What are the critical periods in prenatal development

What are the critical periods in prenatal development? Source: Fetal Development (1999)
Body System
Especially Sensitive
Development up to
Central nervous
4th to 8th weeks
Postnatal, through to
5th to 9th weeks
12th week
Upper limbs
6th to 10th weeks
12th week
6th to 10 weeks
Lower limbs
6th to 10th weeks
12th week
9th to 11th weeks
9th to 11th weeks
16th week
External genitalia
9th to 11th weeks
6th to 11th weeks
13th week
What are the factors that contribute to low birth weight?
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. They
often receive late or inadequate prenatal care, breathe polluted air, live in overcrowded
conditions, move from place to place and ingest unhealthy substances. Poor women have
less access to family planning services, and they live in communities that encourage
higher birth rates, partly because these communities have higher death rates. In this way,
the social context may underlie many of the biological causes of low birth weight.
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 of having low birth weight babies. Research indicates
that obese women should gain seven to 11 kilograms during pregnancy; normal-weight
women 11 to 16 kilograms; and underweight women even more. Beyond overall weight,
certain nutrients defend against low birth weight, including zinc, iron, and folic acid.
Indeed, malnutrition (not age) is the primary reason young teenagers tend to have small
babies. They tend to eat sporadically and unhealthily, and because their own bodies are
still developing their diet is inadequate to support the growth of two.
What are some influences that can have a negative impact on healthy prenatal
Harmful substances such as drugs or radiation that invade the womb and result in birth defects
are called teratogens. Teratogens are especially damaging in the embryonic stage because it is a
critical period in prenatal development. Later, during the fetal stage, the environment provided
by the mother affects the baby’s size, behaviour, intelligence and health, rather than the
formation of organs and limbs (Bernstein and Nash, 1999, p. 333).
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 who eat raw meat. The expectant
mother may pick up the virus through the cat litter box (Santrock, 1999, p. 100).
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, p. 93).
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 seven 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 seven kilograms (Berger, 2000, p.
Infections and Diseases
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 third and fourth weeks of pregnancy. Syphilis is more
damaging later in prenatal development, four 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 her offspring is
nowhere better exemplified than when the mother is infected with HIV (Santrock, 1999,
p. 98).
Drugs include 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 four years of age. Cigarette
smoking by pregnant women can also adversely influence prenatal 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 barbituates may have babies who are addicted or may exhibit
tremors, restlessness, and irritability (Santrock, 1999, p. 98).
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:
o Of the more than 25 million low birth weight infants born worldwide each year
the overwhelming majority are in developing countries.
o Developing countries in the same geographic region, with similar ethnic
populations, have markedly different low birth weight rates when they have
different average incomes.
o Within nations, differences in low birth weight rates among ethnic groups follow
socioeconomic differences among those groups.
o Within the United States low birth weight rates in the poorest states are almost
twice those in some richer states (Berger, 2000, p. 120).
Emotional state
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, p. 93).
What are the factors that influence the degree of affect?
One crucial factor is the time at which the developing organism is exposed to a specific
teratogen. Some teratogens cause damage only during specific days or weeks early in pregnancy,
when a particular part of the body is being formed. 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 prenatal period.
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.
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) and fetuses are at a greater risk than female
in that 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, pp. 106-109).