Although pregnancy induces a state of hypercoagulability, the thromoboembolic risks during a normal pregnancy are minor. This is not the case when the pregnant women has a native valvulopathy in atrial fibrillation, a mechanical prosthesis or a coagulation anomaly. The respective indications for heparin and oral anticoagulants reflect their respective advantages and disadvantages for the mother and child.

Historically, recommendations for anticoagulants during pregnancy have evolved with each new publication of results concerning their use.

Very early, coumarin induced embryopathies,1 especially during the first trimester, led to restricted use of oral anticoagulants in favour of heparin. However, it rapidly became apparent that heparin also had disadvantages, especially in women with prostheses. Heparin in these patients facilitated the evolution of the pregnancy by limiting fetal accidents but increased maternal haemorrhagic and thromboembolic risks.

The ease of use and safety provided by low molecular weight heparins (LMWH) led to their widespread use, other than in pregnancy, to treat and prevent arterial and venous thromboembolic events. At present, their use during pregnancy is hampered by the lack of large randomised prospective studies, which are difficult to conduct in light of the ethical constraints which understandably apply.