Drugs in Pregnancy and Lactation
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Direct answer
Weeks three to eight after conception — organogenesis — carry the highest teratogenic risk; before implantation damage is all-or-none, after week ten growth and organ function bear the damage instead. The safe shortlist writes itself: paracetamol for pain, penicillins and cephalosporins for infection, insulin for gestational diabetes, heparins for thrombosis, labetalol, methyldopa or nifedipine for hypertension, doxylamine-pyridoxine for vomiting. The offenders to refuse: warfarin (nasal hypoplasia, stippled epiphyses), valproate (neural tube defects), ACE inhibitors and ARBs (fetal renal failure), isotretinoin, tetracyclines, and NSAIDs in the third trimester through premature ductal closure. In lactation the working rule is relative infant dose below ten per cent — with codeine, for its ultrarapid-metaboliser hazard, now contraindicated.
What you must remember
- Time windows: weeks 3-8 for structural teratogenesis; CNS growth and function remain vulnerable throughout; the FDA letter categories A-X were abandoned in 2015 for narrative risk summaries — a modern exam point.
- Warfarin versus heparin: warfarin crosses the placenta causing warfarin embryopathy; unfractionated and low-molecular-weight heparins do not — heparin is the anticoagulant of pregnancy.
- Antiepileptic ladder: valproate worst (neural tube and neurodevelopmental harm) — avoid in women; lamotrigine and levetiracetam preferred, monotherapy at lowest dose, with folic acid 5 mg before conception.
- ACE inhibitors and ARBs: fetal renal failure, oligohydramnios, skull hypoplasia — contraindicated from the second trimester, and switched out pre-conception.
- NSAIDs at term: premature ductus arteriosus closure and oligohydramnios in the third trimester — paracetamol is the default analgesic throughout.
- Antithyroid choice: propylthiouracil in the first trimester (less teratogenic than methimazole, which causes aplasia cutis and choanal atresia), switching to methimazole later to spare PTU's liver.
- Lactation avoid-list: codeine and tramadol (CYP2D6 ultrarapid conversion), chloramphenicol, sulphonamides in G6PD or jaundiced infants, amiodarone, methotrexate, and oestrogen-containing pills that suppress milk — progestin-only options are preferred.
- Vaccines: live vaccines (MMR, varicella) contraindicated; inactivated influenza vaccination actively recommended.
Counselling a woman with epilepsy who wants to conceive
Never stop the antiepileptic — an uncontrolled seizure harms two patients at once. Plan before conception: shift off valproate onto lamotrigine or levetiracetam, settle on monotherapy at the lowest effective dose, start folic acid 5 mg daily, and book the anomaly scan. During pregnancy, lamotrigine clearance rises with the blood volume, so levels and clinical state guide dose increments. The baby receives drug through milk in amounts that keep breastfeeding not just permissible but advisable for most agents. The same architecture serves hypertension: replace the ACE inhibitor with labetalol or methyldopa, whose decades of safety data still anchor Indian practice.
Lactation arithmetic completes the counselling: relative infant dose — the infant's milligram-per-kilogram as a percentage of the mother's — below 10 per cent is the compatibility rule; codeine fails catastrophically in ultrarapid metabolisers, whose milk carries a morphine overdose.
Indian prescribing context
Methyldopa remains first-line for pregnancy hypertension across Indian public facilities — cheap, familiar, with the longest fetal safety record — while labetalol and nifedipine carry the escalations. Two Indian realities shape this chapter: over-the-counter self-medication in pregnancy (NSAIDs bought casually for a backache at 34 weeks is a recurring casualty presentation) and untreated infection — asymptomatic bacteriuria screening and treated syphilis do more fetal good than any teratology lecture. Doxylamine-pyridoxine for nausea and penicillins for infection are the rational defaults. Report suspected teratogenic exposures to the PvPI, which feeds pregnancy registries.
Frequently asked questions
Which period of pregnancy carries the highest teratogenic risk?
Weeks three to eight post-conception, during organogenesis; earlier injury is all-or-none, later injury affects growth and function.
Why is heparin preferred over warfarin in pregnancy?
Heparin's large molecule does not cross the placenta, while warfarin causes the warfarin embryopathy — nasal hypoplasia, stippled epiphyses, limb defects.
Why avoid NSAIDs in the third trimester?
Prostaglandin inhibition closes the ductus arteriosus prematurely and reduces fetal urine output, causing oligohydramnios; paracetamol replaces them.
Which antiepileptics are preferred in pregnancy?
Lamotrigine and levetiracetam as monotherapy at the lowest effective dose, with pre-conception folic acid 5 mg; valproate is avoided whenever possible.
Why is codeine contraindicated during breastfeeding?
In CYP2D6 ultrarapid metabolisers, milk morphine concentrations can cause fatal neonatal respiratory depression — a pharmacogenetic contraindication.