Prescribing in pregnancy and breastfeeding
How drugs cross the placenta and into milk, when the embryo is most vulnerable, and how risk is described on modern labels.
Most medicines reach the embryo or fetus to some degree, yet most do not harm it at usual doses, and untreated illness can be more dangerous than the drug. Good decisions weigh the timing of exposure, what is known about the drug and the risk of the disease itself, and they are made together with the patient.
Crossing the placenta
Transfer depends on lipid solubility, ionisation, molecular weight, protein binding and placental transporters. Thiopental crosses almost at once and can sedate the newborn; highly ionised neuromuscular blockers cross slowly. Heparins do not cross, which is why they are used when anticoagulation is needed in pregnancy, while warfarin is teratogenic. Antibodies, including some biologic medicines, are actively carried across from the second trimester.
Timing matters
- Embryonic period (organ formation): the main window for structural malformations.
- Fetal period: effects on growth, function and the developing brain; the brain keeps developing throughout pregnancy.
- Late pregnancy and birth: ACE inhibitors used late in pregnancy harm the fetus, and opioids used for a long time can cause neonatal withdrawal.
Examples to know
Valproate carries a boxed warning for major malformations, especially neural tube defects, and for lower IQ and neurodevelopmental disorders; it is contraindicated for migraine prevention in pregnancy. Tetracyclines given in the second half of pregnancy can discolour the child's teeth and slow bone growth. Prolonged opioid use in pregnancy can cause neonatal opioid withdrawal syndrome. Many other drugs are safe or the safest choice for a condition that must be treated.
How risk is labelled
The FDA's Pregnancy and Lactation Labeling Rule removed the letter categories, which were often misread as a grading of danger. Labels now describe the evidence in words in three sections: Pregnancy (including registries that collect outcomes), Lactation, and Females and Males of Reproductive Potential (pregnancy testing, contraception, fertility).
Breastfeeding
Most drugs pass into milk in small amounts, and breastfeeding usually continues. Concern is greater for drugs that are toxic in small amounts, accumulate in the infant or depress breathing. Codeine is an example: breastfeeding is not recommended during treatment because ultra-rapid metabolisers can pass enough morphine to the baby to cause harm.
Before and during pregnancy
Any medicine, including over-the-counter products, herbs and supplements, is reviewed with a health professional before and during pregnancy. Folic acid before conception and early in pregnancy helps prevent neural tube defects. Medicines for chronic conditions are not stopped without advice.
Test yourself
Think of the answer, then tap the question to see it.
Why can heparin be used for anticoagulation in pregnancy while warfarin is avoided?
Heparin is a large polar molecule that does not cross the placenta; warfarin crosses and is teratogenic.
Which period of pregnancy is the main window for major structural malformations?
The embryonic period of organ formation (organogenesis).
Which two fetal risks appear in the valproate boxed warning?
Major malformations, especially neural tube defects, and decreased IQ or neurodevelopmental disorders.
What replaced the A, B, C, D, X pregnancy categories on US labels?
Narrative Pregnancy, Lactation, and Females and Males of Reproductive Potential sections (PLLR).
Example drugs
Connections
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: Pregnancy and Lactation Labeling Resources (U.S. Food and Drug Administration, 2026); Pregnancy and Medicines | MedlinePlus (MedlinePlus (U.S. National Library of Medicine), 2026); DailyMed - VALPROIC ACID capsule, liquid filled (DailyMed (U.S. National Library of Medicine), 2026); DailyMed - TETRACYCLINE HYDROCHLORIDE capsule (DailyMed (U.S. National Library of Medicine), 2026).
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Representative model, for educational purposes only.