Prenatal development
We don't often discuss prenatal development in psychology courses, but I think that's really
unexcusable. If anything, it is the most significant segment of our development! So I have
included this short chapter, adapted in large part from The Medical Encyclopedia, U.S. National
Library of Medicine, available online at
http://www.nlm.nih.gov/medlineplus/ency/article/002398.htm. I know many of you have seen
all this before in other classes, but it is well worth the review!
After sexual intercourse, sperm travels through the cervix and uterus and into the Fallopian
tubes. Conception usually takes place in the outer third of the Fallopian tube. A single sperm
penetrates that egg and a joining of the genetic information occurs. This resulting single cell is
called a zygote.
The zygote spends the next few days traveling down the Fallopian tube and rapidly multiplying
the number of cells through division. A mulberry-like mass, like a hollow rubber ball, 1/100 inch
wide, results from the cell division. This ball of cells in the Fallopian tube is called a morula.
With additional cell division, the morula becomes a blastocyte, with an inner core and an outer
shell of cells. The outer group of cells become the membranes that nourish and protect the inner
group of cells, which becomes the fetus.
The blastocyte implants in the uterus between the 7th and 9th day after conception. At this point
the endometrium (the lining of the uterus) has grown and is ready to support a fetus. The
blastocyte burrows into the endometrium where it receives nourishment. It is barely visible, but
doubles every 24 hours. The placenta and supporting infrastructure for pregnancy develop at this
time as well. It is estimated that up to 55% of zygotes never reach this phase of growth.
The Embryo
The embryonic stage begins on the 15th day after conception and continues until about the 8th
week, or until the embryo is 1.2 inches in length. During this period the cells of the embryo are
not only multiplying, but they are taking on specific functions. This process is called tissue
differentiation. It is during this critical period of differentiation (most of the first trimester or
three-month period) that the growing fetus is most susceptible to damage from external sources
(teratogens) including viral infections such as rubella, x-rays and other radiation, and poor
nutrition.
A child who has one developmental problem may have other problems that arose at the same
time: Kidney problems and hearing problems, for example, are often found together because
both kidneys and the inner ears develop at the same time.
In Week 3 we see the formation of the heart, the beginning development of the brain and spinal
cord, and the beginning of the gastrointestinal tract.
Teratogens introduced during this period may cause severe problems such as the absence of one
or more limbs or a heart that is outside of the chest cavity at birth.
Weeks 4 and 5 --1/4 inch long: Here we see the beginnings of the vertebra, the lower jaw, the
larynx (voice box), and the rudiments of the ear and eye. The heart, which is still outside body,
now beats at a regular rhythm. Although arm and leg "buds" are visible with hand and foot
"pads," the embryo still has a tail and cannot be distinguished from pig, rabbit, elephant, or
chick embryo.
Teratogens may cause very serious problems involving the esophagus, vertebrae, eyes. The baby
could be born with severe facial clefts or missing hands or feet.
Week 6 -- 1/2 inch, 1/1000 of an ounce: In week 6, we see the formation of the nose, jaw, palate,
lung buds. The fingers and toes form, but may still be webbed. The tail is receding, and the
heart is almost fully developed.
Teratogens at this point may leave the baby with profound heart problems or a cleft lip.
Week 7 -- 7/8 inch, 1/30 ounce (less than an aspirin): This week, the eyes move forward on the
face, and the eyelids and tongue begin to form. All essential organs have begun to form.
Teratogens may cause heart and lung problems, a cleft palate, and ambiguous genitalia (not quite
male or female).
Week 8 --1 inch, 1/15 ounce: The embryo now resembles a human being. The facial features
continue to develop and the external ear appears. Also, we see the beginnings of external
genitalia. By now, the circulation through the umbilical cord is well developed. The long bones
begin to form and the muscles are able to contract.
Teratogens may still cause heart problems and stunting of the fingers and toes.
The Fetus
At this point the embryo is developed enough to call a fetus. All organs and structures found in a
full-term newborn are present.
Weeks 9 to 12 -- 3 inches, 1 ounce: The head comprises nearly half of the fetus’ size and the face
is well formed. The eyelids close now and will not reopen until about the 28th week. The tooth
buds for the baby teeth appear. The genitalia are now clearly male or female.
Weeks 13 to 16 -- 6 inches: These weeks mark the beginning of the second trimester. A lthough
the skin of the fetus is almost transparent, fine hair develops on the head called lanugo. The
fetus makes active movements, including sucking, which leads to some swallowing of the
amniotic fluid. A thin dark substance called meconium is made in the intestinal tract. The heart
beats120-150 beats per minute and brain waves detectable.
Weeks 17 to 20 -- 8 inches: Eyebrows and lashes appear and nails appear on fingers and toes.
This is an exciting time for the parents: The can mother feel the fetus moving ("quickening")
and the fetal heartbeat can be heard with a stethoscope.
Weeks 21 to 24 -- 11.2 inches, 1 lb. 10 oz.: All the eye components are developed, footprints and
fingerprints are forming, and the entire body covered in cream-cheese-like vernix caseosa. The
fetus now has a startle reflex.
Weeks 25 to 28 -- 15 inches, 2 lbs. 11 oz.: Now we are entering the third trimester. During these
weeks, we see rapid brain development. The nervous system is developed enough to control
some body functions, and the eyelids open and close. A baby born at this time may survive, but
the chances of complications and death are high.
Weeks 29 to 32 -- 15 -17 inches, 4 lbs. 6 oz.: These weeks see further development towards
independent life: There is a rapid increase in the amount of body fat and the fetus begins storing
its own iron, calcium, and phosphorus. The bones are fully developed, but still soft and pliable.
There are rhythmic breathing movements present, the fetal body temperature is partially selfcontrolled, and there is increased central nervous system control over body functions.
Weeks 33 to 36 -- 16 -19 inches, 5 lbs. 12 oz. to 6 lbs. 12 oz.: The lanugo (body hair) begins to
disappear. A baby born at 36 weeks has a high chance of survival.
Weeks 37 to 40 -- 19 - 21 inches 7 or 8 pounds: At 38 weeks, the fetus is considered full term. It
fills the entire uterus, and its head is the same size around as its shoulders. The mother supplies
the fetus with the antibodies it needs to protect it against disease.
Birth
Birth has nearly as many dangers as the embryonic stage, mostly from infections and anoxia.
Anoxia means "no oxygen," and can be due to a number of situations: If something prevents the
exchange of blood from mother to child prior to the baby breathing on its own, the lack of
oxygen quickly begins to take its toll, especially on the brain. This is called cord strangulation,
and it can be caused by the cord being pressed between the baby's head and the mother's pelvis.
A breech birth, which involves the baby's buttocks going first instead of its head, can slow the
birth process. And premature separation of the placenta from the mother's uterus can cause
anoxia as well.
Sometimes, it is necessary to perform a Caesarian section (or C-section) when such problems
arise, in order to get the baby out more quickly. It is a fairly routine operation involving cutting
the mother's abdomen and uterine wall. In emergencies, this is done with general anesthesia, but
can also be done with spinal anesthesia, which allows the mother to remain awake and speeds
her recovery time. It is, of course, stills a serious surgical procedure and involves enough risks
that a vaginal birth is usually preferred.
Approximate Timetable of
Prenatal Development
This page presents a detailed overview of human development from the time the sperm is united
with the ovum until birth. Prenatal development is divided into three trimesters. During the
first two months the developing human is referred to as an embryo. The embryo has three layers
from which all body organs develop. During the second trimester the developing human is
referred to as a fetus. During the third trimester, the individual is a baby which if born
prematurely could survive with extra support. Premature births prior to the third trimester are
less likely to survive, even with extraordinary medical care in a neonatal intensive care unit.
First Trimester
First Month
Fertilization, descent of ovum from tube to uterus. Early cell division and formation of
embryonic disc from which new organism will develop. Early formation of three layers of cells:
(1) the ectoderm, from which sense organs and nervous system will develop (2) the mesoderm,
from which circulatory, skeletal and muscular systems will develop (3) the endoderm, from
which digestive and some glandular systems will develop. Special layer of cells formed in the
uterus which will become the placenta and through which nutritive substances will be carried to
the new organism and waste products carried away. Special layer of cells forms the amnion or
water-sac, which will surround the developing embryo except at umbilical cord. Heart tube
forms and begins to pulsate and force blood to circulate through blood vessels in embryonic
disc. Nervous system begins to arise, first in form of neural groove. Development of intestinal
tract, lungs, liver and kidneys begins. By end of one month, the embryo is about one-fourth inch
long, curled into a crescent, with small nubbins on sides of body indicating incipient arms and
legs.
Second Month
Embryo increases in size to about 1½ inches. Bones and muscle begin to round out contours of
body. Face and neck develop and begin to give features a human appearance. Forehead very
prominent, reflecting precocious development of brain in comparison to rest of body. Limb buds
elongate. Muscles and cartilage develop. Sex organs begin to form.
Third Month
Beginning of fetal period. Sexual differentiation continues, with male sexual organs showing
more rapid development and the female remaining more neutral. buds for all 20 temporary teeth
laid down. Vocal cords appear; digestive system shows activity. Stomach cells begin to secrete
fluid; liver pours bile into intestine. Kidneys begin functioning, with urine gradually seeping
into amniotic fluid. Other waste products passed through placenta into mother's blood. Bones
and muscles continue development, and by end of third month spontaneous movements of arms,
legs, shoulders and fingers are possible.
Second Trimester
Fourth Month
Lower parts of body show relatively accelerated rate, so that head size decreases from one-half to
one-fourth of body size. Back straightens, hands and feet are well-formed. Skin appears dark
red, owing to coursing of blood showing through thin skin and wrinkles, owing to absence of
underlying fat. Finger closure is possible. Reflexes become more active as muscular maturation
continues. Fetus begins to stir and so thrust out arms and legs in movements readily perceived
by the mother.
Fifth Month
Skin structures begin to attain final form. Sweat and sebaceous glands are formed and function.
Skin derivatives also appear -- hair, nails on fingers and toes. Bony axis becomes quite straight
and much spontaneous activity occurs. Fetus is lean and wrinkled, about one foot long and
weighs about one pound.
Sixth Month
Eyelids which have been fused shut since third month, reopen; eyes are completely formed.
Taste buds appear on tongue and in mouth and are, in fact, more abundant than in the infant or
adult.
Third Trimester
Seventh Month
Organism capable of independent life from this time on. Cerebral hemispheres cover almost the
entire brain. Seven-month fetus can emit a variety of specialized responses. Generally is about
15 inches long and weighs about three pounds.
Eighth and Ninth Month
During this time, finishing touches are being put on the various organs and functional capacities.
Fat is formed rapidly over the entire body, smoothing out the wrinkled skin and rounding out
body contours. Dull red color of skin fades so that a firth pigmentation of skin is usually very
slight in all races. Activity is usually great and he can change his position within the somewhat
crowded uterus. Periods of activity will alternate with periods of quiescence. Fetal organs step
up their activity. Fetal heart rate becomes quite rapid. Digestive organs continue to expel more
waste products, leading to the formation of a fetal stool, called the meconium, which is expelled
shortly after birth. Violent uterine contractions begin, though milder ones have been tolerated
earlier, and the fetus is eventually expelled from the womb into an independent physiological
existence.
Influences on prenatal development
Abstract: There are many sources of potential harm to both developing baby and mother.
Ranging from environmental concerns such as toxic chemicals, fumes and poisons, to drugs
including smoking and alcohol, the delicate nature of prenatal development can often be
jeopardized. This article describes the various influences on prenatal development.
What are some influences that impact on healthy prenatal development?
Teratogens are the broad range of substances (such as drugs and pollutants) and conditions (such
as severe malnutrition and extreme stress) that increase the risk of prenatal abnormalities. These
abnormalities include obvious physical problems (such as missing limbs) and more subtle
impairments such as brain damage that first appears in elementary school. A specific teratogen
may damage the body structures, the growth rate, the neurological networks, or all three.
Teratogens that harm the brain, and therefore make a child hyperactive, antisocial, and retarded
and so on, are called behavioural teratogens; their effects can be far more damaging over the life
of a person than physical defects. (Berger, 2000)
What are the factors that influence the degree of affect?
One crucial factor is when the developing organism is exposed to which teratogen. Some
teratogens cause damage only during specific days or weeks early in pregnancy, when a
particular part of the body is undergoing formation. Others can be harmful at any time, but how
severe the damage is depends on when the exposure occurred. The time of greatest susceptibility
is called the critical period. Each body structure has its own critical period. As a general rule, for
physical defects the critical period is the entire period of the embryo. (Berger, 2000) A second
important factor is the dose and/or frequency of exposure to a teratogen. For most teratogens,
experts are reluctant to specify a threshold below which the substance is safe. One reason is that
many teratogens have an interaction effect; that is, one poison intensifies the effects of another.
(Berger, 2000) A third factor that determines whether a specific teratogen will be harmful, and to
what extent, is the developing organism's genes. In some cases, genetic vulnerability is related to
the sex of the developing organism. Generally, male embryos (XY) embryos and fetuses are at a
greater risk than female in that more male embryos are more often aborted spontaneously. In
addition, newborn boys have more birth defects, and older boys have more learning disabilities
and other problems caused by behavioural teratogens. (Berger, 2000)
What are some of the specific influences that may affect prenatal development?
Radiation, chemicals and other hazards in the environment can endanger the fetus.
Chromosomal abnormalities are higher among the offspring of fathers exposed to high levels of
radiation in their occupations. Environmental pollutants and toxic wastes are also sources of
danger to unborn children. Among the dangerous pollutants and wastes are carbon monoxide,
mercury and lead. Another environmental concern is toxoplasmosis, a mild infection that causes
cold-like symptoms or no apparent illness in adults, but can cause eye defects, brain defects and
premature birth. Cats are common carriers of toxoplasmosis, especially outdoor cats that eat raw
meat. The expectant mother may pick up the virus through the cat litter box. (Santrock, 1999)
In terms of the mother’s age, two time periods are of special interest: adolescence and the
thirties and beyond. Infants born to adolescents are often premature. The mortality rate of infants
born to adolescent mothers is double that of infants born to mothers in their twenties. Down
Syndrome, a form of mental retardation, is related to the mother's age. By age 40, the probability
is slightly over 1 in 100. By age 50, it is almost 1 in 10. The risk is also higher before age 18.
Women also have more difficulty in becoming pregnant after the age of 30. (Santrock, 1999)
A developing fetus depends completely on its mother for nutrition, which comes from the
mother's blood. Among the important factors are the total number of calories and the appropriate
levels of protein, vitamins and minerals. The mother's nutrition even influences her ability to
reproduce. In extreme instances of malnutrition, women stop menstruating. Also children born to
malnourished mothers are more likely to be malformed. (Santrock, 1999) Another common
reason for slow fetal growth - and hence low birth weight - is maternal malnutrition, a problem
that has many specific causes. Women who begin pregnancy underweight, eat poorly during
pregnancy, and consequently do not gain at least 1.5 kilograms per month in the second and third
trimesters run a much higher risk than others of having a low-birth weight infant. Indeed, women
who gain less than 7 kilograms, even if they are non-smokers who begin pregnancy overweight,
still have a higher risk of preterm and smaller babies than those who gain at least 7 kilograms.
(Berger, 2000)
Maternal diseases and infections can produce defects by crossing the placental barrier. For
example, the greatest damage to the fetus from the mother contracting German measles occurs
during the 3rd and 4th weeks of pregnancy. Syphilis is more damaging later in pre-natal
development - 4 months or more after conception. Rather than affecting organ development as
Rubella does, syphilis damages organs after they have formed. The importance of the mother's
health to the health of their offspring is nowhere better exemplified than when the mother is
infected with HIV. (Santrock, 1999)
Drugs include the use of tobacco, alcohol, prescription or illegal drugs. For example, the effects
of thalidomide during the fourth week of development had devastating effects. Heavy drinking
by an expectant mother can also be devastating. Fetal alcohol syndrome is a cluster of
abnormalities that appear in the offspring of mothers who drink alcohol heavily during
pregnancy. The abnormalities include facial deformities and defective limbs, face and heart.
Most of these children are below average in intelligence. In one study, however, even mothers
who drank moderately during pregnancy had babies who were less attentive and alert, with the
effects still present at 4 years of age. Cigarette smoking by pregnant women can also adversely
influence pre-natal development, birth and postnatal development. Fetal and neonatal deaths are
higher among smoking mothers. Also prevalent are a higher incidence of preterm births and
lower birth weights. Respiratory problems and sudden infant death syndrome are also more
common among the offspring of mothers who smoked during pregnancy. Tranquilizers taken
during the first three months may cause cleft palate or other congenital malformations. Mothers
who take large amounts of barbiturates may have babies who are addicted or may exhibit
tremors, restlessness and irritability. (Santrock, 1999)
Usage
Effects
Drug
3 or more drinks daily, or binge Causes fetal alcohol syndrome (FAS).
drinking of 5 or more drinks on Symptoms include abnormal facial
one occasion early in pregnancy characteristics (small head, wide spacing
between the eyes, a flattened nose, a
narrow upper lip, unusual eyelids), overall
growth retardation, learning disabilities
and behaviour problems.
More than ½ ounce of absolute Causes fetal alcohol effects (FAE). FAE
alcohol a day
does not obviously affect facial appearance
or physical growth, but it affects brain
functioning.
Moderate drinking: less than 1 or Probably has no negative effects on
2 servings of beer or wine or 1 prenatal development, although this is
mixed drink on a few days per controversial
week
Tobacco Maternal smoking early in Increases risk of abnormalities, including
pregnancy
malformations of the limbs and the urinary
tract
Maternal
smoking
late
in Reduces birth weight and size. Babies born
pregnancy
to habitual smokers weigh, on average,
about 250 grams less than they would
otherwise be expected, and they are
shorter, both at birth and in the years to
come. They may have childhood problems,
particularly with respiration and, in
adulthood, increased risk of becoming
smokers themselves.
Paternal smoking
Reduces birth weight by about 45 grams on
average
Alcohol
Compared with women of higher socioeconomic status, pregnant women at the bottom of the
economic ladder are more likely to be ill, malnourished, teenaged, and stressed. Physical
difficulty like malfunction of the placenta or the umbilical cord is likely when pregnancies are
closely spaced and close spacing correlates with poverty. Poverty helps explain the wide
national and international variations in the following statistics:



Of the more than 25 million low-birth weight infants born worldwide each year, the
overwhelming majority are in developing countries.
Developing countries in the same geographic region, with similar ethnic populations,
have markedly different low-birth weight rates when they have different average
incomes.
Within nations, differences in low-birth weight rates among ethnic groups follow
socioeconomic differences among those groups.

Within the United States, low birth weight rates in the poorest states are almost twice
those in some richer states. (Berger, 2000)
The mother's stress can be transmitted to the fetus. When a pregnant woman experiences intense
fears, anxieties and other emotions, physiological changes occur in the fetus. These include
changes in respiration and glandular secretions. For example, producing adrenaline in response
to fear restricts blood flow to the uterine area and may deprive the fetus of adequate oxygen.
Also, reassuring the mother of fetal well-being has positive outcomes for the infants in the study.
(Santrock, 1999)
Are there times during pregnancy when the effect of teratogens is especially important?
Not only is the specific teratogen important in determining the effects on prenatal development,
but so is the time during pregnancy when the teratogen influences the fetus. This is referred to as
a critical period. The table below describes the critical periods for fetal development for the
major organs and body systems.
Body System
Central Nervous System/Brain
Heart
Upper limbs
Eyes
Lower limbs
Teeth
Palate
External Genitalia
Ears
Especially Sensitive
4th to 8th weeks
5th to 9th weeks
6th to 10th weeks
6th to 10 weeks
6th to 10th weeks
9th to 11th weeks
9th to 11th weeks
9th to 11th weeks
6th to 11th weeks
Development up to …
Postnatal, through to adulthood
12th week
12th week
Term
12th week
Term
16th week
Term
13th week
Summary
Teratogens represent the broad range of substances and conditions that can seriously impact
prenatal development. Ranging from environmental conditions such as toxic chemicals and
radiation to specific substances such as tobacco and alcohol, the delicate nature of prenatal
development can be compromised and the effects of that impairment last a lifetime.