michael s
(Michael S)
#1
94 A patient is on life-long warfarin and wishes to
become pregnant. How should she be managed?
Rachael James
All anticoagulant options during pregnancy are associated with
potential risks to the mother and fetus. Any woman on warfarin
who wishes to become pregnant should ideally be seen for pre-
pregnancy counselling and should be involved in the anti-
coagulation decision as much as possible. Potential risks to the
fetus need to be balanced against the increased maternal throm-
botic risk during pregnancy. Anticoagulation for mechanical heart
valves in pregnancy remains an area of some controversy.
The use of warfarin during pregnancy is associated with a low
risk of maternal complications^1 but it readily crosses the placenta
and embryopathy can follow exposure between 6–12 weeks’
gestation, the true incidence of which is unknown. A single study
has reported that a maternal warfarin dose 5mg is without this
embryopathy risk.^2 As pregnancy progresses, the immature
vitamin K metabolism of the fetus can result in intracranial haem-
orrhage even when the maternal INR is well controlled. In
addition, a direct CNS effect of warfarin has been described,
resulting in structural abnormalities. Conversion to heparin in
the final few weeks of pregnancy is recommended to prevent the
delivery of, what is in effect, an anticoagulated fetus.^3
In contrast, unfractionated heparin (UFH) is free from direct fetal
harm but it has varied pharmacokinetic and anticoagulant effects
and adequate maternal anticoagulation can be difficult to achieve.
The use of UFH in women with mechanical valve replacements
during pregnancy has been associated with increased maternal
thrombosis and bleeding. Studies have been criticised for the use
of inadequate heparin dosing and/or inadequate therapeutic
ranges^4 although a recent prospective study which used heparin in
the first trimester and in the final weeks of pregnancy reported fatal
valve thromboses despite adequate anticoagulation.^5 Long term
heparin use risks osteoporosis and heparin-induced thrombo-
cytopenia (HIT).^4 Intensive monitoring is required in pregnancy
and the use of anti-Xa assays may be necessary.
Low molecular weight heparins (LMWH) have a more reliable
anticoagulant effect.^6 The dose is adjusted according to anti-Xa
levels. Use in pregnancy is mainly for thromboprophylaxis rather
