referred to as a fetus. During this stage, the
major structures are continuing to develop. By the third month, the fetus has all its body parts
including external genitalia. In the following weeks, the fetus will develop hair, nails, teeth and
the excretory and digestive systems will continue to develop. The fetus is about 3 inches long
and weighs about 28 grams.
During the 4th – 6th months, the eyes become more sensitive to light and hearing develops. The
respiratory system continues to develop, and reflexes such as sucking, swallowing and
hiccupping, develop during the 5th month. Cycles of sleep and wakefulness are present at this
time as well. The first chance of survival outside the womb, known as the age of viability is
reached at about 24 weeks (Morgan, Goldenberg, & Schulkin, 2008). Many practitioners hesitate
to resuscitate before 24 weeks. The majority of the neurons in the brain have developed by 24
weeks, although they are still rudimentary, and the glial or nurse cells that support neurons
continue to grow. At 24 weeks the fetus can feel pain (Royal College of Obstetricians and
Gynecologists, 1997).
Figure 2.7 Fetus
Source
Between the 7th – 9th months, the fetus is primarily
preparing for birth. It is exercising its muscles and its
lungs begin to expand and contract. The fetus gains
about 5 pounds and 7 inches during this last trimester
of pregnancy, and during the 8th month a layer of fat
develops under the skin. This layer of fat serves as
insulation and helps the baby regulate body
temperature after birth.
At around 36 weeks the fetus is almost ready for birth.
It weighs about 6 pounds and is about 18.5 inches
long. By week 37 all of the fetus’s organ systems are
developed enough that it could survive outside the
mother’s uterus without many of the risks associated
with premature birth. The fetus continues to gain
weight and grow in length until approximately 40
weeks. By then the fetus has very little room to move
around and birth becomes imminent. The progression
through the stages is shown in Figure 2.8.
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Figure 2.8 Prenatal Development Age Milestones
Source
Prenatal Brain Development
Prenatal brain development begins in the third gestational week with the differentiation of stem
cells, which are capable of producing all the different cells that make up the brain (Stiles &
Jernigan, 2010). The location of these stem cells in the embryo is referred to as the neural
plate. By the end of the third week, two ridges appear along the neural plate first forming the
neural groove and then the neural tube. The open region in the center of the neural tube forms
the brain’s ventricles and spinal canal. By the end of the embryonic period, or week eight, the
neural tube has further differentiated into the forebrain, midbrain, and hindbrain.
Brain development during the fetal period involves neuron production, migration, and
differentiation. From the early fetal period until midgestation, most of the 85 billion neurons
have been generated and many have already migrated to their brain positions. Neurogenesis, or
the formation of neurons, is largely completed after five months of gestation. One exception is
in the hippocampus, which continues to develop neurons throughout life. Neurons that form the
neocortex, or the layer of cells that lie on the surface of the brain, migrate to their location in an
orderly way. Neural migration is mostly completed in the cerebral cortex by 24 weeks (Poduri &
Volpe, 2018). Once in position, neurons begin to produce dendrites and axons that begin to form
45
the neural networks responsible for information processing. Regions of the brain that contain
the cell bodies are referred to as the gray matter because they look gray in appearance. The
axons that form the neural pathways make up the white matter because they are covered in
myelin, a fatty substance that is white in appearance. Myelin aids in both the insulation and
efficiency of neural transmission. Although cell differentiation is complete at birth, the growth
of dendrites, axons, and synapses continue for years.
Teratogens
Good prenatal care is essential. The developing child is most at risk for some of the severe
problems during the first three months of development. Unfortunately, this is a time at which
many mothers are unaware that they are pregnant. Today, we know many of the factors that can
jeopardize the health of the developing child. The study of factors that contribute to birth defects
is called teratology. Teratogens are environmental factors that can contribute to birth defects,
and include some maternal diseases, pollutants, drugs and alcohol.
Factors influencing prenatal risks: There are several considerations in determining the type
and amount of damage that might result from exposure to a particular teratogen (Berger, 2005).
These include:
•
•
•
•
•
The timing of the exposure: Structures in the body are vulnerable to the most severe
damage when they are forming. If a substance is introduced during a particular structure’s
critical period (time of development), the damage to that structure may be greater. For
example, the ears and arms reach their critical periods at about 6 weeks after conception.
If a mother exposes the embryo to certain substances during this period, the arms and ears
may be malformed.
The amount of exposure: Some substances are not harmful unless the amounts reach a
certain level. The critical level depends in part on the size and metabolism of the mother.
The number of teratogens: Fetuses exposed to multiple teratogens typically have more
problems than those exposed to only one.
Genetics: Genetic make-up also plays a role on the impact a particular teratogen might
have on the child. This is suggested by fraternal twins exposed to the same prenatal
environment, but they do not experience the same teratogenic effects. The genetic makeup of the mother can also have an effect; some mothers may be more resistant to
teratogenic effects than others.
Being male or female: Males are more likely to experience damage due to teratogens
than are females. It is believed that the Y chromosome, which contains fewer genes than
the X, may have an impact.
Figure 2.9 illust