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EPISODE 5 - CH. 3 - PRENATAL DEVELOPMENT
We never see more rapid and fascinating development than what occurs
during the 38 weeks of pregnancy after sperm meets egg.
From Zygote to Newborn
It takes 38 weeks to make a baby, from zygote to newborn. Sometimes youʼll
hear people talk about a 40-week pregnancy. Whatʼs the difference? It depends
on when you starting counting. If you start with the date of the womanʼs last
period, itʼs a 40-week pregnancy; if you start with the date of conception, itʼs a
38-week pregnancy. Sometimes doctors prefer to use the 40-week schedule
because they feel that day of the womanʼs last period is more easily
determined than the exact day conception occurred. It could take several days
for sperm to reach the egg, so just because a couple had sex, it in 1no way
indicates that thatʼs the day conception occurred. In fact we can be pretty
confident that is not the day that conception occurred.
We follow the scientistsʼ lead in this class, referring to a 38-week pregnancy
that can be broken down into three distinct periods of development: zygotic,
embryonic, and fetal. This isnʼt the same thing as trimesters, by the way,
although itʼs easy to learn how they overlap: All three prenatal periods – the
zygote, the embryo, and the fetus – occur during the first three months, the first
trimester. The last period, the period of the fetus, begins during the first
trimester, yes, but extends from the end of the first trimester all the way through
to birth. The nice thing about prenatal periods, in contrast to trimesters, is that
they are divided by the activities going on. The period of the zygote extends
from conception to implantation, the period of the embryo involves the
beginnings of all major bodily systems (organs, circulatory system, etc.), and
the period of the fetus is when all major body systems become fully functional.
Letʼs talk more in depth about each.
Most pregnant women donʼt even know theyʼre pregnant until after the germinal
period has passed. Nausea and other symptoms of pregnancy typically donʼt
begin until after the zygote has become embedded in the uterus lining around 2
weeks after conception (and this is the event that ends the germinal period).
But, rewinding a little bit, after conception and prior to implantation, the main
action of the germinal period is cell division. Lots and lots of cell division. Very
early on, before the zygote reaches the 8-cell stage, the cells could become
anything at all. They are called stem cells. Scientists hope someday to be able
to use stem cells to generate cells to cure many of our most vexing disorders
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and diseases. After the 8-cell stage, DNA instructions for the cells kick in, and
cells start to differentiate (specialize) and then move where they are needed.
The embryonic period follows the germinal period. It begins once implantation
is completed, which is around two weeks, and lasts through week 8.
Remember, weʼre still in the first trimester here. Some women will experience
“morning sickness” during this time, although itʼs really more accurate just to
call it nausea because many women feel it at different times of the day. Some
women will not even experience much nausea at all, or any. Others may notice
their sense of smell or taste becomes unbearably acute, which, in it itself, can
also bring on feelings of nausea! The embryo is really busy developing the start
of all major body systems. The cardiovascular system begins to function. All
organs get their start. Regardless of gender, the embryo at this point in time
has a genital tubercle right now, which is a little flap of tissue between the legs
that will develop into the sexual organs during the fetal period. Throughout
pregnancy, development proceeds in a proximodistal fashion, meaning that the
inner parts (organs) are maturing earlier than the outer parts (fingers).
Moving along onto the fetal period. The fetal period is long. It lasts from 9
weeks to 38 weeks. Notice again the start of the fetal period is within the first
trimester and then it extends throughout the entire remainder of the pregnancy.
The fetal period is a time for all major body systems and organs to gradually,
gradually reach maturity. Fetuses with chromosomes XY develop testes, which
begin to produce testosterone, which turns the genital tubercle into male sexual
organs and influences brain development as well. You know that sense of ʻI am
a boyʼ or ʻI am a girl,ʼ that the vast majority of us just feel when we donʼt have
confusion about our gender? That has to do with the way that hormones
influence our brains during prenatal development. A very small portion of
individuals have different experiences where their hormonal production during
this time is not gender typical. That can help you understand why a person can
honestly feel like they are a different gender then their body dictates. But like I
said, for the majority of us this all lines up together. The testosterone influences
the development of organs, and also influences brain development. Without
testosterone, female sex organs develop and a female-ish brain develops as
well. By the way, let me just stop here though and clarify that although there
are some differences between the male and female brain, there are fewer
differences than you probably think, Weʼll talk about what things differ and what
things donʼt across the semester. The fetus experiences dramatic growth,
from .25 oz (on average) at week 8 to 7.5 lbs (on average) at week 38. Fat
layers develop near the very end of the pregnancy as well.
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The organ that takes the longest to develop is the brain, as neurons are
created, sent to their designated positions, and begin making connections with
other neurons. Brain development progresses gradually from back to front, so
gradually, in fact, that the very front portion of the cortex (the prefrontal cortex)
does not become fully mature until late adolescence or early adulthood!
Prenatal development, in general, is cephalocaudal, meaning that the brain is a
priority. The age of viability, a time when a preterm baby might be able to
survive outside of the womb, is determined largely by brain development. This
occurs somewhere between 22-26 weeks after conception, so thatʼs during the
fetal period. Thinking about the cephalocaudal development, the priority of the
brain, notice how the proportion of the fetusʼs body is so different from ours,
with a huge head compared to the rest of the body. That huge head is
containing that prioritize brain. Eventually, as the infant develops into
childhood, the rest of the body will catch up to the brain and we will see more
adult-like proportions.
The priority of brain development, and our enormous heads that go with it, can
make birth much more difficult for us compared to other species. And, the fact
that we walk upright (instead of on all fours) led us to evolve with more narrow
hips than our primate ancestors. A large fetus head and narrow maternal hips =
a challenging birth. Luckily, in the vast majority of cases today, birth proceeds
with a hitch. Although this certainly was not the case in the past when it was
much more common for the mother or the baby or both to die during childbirth.
Thankfully we have c- sections and other medical interventions to help if there
are problems during birth. Some, though, would suggest that the modern
hospital birth, with hormones used to hasten contractions, etc., may actually
prolong and complicate the birth process and lead to more c-sections than
would be necessary. Itʼs true that c-section births have increased, and thatʼs
something to be aware of and to keep investigating, but itʼs also true that
without procedures like c-sections, many lives wouldʼve been lost.
Complications
One troubling aspect of the birthing experience in the United States is the
increasing rates of babies born weighing less than 5.5 lbs, a marker for low
birthweight. A low birthweight baby may be preterm (born early). Some are not
born early, but are born small for their gestational age. They may be 38 weeks
of age, but still less than 5.5 lbs. This is often due to substance abuse
(including smoking) or malnutrition thatʼs occurred during the pregnancy. In
general, nurture plays a stronger role in the low birthweight experience than
nature does, especially considering that rates of low birthweight are decreasing
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in other developed nations while they increase here in the U.S. Stress may
also impact prenatal development. When a pregnant mother is extremely
stressed out, her bodyʼs resources are diverted to her brain and to her muscles
for survival. Itʼs like the body prioritizes things and says, “We canʼt focus that
much on this developing baby in the uterus; weʼve got to make sure to keep
mom alive.” So what does this mean for the motherʼs body? Well, blood flow to
the uterus is decreased when a mother is extremely stressed out and the fetus,
then, is deprived of oxygen and nutrients.
One final reason for an increased rate of low birthweight is infertility treatments
that result in multiple births. A fetus who has had to share the uterus with
others is almost always smaller than one who had the whole place to him- or
herself. Sometimes itʼs just a matter of space.
I mentioned how substance abuse can impact prenatal development. Letʼs look
more closely at things like that. Today, the idea that the developing human is
affected by whatever the mother ingests or is exposed to seems obvious. But it
wasnʼt always that way. For a long time, scientists incorrectly believed that the
placenta shielded the developing human against harmful substances. It wasnʼt
until 1941 that the first teratogen was identified: rubella (German measles). In
the early 1960s, the thalidomide disaster occurred. Thalidomide was a drug
prescribed to relieve morning sickness during pregnancy. Thousands of
children across the world were born with limb deformities due to thalidomide.
After this, scientists started looking more closely at other potential teratogens,
such as alcohol. Fetal Alcohol Syndrome was officially recognized in 1973.
Even a small amount of alcohol can take a toll on prenatal development
(especially on the brain), as evidenced by the fact that alcohol is the leading
teratogenic cause of mild mental retardation. Teratogens that influence the
brain, by the way, are called behavioral teratogens.
The impact a teratogen has on the developing human depends upon three key
factors: genetic vulnerability, the timing of the exposure, and the amount of the
exposure. The embryo or fetusʼs genes may make them more vulnerable to the
effects of a teratogen. Take alcohol, for example. Some of us metabolize
alcohol more quickly than others, and this is influenced by our genes. An
embryo without such genes metabolizes alcohol more slowly, meaning that it
stays in his/her system longer and has a more devastating impact on
development. So, genetic vulnerability certainly matters, although the best
advice for a pregnant woman is to avoid this risk altogether by not drinking
during pregnancy. How about the timing of the exposure? Again, letʼs focus on
alcohol. Maternal drinking during the embryonic period will impact the
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development of facial features and the brain. In fact, children with FAS (Fetal
Alcohol Syndrome) have unique facial features because of this. But, if a mother
drinks during the fetal period only, the child may be born with typical facial
features but have significant brain damage nevertheless. The amount of the
teratogen the developing human was exposed to plays an important role as
well. With alcohol, sustained heavy drinking results in FAS, whereas light or
moderate drinking may have a lesser impact (although that impact – usually
mental retardation – is still quite significant). Some substances do not become
teratogenic at all until they reach a certain dosage, something referred to as a
threshold effect. Take vitamin A, which is healthy in small doses but can result
in extreme birth defects if taken in large doses. Accutane is a medication for
acne and itʼs basically a megadose of vitamin A, which is why many doctors
require women on Accutane to be on birth control at the same time.
Even if the beginning of life involves complications, thatʼs not the end of the
story. With low birthweight babies and those exposed to teratogens, we still
have all of the years ahead for a positive environment and responsive
parenting to make a difference.
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