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3.5: Periods of Prenatal Development

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    Now we turn our attention to prenatal development which is divided into three periods: the germinal period, the embryonic period, and the fetal period. Here is an overview of some of the changes that take place during each period.

    Environmental Risks

    Good prenatal care is essential. The developing child is most at risk for some of the most severe problems during the first three months of development. Unfortunately, this is a time at which many mothers are unaware that they are pregnant. Today, we know many of the factors that can jeopardize the health of the developing child. The study of factors that contribute to birth defects is called teratology. Teratogens are factors that can contribute to birth defects which include some maternal diseases, pollutants, drugs and alcohol.

    Factors influencing prenatal risks: There are several considerations in determining the type and amount of damage that might result from exposure to a particular teratogen (Berger, 2004). These include:

    • The timing of the exposure: Structures in the body are vulnerable to the most severe damage when they are forming. If a substance is introduced during a particular structure’s critical period (time of development), the damage to that structure may be greater. For example, the ears and arms reach their critical periods at about 6 weeks after conception. If a mother exposes the embryo to certain substances during this period, the arms and ears may be malformed.
    • The amount of exposure: Some substances are not harmful unless the amounts reach a certain level. The critical level depends in part on the size and metabolism of the mother. In other words, the dose of a teratogen that a pregnancy is exposed to is a determinant of the effects and that dose can be modified by not only the mother's exposure but by characteristics of the mother.
    • Genetics: Genetic make-up also plays a role on the impact a particular teratogen might have on the child. Just as genes later in life make us more or less susceptible to various disorders, a child's genes may make it more or less likely to exhibit the effects of exposure to a teratogen. This is suggested by fraternal (dizygotic) twin studies who are exposed to the same prenatal environment, yet do not experience the same teratogenic effects. The genetic make-up of the mother can also have an effect; some mothers may process a teratogen such that its effect on her offspring is minimized - effectively decreasing exposure that occurs.

    It should be noted that the effects of teratogens are not something that can be studied experimentally in humans. In other words, as it would be unethical to expose a human embryo or fetus to radiation, alcohol, cocaine, or high doses of caffeine, our understanding of the danger of such things has developed through the use of correlational studies and animal studies.

    Chart showing stages of prenatal development, beginning with the dividing zygote and implantation within the first two weeks, then the CNS and heart formation in week 3, then eyes, the heart, limbs, and ears between weeks 3 and 8, then genitals and increase brain development after week 9.
    Figure \(\PageIndex{6}\): Critical Periods of Prenatal Development. (Unknown; Not Specified via Lumen Learning)

    This page titled 3.5: Periods of Prenatal Development is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by Laura Overstreet via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.