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Drugs and teratogens in pregnancy
The harm a drug does to the fetus depends on the drug and the timing: in the first two weeks after conception "all or nothing", in weeks 3–8 after conception (organogenesis) the risk of malformations is highest, and afterwards the harm is mainly to function and growth.
Why it matters in the exam
Teratogen questions present a woman taking a drug who discovers she is pregnant, or a woman planning pregnancy, and ask what malformation is expected and what to switch to. Know the classic pairs: isotretinoin — brain, heart and ear malformations; valproate — neural tube defects and cognitive harm; warfarin at weeks 6–12 — nasal hypoplasia and stippled epiphyses; ACE inhibitors and ARBs in the second and third trimesters — fetal renal failure and oligohydramnios; methimazole in the first trimester — aplasia cutis; lithium — Ebstein anomaly (small risk); tetracyclines — tooth staining; misoprostol — Möbius syndrome; thalidomide — limb defects.
It is just as important to know what is safe, so that necessary treatment is not stopped: paracetamol, penicillins and cephalosporins, labetalol, nifedipine, methyldopa, insulin, low-molecular-weight heparin and levothyroxine. In most chronic conditions — epilepsy, depression, hypertension — it is better to change treatment before pregnancy than to stop it abruptly. NSAIDs are avoided from mid-pregnancy (oligohydramnios) and in the third trimester (ductal closure). Folic acid 0.4 mg for every woman before pregnancy, and a high dose (4 mg) for a woman with a previous fetus with a neural tube defect or taking antiepileptic drugs. Since 2015 the FDA letter categories have been replaced by a detailed description.
Key points
- "All or nothing" in the first two weeks; malformations in weeks 3–8 after conception.
- Isotretinoin, valproate, warfarin, ACE inhibitors, methimazole — the classic pairs.
- Safe: paracetamol, penicillins, labetalol, nifedipine, insulin, LMWH.
- Folic acid 0.4 mg; 4 mg at high risk.
- NSAIDs avoided from week 20.
Summary table
| Drug | Malformation or harm | Timing | Alternative or note |
|---|---|---|---|
| Isotretinoin | Brain, heart, ear and facial malformations | First trimester | Two methods of contraception; stop at least a month before pregnancy |
| Valproate | Neural tube defects, cognitive harm | First trimester and throughout | Lamotrigine or levetiracetam; high-dose folic acid |
| Warfarin | Nasal hypoplasia, stippled epiphyses, fetal brain haemorrhage | Weeks 6–12 onwards | Low-molecular-weight heparin (exceptions with a mechanical valve) |
| ACE inhibitors and ARBs | Fetal renal failure, oligohydramnios, skull defect | Second and third trimesters | Labetalol, nifedipine, methyldopa |
| Methimazole | Aplasia cutis, choanal atresia | First trimester | Propylthiouracil in the first trimester, then switch to methimazole |
| NSAIDs | Oligohydramnios; ductal closure | From week 20; mainly from week 30 | Paracetamol |
| Generally safe | — | — | Paracetamol, penicillins, cephalosporins, labetalol, nifedipine, insulin, LMWH, levothyroxine |
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Sources
- ACOG Committee Opinion No. 762, 2019 — Prepregnancy Counseling
- FDA Pregnancy and Lactation Labeling Rule (PLLR), 2015 — Content and format of labeling for human prescription drugs: pregnancy and lactation
- Briggs' Drugs in Pregnancy and Lactation, 12th edition (2021) — Individual drug monographs
- Williams Obstetrics, 26th edition (2022) — Teratology, Teratogens, and Fetotoxic Agents
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.