Anticoagulation (Blood thinning) in Pregnancy

What is Anticoagulation?

There are a number of medical conditions where blood is "too thick". Sometimes these conditions are associated with blood clots forming in the blood vessels. It is sometimes necessary to give patients anticoagulation (blood thinning) to prevent or treat clots that have formed in the blood vessels.

How does blood thinning work?

Blood clots in the circulation by a combination of proteins and cells. Blood thinning generally stops some of the blood clotting proteins from working properly. Blood thinning is usually achieved by either injections with a medication called heparin (Clexane or Fragmin) or by a tablet (Warfarin).

Why do I need to have my blood thinned when I am pregnant?

Blood thinning may be necessary in women who are pregnant for a number of reasons.

These include:

  1. Developing a blood clot when pregnant – the treatment for this involves blood thinning during pregnancy
  2. Some women have abnormal blood clotting (thicker blood) and need blood thinning when they are pregnant to prevent blood clots; having a baby can make the blood even thicker in some women
  3. Women who have abnormal heart valves or artificial heart valves will need blood thinning to prevent blood clots in pregnancy.

Are there any side effects of blood clotting I need to worry about in pregnancy?

Blood thinning is generally safe but there are a number of important precautions to take in pregnancy. One of the blood thinning medications (Warfarin) can sometimes cause problems in the baby – it is important to stop warfarin before getting pregnant or at least to stop warfarin as soon as you become pregnant. Warfarin can sometimes interfere with normal bone growth in babies. Heparin injections are safer for the baby because this medication does not cross the placenta into the baby's circulation. If you are already on Warfarin and you think you may be pregnant, it is a good idea to do an early pregnancy test and stop the Warfarin as soon as possible.

Usually, warfarin should not be used when you are pregnant. Heparin injections can be used safely throughout the pregnancy but require you to inject yourself every day.

Is there anything else I can do?

It is important that the heparin injections (Clexane) are monitored when you are pregnant so your blood thinning is at the right level. This will make sure your blood is not too thin which can cause bleeding or not thin enough which can cause blood clots.
It is also important to maintain a healthy weight during pregnancy and to exercise a little – this reduces the risk of blood clots developing.

What happens around the time I have the baby? Do I have to stop the blood thinning?

This depends on why you are having blood thinning – sometimes it is safe for you to stop the blood thinning 2 – 3 days before you plan to have the baby. Other times it may be necessary for you to come into hospital to have a different sort of blood thinning which can be stopped immediately before you have your baby. It is best you discuss this with your doctor.

What happens after I have my baby?

It is usual for women who need blood thinning when pregnant to need blood thinning for a short time (usually 6 weeks) after delivery. The type and duration of blood thinning will depend on your medical condition.

Is it safe to breast feed when on blood thinning?

The blood thinning medication (both heparin injections and Warfarin) are not found in breast milk and so breast feeding is safe whilst on blood thinning.

Resources used to produce this information sheet.

  1. Bates SM, Greer IA, Hirsh J, Ginsberg JS. Use of antithrombotic agents during pregnancy: the Seventh ACCP Conference on Antithrombotic and Thrombolytic Therapy. Chest 2004;126(3 Suppl):627S-644S.


The information presented in this fact sheet is intended as a general guide only. Patients should seek further advice and information about anticoagulation (blood thinning) in pregnancy and their individual condition from their treating haematologist or doctor.

You are here: Haematology Fact Sheets Anticoagulation in Pregnancy

Melbourne Haematology

Melbourne Haematology is a group of clinical and laboratory trained specialist haematologists based in Melbourne dedicated to provide comprehensive management of patients with haematological disorders

Read more

Find a Haematologist

Melbourne Haematology currently has over 30 Haematologists practicising in over 20 locations throughout Melbourne and surrounding suburbs including Geelong.

View Our Locations page to quickly find your nearest Haematologist.