Articles

Key takeaways

• A deep vein thrombosis (DVT) is a blood clot, usually in the leg.
• Pregnancy increases your risk of DVT, but it is uncommon.
• Your DVT risk is assessed throughout your pregnancy and after birth.
• Staying active and hydrated helps reduce your risk of blood clots.

Understanding your risk for blood clots in pregnancy

A thrombosis is a blood clot that forms in a blood vessel. A venous thrombosis occurs in a vein. Veins are the blood vessels that carry blood back to your heart and lungs.

A blood clot that forms in a deep vein is called a deep vein thrombosis (DVT). This usually happens in your leg, calf, or pelvis. If a DVT clot breaks off, it can travel to your lungs and block a blood vessel. This is called a pulmonary embolism and needs emergency treatment.

DVT is not common in pregnancy. However, you are more likely to develop DVT at any stage of your pregnancy than people of the same age who are not pregnant.

Your risk is highest just after you have had your baby. DVT can occur at any time during your pregnancy, including the first three months.

How your risk for DVT is assessed

Your midwife or doctor will carry out a risk assessment for DVT. This happens at your first antenatal booking appointment and again at around 28 weeks of pregnancy. A further risk assessment should be done after your baby is born.

Your risk will also be assessed if your situation changes during pregnancy. This includes if you are admitted to hospital. During the assessment, your doctor or midwife will ask about any risk factors you may have.

Risk factors for DVT include:

• Personal or family history – if you or a close family member have had a blood clot before.
• Age – if you are over 35.
• A high BMI – if you have a high body mass index (BMI) of 30 or more.
• Medical conditions – such as thrombophilia (a condition that makes clots more likely) or sickle cell disease.
• Severe infection – or a recent serious injury, such as a broken leg.
• Multiple pregnancy – if you are carrying twins or multiple babies.
• Previous births – if you have had three or more children.
• Pre-eclampsia – a condition that can occur during pregnancy.
• Fertility treatment – if you have had fertility treatment.
• Caesarean section – if you are having a caesarean section.
• Smoking – if you smoke.
• Severe varicose veins – especially if they are painful or above the knee. Also look for redness or swelling.
• Dehydration – if you become dehydrated.
• Long labour – if you have a labour lasting more than 24 hours.
• Blood loss – if you lose a lot of blood after having your baby or need a blood transfusion.
• Hospital admission – if you are admitted to hospital.
• Reduced mobility – due to severe vomiting, infection, or illness from fertility treatment (ovarian hyperstimulation syndrome).
• Long periods of immobility – for example, after an operation or when travelling for four hours or longer.

Prevention for those at higher risk

If you have a higher risk of DVT, your healthcare team may suggest specific steps to help prevent it. This might include taking a blood-thinning medicine called heparin. Specifically, your doctor may prescribe low-molecular-weight heparin (LMWH).

LMWH is a type of anticoagulant (blood thinner) that works by reducing the blood's ability to clot. It is preferred during pregnancy because it does not cross the placenta, making it safe for the baby.

How heparin is used

LMWH is typically administered as a daily injection under the skin (subcutaneously), which you or a family member can be taught to do at home. This treatment helps to significantly lower your risk of developing DVT without posing a risk to your pregnancy.

If you are taking heparin and are having a planned caesarean, your team will give you clear instructions on when to take your last dose before the birth. They will also advise when to start again afterwards.

General prevention tips for a healthy pregnancy

The good news is that most blood clots that happen during pregnancy and after birth can be prevented. There are several things you can do to help reduce your risk.

General prevention tips include:

• Stay active – your midwife or doctor can advise you about safe exercise during pregnancy.
• Wear compression stockings – wear special stockings (graduated elastic compression stockings) if prescribed, to help blood circulation in your legs.
• Stay hydrated – drink plenty of fluids throughout the day, like water.
• Stop smoking – get support to stop smoking.
• Maintain a healthy weight – Talk to your midwife or doctor about managing your weight safely during pregnancy.

Travel and DVT

Travelling for longer than four hours can increase your risk of developing DVT. It is not known if this risk is greater during pregnancy.

To help reduce your risk of DVT while travelling:

• Drink plenty of water.
• Do simple leg exercises, such as regularly flexing your ankles.
• Walk about during refuelling stops or when it is safe to do so on the bus, train, or plane.
• Avoid drinking alcohol.