Embryology · Teratology and prenatal diagnosis
Principles of teratology: critical periods, dose and genotype !
- Topic
Why the same drug, virus or exposure can do nothing, cause a miscarriage or cause a birth defect, depending on the week of pregnancy.
Updated 2026-11-01
What it is
Teratology studies how outside agents (drugs, infections, radiation, maternal disease) disturb development. The effect depends mostly on timing, dose and the genotype of mother and embryo. The embryonic period, weeks 3 to 8 after fertilization, carries the highest risk of major malformations, often before a woman knows she is pregnant.
Key facts
- About 3% of live-born infants have a major structural anomaly.
- Causes: roughly 15% environmental, 30% genetic, and most are multifactorial or unknown.
- Each organ has its own sensitive window: the neural tube closes in week 4 and the palate fuses around weeks 7–8 (embryonic age).
- The placenta is not a barrier: most small, lipid-soluble drugs cross it.
- The four possible outcomes are death, malformation, growth restriction and functional disorder.
Step by step
- An agent reaches the embryo, almost always across the placenta.
- It acts by a specific mechanism: cell death, reduced proliferation, altered signalling or vascular damage.
- The tissue forming at that moment is the most vulnerable.
- Dose, duration and maternal and fetal genotype modulate the result.
- It shows as pregnancy loss, malformation, smaller growth or functional impairment.
Timing of exposure and typical effect
| Period | Weeks (from fertilization) | Typical effect |
|---|---|---|
| Pre-implantation | 1–2 | All-or-none: loss or an unharmed embryo |
| Embryonic | 3–8 | Major structural malformations |
| Fetal | 9 to term | Growth restriction, functional and central nervous system defects |
Clinical notes
- Assessing an exposure means asking the exact date, agent, dose and duration, and working out the embryonic age.
- Decisions about medicines in pregnancy are made with the treating professional: stopping a needed drug can also harm mother and fetus.
- Many defects arise before the first prenatal visit, which is why preconception counselling matters.
Test yourself
Think of the answer, then tap the question to see it.
Which period of development carries the highest risk of major structural malformations?
The embryonic period, weeks 3 to 8 after fertilization, when the organs form.
What does the 'all-or-none' effect mean?
In the first two weeks a harmful exposure either causes loss of the embryo or the embryo recovers completely without malformations.
Which kind of damage predominates when exposure happens in the fetal period?
Growth restriction and functional problems, especially of the central nervous system, rather than major malformations.
Besides timing, which factors determine a teratogen's effect?
Dose and duration of exposure, the genotype of mother and embryo, and the agent's mechanism of action.
Related
- Weeks 3–8: the embryonic period and organogenesis
- Gestational age vs fertilization age: how pregnancy is dated
- Malformation, disruption, deformation, sequence, syndrome, association
- Prenatal diagnosis: ultrasound, serum screening, cell-free DNA, CVS, amniocentesis
- Thalidomide and limb reduction defects
- Ionizing radiation, tobacco and cocaine
- Valproate, carbamazepine and phenytoin
- Placenta: structure, circulation and functions
Related conditions
Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given.
Sources: Langman's Medical Embryology, 13th ed., ch. 9: Birth Defects and Prenatal Diagnosis (Wolters Kluwer, 2015); About Birth Defects (CDC, 2024); Radiation Effects on the Fetus (StatPearls Publishing (NCBI Bookshelf), 2023); Teratogenic Medications (StatPearls Publishing (NCBI Bookshelf), 2024).
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Representative model, for educational purposes only.