Reviewed by Shameer Deen, ST5 Urology Registrar
Approximately 90% of pregnant women take at least one medication during their pregnancy, yet the evidence base for drug safety in this population remains incomplete for many agents [1]. Most randomised controlled trials exclude pregnant women, leaving clinicians to navigate drug safety decisions using observational data, animal studies, and extrapolated pharmacological principles. Getting these decisions right is critical — both over-treatment and under-treatment carry real risks for mother and fetus.
Understanding Teratogenicity
A teratogen is any agent that can cause structural or functional abnormalities in a developing embryo or fetus. The risk of teratogenic harm depends on several factors [1]:
- Gestational age at exposure — the embryonic period (weeks 3-8) is the most sensitive, as organogenesis is occurring. Exposure before implantation (weeks 1-2) typically follows an all-or-nothing principle. After organogenesis, structural malformations are less likely but functional effects remain possible throughout pregnancy.
- Dose and duration — single brief exposures carry lower risk than sustained therapy
- Individual susceptibility — genetic variation in drug metabolism affects fetal drug exposure
- Drug physicochemical properties — lipid-soluble, low-molecular-weight drugs cross the placenta most readily
Risk Classification Systems
The FDA's legacy A/B/C/D/X classification has been replaced by the Pregnancy and Lactation Labeling Rule (PLLR), which requires narrative risk summaries for human data, animal data, and clinical considerations. In practice, clinicians often still encounter the older letter categories in legacy references [1]. The most important categories to recognise are:
- Category X — studies in animals or humans have demonstrated fetal abnormalities and the risks clearly outweigh any possible benefit (e.g. thalidomide, isotretinoin, methotrexate, warfarin in the first trimester)
- Category D — evidence of human fetal risk exists, but potential benefits may warrant use in serious conditions (e.g. phenytoin, lithium, tetracyclines)
Drugs Considered Safe in Pregnancy
Several drug classes have extensive safety records in pregnancy [1]:
- Paracetamol — first-line analgesic and antipyretic throughout pregnancy. Recent observational data has raised questions about associations with neurodevelopmental outcomes, but current consensus supports use at the lowest effective dose for the shortest duration.
- Penicillins — the most widely studied antibiotic class in pregnancy; amoxicillin and ampicillin are safe across all trimesters
- Cephalosporins — considered safe; no pattern of malformation identified in surveillance studies
- Labetalol and nifedipine — first-line antihypertensives in gestational hypertension and pre-eclampsia
- Low-dose aspirin — recommended from 12 weeks for women at high risk of pre-eclampsia
- Heparin (unfractionated and LMWH) — do not cross the placenta; anticoagulants of choice in pregnancy
Drugs to Avoid
Several commonly used drugs carry clear evidence of fetal harm and should be avoided or discontinued before conception [1]:
- Warfarin — warfarin embryopathy (nasal hypoplasia, stippled epiphyses) with first trimester exposure; intracranial haemorrhage risk later in pregnancy
- ACE inhibitors and ARBs — fetopathy with second and third trimester exposure including renal tubular dysgenesis, oligohydramnios, and limb contractures
- NSAIDs — premature closure of the ductus arteriosus with third trimester use; avoid after 20 weeks
- Isotretinoin — category X; highly teratogenic; requires pregnancy prevention programme
- Methotrexate — category X; abortifacient and teratogen; requires 3-6 month contraception washout
- Tetracyclines — dental discolouration and bone deposition in second and third trimesters
- Fluoroquinolones — arthropathy in animal models; avoid unless no alternative
MedNext Pregnancy Safety Data
MedNext Formulary includes pregnancy safety information for all drugs in the MedNext Audited Proprietary Dataset, drawing on the most current evidence. Each drug monograph clearly flags pregnancy risk and provides trimester-specific guidance where data supports it, helping prescribers make rapid, evidence-based decisions in clinical practice.
References
- Briggs GG, Freeman RK, Towers CV, Forinash AB. Drugs in Pregnancy and Lactation: A Reference Guide to Fetal and Neonatal Risk. 12th ed. Wolters Kluwer, 2021.