Heparin During Pregnancy: Safety, Uses, and What You Need to Know

When you're pregnant and need a blood thinner, heparin, a low-molecular-weight anticoagulant used to prevent dangerous clots during pregnancy. Also known as low molecular weight heparin (LMWH), it's one of the few blood thinners considered safe for both mother and baby. Unlike warfarin, which can cross the placenta and harm fetal development, heparin stays in the mother’s bloodstream—making it the go-to choice for conditions like deep vein thrombosis, pulmonary embolism, or recurrent miscarriages linked to clotting disorders.

Doctors often prescribe heparin during pregnancy if you have a history of blood clots, antiphospholipid syndrome, or a genetic clotting disorder like Factor V Leiden. It’s also used after cesarean sections or in high-risk pregnancies where clotting risk rises. The two main types are unfractionated heparin and low molecular weight heparin (like enoxaparin or dalteparin). Most pregnant women get the low molecular weight version because it’s easier to use—once-daily injections, fewer lab checks, and lower risk of side effects like osteoporosis or heparin-induced thrombocytopenia.

It’s not all smooth sailing, though. Some women report bruising at the injection site, mild nausea, or a drop in platelet count. Rarely, long-term use can weaken bones, so your doctor might monitor your calcium and vitamin D levels. You won’t need frequent blood tests like with warfarin, but you’ll still get periodic checks to make sure the dose is right. And while heparin doesn’t enter breast milk, you’ll still want to talk to your provider if you plan to nurse after delivery.

What’s missing from most discussions is how heparin fits into the bigger picture of pregnancy care. It’s not just about preventing clots—it’s about keeping you alive so you can carry your baby to term. That’s why it’s often paired with other support: folic acid, prenatal vitamins, and sometimes aspirin. If you’re on heparin, your OB-GYN and hematologist will likely work together to adjust your plan as your pregnancy progresses. Your risk changes in each trimester, and so does the ideal dose.

There’s no one-size-fits-all approach. Some women start heparin right after a positive pregnancy test. Others begin only after a confirmed diagnosis of a clotting issue. If you’ve had a previous pregnancy loss linked to clots, your doctor might start early—even before you know you’re pregnant. The key is knowing your history and speaking up if you notice unusual swelling, chest pain, or shortness of breath.

Below, you’ll find real-world advice from women who’ve been through it, clear comparisons between heparin and other options, and practical tips on managing injections, side effects, and anxiety during treatment. No fluff. Just what you need to stay safe and informed.