Anatomy chapters
Teratology
How drugs, infections, radiation and maternal illness cause birth defects, why timing matters, and the classic teratogen and defect pairs examiners use.
- A teratogen is anything that causes a structural or functional birth defect.
- The embryonic period (weeks 3 to 8) is the most sensitive time, and each organ has its own critical period.
- Learn the classic pairs: thalidomide-limbs, rubella-heart, eye, ear, valproate-neural tube, alcohol-face and brain.
On this page
Overview
Teratology is the study of birth defects and their causes. A teratogen is any agent that can cause a defect in the developing baby.
Teratogens include drugs, infections, radiation, chemicals and some maternal illnesses. Many defects have a genetic cause or a mix of genes and environment.
Principles of teratology
Whether a teratogen causes harm depends on several things.
- The timing of exposure. This is the most important factor.
- The dose and how long the exposure lasts.
- The genes of the mother and embryo, which make some more susceptible.
- The way the agent acts, which shapes the pattern of the defect.
Timing
| Period | Effect of a teratogen |
|---|---|
| Weeks 1–2 (before the germ layers) | "All or nothing": the embryo dies or recovers fully |
| Weeks 3–8 (embryonic period) | Major structural defects; the most sensitive time |
| Week 9 to birth (fetal period) | Growth problems, functional defects, damage to the brain, eyes and genitalia that are still developing |
The embryonic period (weeks 3 to 8) is the most sensitive time, because all organs are forming.
Each organ has its own critical period. The heart is most sensitive around weeks 3 to 6, the limbs around weeks 4 to 7. The brain keeps developing all through pregnancy, so it can be harmed at any time.
The central nervous system stays sensitive throughout pregnancy, not just in the embryonic period.
Infections (TORCH)
| Infection | Main defects |
|---|---|
| Rubella | Cataract, sensorineural deafness, heart defects (patent ductus arteriosus, pulmonary stenosis) |
| Cytomegalovirus | The commonest congenital viral infection; deafness, small head, brain calcification around the ventricles |
| Toxoplasmosis | Hydrocephalus, calcification scattered in the brain, chorioretinitis |
| Syphilis | Deafness, notched teeth, saddle nose, bone changes |
| Herpes simplex | Usually caught at birth; skin, eye and brain infection |
| Varicella | Scarred skin, limb defects, eye and brain damage |
Congenital rubella triad: cataract, deafness and heart defects (often a patent ductus arteriosus).
The Zika virus has also been linked with microcephaly.
Drugs
| Drug | Main defects |
|---|---|
| Thalidomide | Phocomelia (absent or very short limb segments), heart and ear defects |
| Alcohol | Fetal alcohol syndrome: small head, smooth philtrum, thin upper lip, short palpebral fissures, learning disability |
| Valproate | Neural tube defects (spina bifida), facial and heart defects, developmental delay |
| Phenytoin | Fetal hydantoin syndrome: facial changes, small nails and finger ends, learning problems |
| Warfarin | Nasal hypoplasia and stippled epiphyses in the first trimester; brain bleeding later |
| Isotretinoin (vitamin A analogue) | Ear, face, heart and brain defects |
| Tetracyclines | Yellow-brown staining of the teeth; bone growth affected |
| ACE inhibitors | Kidney damage and too little amniotic fluid in later pregnancy |
| Diethylstilboestrol (DES) | Abnormal uterus and vaginal clear-cell cancer in daughters |
| Lithium | Heart defects, classically Ebstein anomaly |
Thalidomide, taken for morning sickness in the 1950s and 1960s, caused phocomelia and led to modern drug-safety testing.
Heparin does not cross the placenta, so it is preferred to warfarin for anticoagulation in pregnancy.
Radiation and chemicals
High-dose ionising radiation can cause small head size, learning disability and growth restriction. It can also damage germ cells.
Methylmercury (Minamata disease) causes brain damage similar to cerebral palsy. Lead harms brain development.
Maternal factors
- Diabetes: poorly controlled diabetes causes heart defects, neural tube defects and caudal regression (dysgenesis). Babies are often large (macrosomia).
- Phenylketonuria that is not controlled: small head, heart defects, learning disability.
- Obesity: linked with neural tube and heart defects.
- Low folic acid: raises the risk of neural tube defects.
Taking folic acid before conception and in early pregnancy lowers the risk of neural tube defects.
Clinical relevance
Preventing birth defects
How it happens: many teratogens can be avoided or replaced.
What you see: fewer defects when women are vaccinated against rubella before pregnancy, take folic acid, control diabetes before conceiving and avoid alcohol and known teratogenic drugs.
Counselling women on medicines
How it happens: a woman on a known teratogen (such as valproate or isotretinoin) plans a pregnancy or becomes pregnant.
What you see: the drug is reviewed, ideally before conception, and a safer option is chosen where possible. Specialist advice and local guidance should always be checked, as recommendations change.
Most questions give a drug or infection and ask for the defect, or give a defect and ask for the cause. Know the timing principles (weeks 3 to 8, all or nothing before), the rubella triad, thalidomide and phocomelia, valproate and neural tube defects, warfarin embryopathy and maternal diabetes with caudal regression.
Quick revision
- Weeks 1–2: all or nothing. Weeks 3–8: major defects. Fetal period: growth and functional defects.
- Brain stays sensitive throughout pregnancy.
- Rubella: cataract, deafness, heart defects.
- Thalidomide: phocomelia. Valproate: neural tube defects. Warfarin: nasal hypoplasia.
- Alcohol: smooth philtrum, thin upper lip, small head.
- Maternal diabetes: caudal regression, heart defects, large baby.
- Folic acid prevents neural tube defects.
Written by The House Job from standard embryology references (Langman's Medical Embryology).
FAQ
What is the most sensitive period for teratogens?
The embryonic period, weeks 3 to 8 after fertilisation, when organs are forming.
Which drug causes phocomelia?
Thalidomide.
What are the features of congenital rubella?
Cataract, sensorineural deafness and heart defects, especially patent ductus arteriosus.
Why is warfarin avoided in early pregnancy?
It can cause warfarin embryopathy, with nasal hypoplasia and stippled epiphyses.