What are the critical periods in prenatal development? Source: Fetal Development (1999)
Body System Especially Sensitive Development up
ththCentral nervous 4 to 8 weeks Postnatal, through to
thththHeart 5 to 9 weeks 12 week
thththUpper limbs 6 to 10 weeks 12 week
thEyes 6 to 10 weeks Term
thththLower limbs 6 to 10 weeks 12 week
ththTeeth 9 to 11 weeks Term
thththPalate 9 to 11 weeks 16 week
ththExternal genitalia 9 to 11 weeks Term
thththEars 6 to 11 weeks 13 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. Malnutrition
; 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 development?
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).
; 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).
; 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).