What are the critical periods in prenatal development

By Travis Washington,2014-04-06 20:05
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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 …

    ththCentral nervous 4 to 8 weeks Postnatal, through to

    system/Brain adulthood

    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).

    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).

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