PharmacologyTier 1Medical Sciences concept

Anticoagulant safety in pregnancy (warfarin teratogenicity vs heparins)

Core concept

Warfarin crosses the placenta -> teratogenic; heparins (UFH, LMWH) do not cross -> preferred in pregnancy
Warfarin embryopathy risk is dose- and timing-dependent, highest with exposure in weeks 6-12 (organogenesis)
Mechanism: warfarin inhibits vitamin K epoxide reductase in the fetus too -> abnormal bone/cartilage mineralisation

Warfarin embryopathy: nasal hypoplasia, stippled epiphyses (chondrodysplasia punctata), limb hypoplasia
Later exposure (2nd/3rd trimester): fetal/neonatal haemorrhage, CNS abnormalities (dorsal midline dysplasia) -- risk continues throughout pregnancy, not just first trimester
LMWH does not cross placenta (large, charged molecule) -> no direct fetal anticoagulant effect or teratogenicity

LMWH is standard for VTE prophylaxis/treatment in pregnancy; warfarin reserved for situations where LMWH is inadequate (e.g. some mechanical heart valves, where warfarin may have superior valve thrombosis prevention despite fetal risk)
Mechanical valve dilemma: warfarin may better protect the mother from valve thrombosis but risks the fetus -- decision requires shared, specialist input, often warfarin substitution during weeks 6-12 only
DOACs (direct oral anticoagulants) avoided in pregnancy -- insufficient safety data, animal studies show placental transfer/harm

Contrast with breastfeeding: warfarin and heparins both considered compatible (minimal transfer into milk); DOACs generally avoided
Same vitamin K epoxide reductase mechanism underlies warfarin's need for bridging with heparin at initiation (transient procoagulant effect from protein C/S depletion) -- separate from the pregnancy issue
Fetal warfarin syndrome overlaps phenotypically with other chondrodysplasia punctata causes (peroxisomal disorders) -- history of exposure is the discriminator

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