Pregnancy requires special consideration with respect to the prevention and treatment of blood clots. Pregnancy does not cause blood clots, but increases the chance that a blood clot will develop by four- to five-fold. Pregnancy’s tendency to form clots is an evolutionary response to protect women against the bleeding challenges of miscarriage and childbirth.

In general:

  • Women who have had a blood clot in the past and are on anticoagulation will need to continue their anticoagulation during pregnancy.
  • Most women who have had a blood clot in the past, but are not currently on anticoagulation, will need to be restarted on anticoagulation during pregnancy.
  • Women who develop a blood clot during pregnancy will need to be started on anticoagulation.

Warfarin, the most commonly prescribed anticoagulant, or the new oral anticoagulants (dabigatran, rivaroxaban and apixaban), are convenient to take because they do not have to be injected, but they are not considered safe for unborn babies. Standard or “unfractionated” heparin and low-molecular-weight heparin (LMWH) are safe in pregnancy because they do not cross the placenta and, therefore, do not enter the blood stream of unborn babies.  In fact, in women who have thrombophilia, heparin or LMWH may actually improve the outcome of pregnancy in women who have had a previous pregnancy that was complicated by severe high blood pressure of pregnancy (severe preeclampsia).  Heparin and LMWH have been used in pregnancy by thousands of women with no birth defects or bleeding problems in their unborn babies.

Whether women are treated with heparin, or LMWH, they will ultimately need to receive once or twice-daily injections until at least 6 weeks after delivery of the baby.

Women are sometimes concerned about having to give injections into their abdomens while they are pregnant.

Women can be reassured that the needles are very short and never go below the fatty layer of tissue underneath the skin.

Special plans have to be made around the time of delivery.

LMWH, if it is in the mother’s system at the time of childbirth, or when an epidural or spinal anesthetic is placed, may increase the risk of bleeding complications.  Therefore, the LMWH is held 24 hours prior to anticipated delivery. Alternatively, heparin, which is shorter acting, can be used during the last few weeks of pregnancy and held a few hours before delivery.  LMWH can be resumed after delivery.

Women can remain on LMWH for the duration of their anticoagulation, or until one to two weeks postpartum, when the risk of bleeding has subsided, at which time women can be restarted on oral anticoagulants. Warfarin is compatible with breastfeeding, but there is insufficient information about the new oral anticoagulants as to whether they are safe in breastfeeding.

Andra H. James, MD, MPH

Professor of Obstetrics & Gynecology

University of Virginia


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