Prenatal Development Stages and Teratogen Risks

Posted on Mar 22, 2025 in Biology

Prenatal Development Stages

There are three main stages of prenatal development:

  1. Germinal Period (First 2 Weeks)

    • Begins with the fusion of ovum and sperm (fertilization).
    • The fertilized egg (zygote) travels down the Fallopian tubes and attaches to the endometrial lining.
    • The zygote becomes multicellular, now referred to as a blastocyst.
    • Potential Complications:
      • Ectopic Pregnancy: Zygote attaches to the Fallopian tube instead of the uterine lining, posing a serious risk to the mother.
      • Zygote fails to attach and is passed with the shed endometrial lining.

  2. Embryonic Period (Weeks 3-8)

    • Rapid development where all organ systems are laid down in a rudimentary form.
    • Organ systems are interdependent.
    • All sensory systems begin forming.
    • Critical Period: This is the most vulnerable time for teratogen exposure, as the embryo is highly susceptible to damage.

  3. Fetal Period (Week 9 to Birth)

    • Longest prenatal period, lasting approximately 7 months.
    • Characterized by rapid growth and refinement of organ systems, especially between the 9th and 20th week.

Teratogens and Their Impact

Teratogen: Any environmental agent that can cause harm to the developing embryo or fetus.

  • External toxins and dangers can be transmitted through the mother’s blood (e.g., alcohol, nicotine, caffeine).

Factors Determining Teratogen Impact

  • Dose: Larger doses over longer periods usually have more negative effects.
  • Heredity: Genetic makeup of the mother and fetus influences susceptibility.
  • Other Negative Influences: Presence of multiple factors (e.g., poor nutrition, lack of medical care) can worsen effects.
  • Age: Effects vary depending on the organism’s age at exposure; the embryonic period is the most critical.

Examples of Teratogens

  1. Tobacco

    • Often results in low birth weight.
    • Can lead to miscarriage, prematurity, cleft lip/palate, blood vessel abnormalities, impaired heart rate, breathing difficulties, infant death, and asthma/cancer later in life.
    • The more cigarettes a mother smokes, the greater the risk to the baby.
    • Babies of smoking mothers may be less attentive to sounds and display more muscle tension.
    • Quitting smoking at any point during pregnancy reduces risks.
    • Nicotine constricts blood vessels, reducing blood flow and nutrient transfer to the fetus.
    • Nicotine increases carbon monoxide in the bloodstream, damaging the central nervous system.

  2. Alcohol

    • Fetal Alcohol Spectrum Disorders (FASD): A range of physical, mental, and behavioral outcomes caused by prenatal alcohol exposure.
    • Even with enriched diets, FAS babies may not catch up in physical size.
    • Mental impairment associated with FASD is permanent.
    • Alcohol interferes with neuron production and migration, reduces brain size, and damages the brain.
    • Heavy drinking deprives the fetus of oxygen.
    • No amount of alcohol is safe during pregnancy.