Medication Teratogens

Medication teratogens are the most clinically prominent teratology category because the embryopathies are well-characterized and many drugs have safer alternatives that prompt "what should you do preconceptionally" decisions. Each major drug below has a recognizable injury pattern keyed to its mechanism: folate antagonism, retinoid-receptor binding, vitamin K antagonism, ACE-pathway disruption, methylation interference. Knowing the mechanism predicts the timing window and the surveillance plan.

The classic teratogenic AEDs cluster around folate antagonism and HDAC inhibition; switching to a safer agent preconceptionally is the high-leverage move because organogenesis happens before most patients know they're pregnant.

Valproate (valproic acid, divalproex)

• Mechanism: folate antagonism + HDAC inhibition (epigenetic disruption).
• Phenotype:
  • Neural tube defects (~1–2% absolute risk; up to 10× background)
  • Distinctive facial dysmorphism (small mouth, thin upper lip, midface hypoplasia, broad nasal bridge)
  • Cleft palate
  • Cardiac defects
  • IQ reduction (~7–10 points on average vs. unexposed children)
  • Increased autism risk (~3× baseline)
• Counseling: switch preconceptionally to a safer AED (lamotrigine, levetiracetam) when seizure control allows. If valproate is unavoidable, use the lowest effective dose, supplement with high-dose folate (4 mg/day), and provide targeted ultrasound + fetal echo.

Phenytoin

• Mechanism: folate antagonism; oxidative metabolite (arene oxide) toxicity.
• Fetal hydantoin syndrome: distal phalangeal hypoplasia, nail hypoplasia, midface hypoplasia, cleft lip/palate, growth restriction, mild intellectual disability.
• ~5–10% of exposed pregnancies show some features.

Carbamazepine

• Mechanism: similar to phenytoin (folate-related).
• Phenotype: ~1% NTD risk (mostly spina bifida); craniofacial features less marked than valproate. Lower risk than valproate but not negligible.

Phenobarbital, Topiramate

• Phenobarbital: cleft lip/palate, cardiac defects.
• Topiramate: cleft lip/palate (FDA pregnancy category change in 2011 based on this signal).

Lamotrigine, Levetiracetam

• Generally considered safer. Lamotrigine: small early signal for cleft palate that hasn't been confirmed. Levetiracetam: no clear teratogenic signal in current data. These are typical preferred AEDs in pregnancy.

Warfarin

• Mechanism: vitamin K antagonism disrupts γ-carboxylation of bone-matrix proteins; small molecule, freely crosses placenta.
• Critical window: weeks 6–9 post-conception (most teratogenic).
• Warfarin embryopathy: nasal hypoplasia (saddle nose), stippled epiphyses (chondrodysplasia punctata), CNS abnormalities (Dandy-Walker, agenesis of corpus callosum), optic atrophy, intellectual disability.
• Late-pregnancy exposure carries fetal/maternal hemorrhage risk and CNS bleeding.
• Counseling: switch to LMWH preconceptionally; LMWH is highly polar and does not cross the placenta. Heparin is acceptable but requires more frequent monitoring.

Direct oral anticoagulants (DOACs: apixaban, rivaroxaban, dabigatran)

• Contraindicated in pregnancy. Limited human data; animal data show fetal toxicity. Switch to LMWH.

Isotretinoin (Accutane)

• Mechanism: retinoic-acid-receptor activation disrupts neural crest migration and pharyngeal-arch patterning.
• Critical window: anytime in first trimester; extremely high penetrance.
• Isotretinoin embryopathy:
  • Microtia / external ear anomalies (most characteristic)
  • Conotruncal cardiac defects
  • Thymic hypoplasia
  • CNS abnormalities (hydrocephalus, posterior fossa)
  • Cleft palate
  • High rate of spontaneous loss
• iPLEDGE program: REMS-mandated registry requiring pregnancy testing and contraception for all isotretinoin patients of reproductive potential. Driven precisely by this teratogenicity.

Other retinoids

• Etretinate / acitretin: long half-life (etretinate stored in adipose for years); even longer washout requirement than isotretinoin.
• Topical retinoids (tretinoin, adapalene): minimal systemic absorption; generally considered low-risk but typically avoided in pregnancy as a precaution.
• Vitamin A: high-dose (>10,000 IU/day) is teratogenic; routine prenatal vitamins are safe.

• Mechanism: disruption of the fetal renin-angiotensin system, which is required for nephrogenesis and amniotic fluid production (fetal urine output).
• Timing: especially second and third trimesters; late-trimester exposure is the classic case.
• ACE inhibitor fetopathy: oligohydramnios → fetal lung hypoplasia, limb contractures, calvarial hypoplasia, neonatal renal failure, hypotension. Mortality is significant.
• First-trimester exposure carries an early-cardiac-malformation signal of debated magnitude.
• Counseling: switch to pregnancy-safe antihypertensive (labetalol, methyldopa, nifedipine) preconceptionally. Avoid throughout pregnancy.

Methotrexate

• Mechanism: folate antagonism (DHFR inhibition).
• Phenotype: aminopterin-like syndrome (craniofacial dysmorphism, limb defects, growth restriction, CNS abnormalities). Highly teratogenic in first trimester.
• Used as an abortifacient; reflects the teratogenic potency.
• Mycophenolate similarly teratogenic (cleft lip/palate, microtia).

Cyclophosphamide / alkylating agents

• DNA-damage mechanism; multi-system malformations possible. Generally avoided in pregnancy unless oncology benefit clearly outweighs.

Lithium

• Mechanism: not fully understood; possibly inositol depletion.
• Ebstein anomaly (RV-side congenital heart defect, displaced tricuspid valve): classic association, originally reported as 400× background risk; subsequent data suggest closer to 10–20× (~1/1000 absolute).
• Counseling: weigh against bipolar relapse risk; if continued, fetal echo at 18–22 weeks.

SSRIs

• Most SSRIs are not strongly teratogenic. Paroxetine has the strongest signal for cardiac malformations (~1.5–2× background) and is generally avoided in pregnancy.
• Late-trimester SSRI exposure → neonatal adaptation syndrome (transient irritability, jitteriness, feeding difficulty) and rare persistent pulmonary hypertension of the newborn.

• NSAIDs after 30 weeks: ductal constriction → premature closure of the ductus arteriosus, fetal pulmonary hypertension, oligohydramnios. The injury is fetal-period, not teratogenic.
• Tetracyclines after week 16: tooth discoloration, enamel hypoplasia, transient bone-growth inhibition.
• Aminoglycosides (streptomycin, gentamicin): ototoxicity; avoid when alternatives exist.

Diethylstilbestrol (DES)

• Historic teratogen (banned 1971). Daughters: clear-cell vaginal/cervical adenocarcinoma, T-shaped uterus, cervical incompetence. Sons: epididymal cysts, testicular abnormalities. Effect is transgenerational endocrine disruption.

Androgens (testosterone, danazol)

• Female fetus: virilization (clitoromegaly, labioscrotal fusion).

Misoprostol

• Mechanism: vascular disruption from uterine contractions during early development.
• Möbius sequence (cranial-nerve VI/VII palsies), limb-reduction defects. Reported after first-trimester misoprostol use as an abortifacient.