Heparin does not cross the placenta. This applies to both unfractionated heparin (UFH) and low molecular weight heparin (LMWH), making heparin the preferred type of blood thinner during pregnancy. Its large molecular size and strong negative electrical charge prevent it from passing through the placental barrier to reach the fetus.
Why Heparin Cannot Cross the Placenta
Heparin is a large, heavily charged molecule. Unfractionated heparin consists of long sugar chains with molecular weights between 12,000 and 15,000 daltons. Even the smaller version, LMWH, weighs in at 3,000 to 6,000 daltons. For context, most drugs that freely cross the placenta are much smaller and carry less electrical charge. Heparin’s sugar chains are densely coated with sulfate groups, giving the molecule a strong negative charge that essentially repels it from the placental membrane. The combination of size and charge makes placental transfer negligible.
This is the key reason heparin is favored over other blood thinners during pregnancy. Warfarin, for example, freely crosses the placenta and can cause birth defects, pregnancy loss, fetal bleeding, and developmental problems. Newer oral blood thinners like rivaroxaban, apixaban, and dabigatran also likely cross the placenta, and their effects on a developing fetus are unknown.
Both UFH and LMWH Are Safe for the Fetus
Neither form of heparin reaches the baby in meaningful amounts. LMWH is classified as a Category B drug by the FDA, meaning animal studies have shown no danger to the fetus. In human studies, fetal deaths that occurred during LMWH use showed no causal link to the medication, and the majority of newborns in these studies had excellent health scores at birth.
Between the two forms, LMWH is the strongly preferred option. The American Society of Hematology and the American College of Chest Physicians both recommend LMWH over UFH for preventing and treating blood clots during pregnancy. LMWH wins on several fronts: it’s more predictable in how it works, requires less monitoring, and carries fewer maternal side effects.
What About Fondaparinux?
Fondaparinux is a synthetic drug related to heparin but with a much smaller molecular structure. Unlike true heparins, it does cross the placenta in small amounts. A study published in the New England Journal of Medicine found that fondaparinux concentrations in umbilical cord blood were roughly one-tenth the level in the mother’s blood. While no adverse effects were seen in the newborns in that study, the possibility of harm hasn’t been ruled out. Current guidelines reserve fondaparinux for pregnant patients who cannot tolerate heparin at all, such as those with a severe allergic reaction or a dangerous immune response called heparin-induced thrombocytopenia.
When Heparin Is Used During Pregnancy
The most common reason for heparin use in pregnancy is preventing or treating blood clots. Pregnancy increases clotting risk significantly, and women with a history of clots or inherited clotting disorders often need anticoagulation throughout their pregnancy. Heparin is also used in women with mechanical heart valves, who need continuous blood thinning to prevent valve-related clots. A third major use is in combination with aspirin for women with antiphospholipid syndrome, an immune condition linked to recurrent pregnancy loss.
Maternal Side Effects to Know About
While heparin is safe for the fetus, it does carry risks for the mother, particularly with long-term use. Unfractionated heparin causes symptomatic spinal fractures in up to 3% of people on extended therapy. As many as 30% experience significant bone density loss leading to weakened bones. LMWH appears to carry a lower risk of both bone loss and the immune-mediated drop in platelets that can occur with UFH, which is another reason it’s the preferred choice.
Timing Around Delivery
One practical consideration is that heparin must be stopped before delivery, particularly if you want an epidural or spinal anesthesia. For standard-dose unfractionated heparin given by injection, the recommended gap is 4 to 6 hours between the last dose and placement of a spinal or epidural needle. For higher therapeutic doses, the wait extends to 12 or even 24 hours depending on the amount used. Your medical team will typically plan the timing of your last dose around your expected delivery window.
Safety During Breastfeeding
Heparin is also safe while nursing. Unfractionated heparin’s large molecular size means it would not be expected to pass into breast milk in any meaningful quantity. LMWH does appear in breast milk in trace amounts, but studies of nursing mothers found either undetectable or extremely low levels. Even the small amount that does appear in milk has very limited ability to be absorbed through the infant’s digestive tract, so it poses no realistic bleeding risk to the baby. No special precautions are needed for breastfeeding mothers on either form of heparin.