Why pregnancy raises the risk

The RCOG explains what a venous thrombosis is: a blood clot in a vein. "A deep vein thrombosis (DVT) is a blood clot that forms in a deep vein of the leg, calf or pelvis."

The HSE lists the reasons pregnancy makes it more likely: changes to your hormones, changes to your circulation, and pressure from your growing baby on the veins in the lower part of your tummy and pelvis. It adds the part people miss: "The risk continues for 6 weeks after you give birth."

The RCOG gives both the direction and the scale: "Pregnancy increases your risk of a DVT, with the highest risk being just after you have had your baby. However, venous thrombosis is still uncommon in pregnancy or in the first 6 weeks after birth, occurring in only 1-2 in 1000 women." It also warns against assuming this is a third-trimester problem: "A DVT can occur at any time during your pregnancy, including the first 3 months."

The symptoms — leg first, then lungs

The RCOG lists DVT symptoms, noting they usually occur in only one leg:

a red and hot swollen leg
swelling of the entire leg or just part of it, or a leg that just feels heavy
pain and/or tenderness, which you may only notice when standing or walking

The HSE adds pain in your calf that gets worse when you bend your foot up towards your knee.

The RCOG is careful to give the counterweight: "During pregnancy, swelling and discomfort in both legs is common and does not always mean that there is a problem." The distinguishing feature is usually that it is one leg, not two.

Contact your midwife, GP or maternity unit the same day if you have any of those leg symptoms. The RCOG's wording is "seek advice immediately". Do not wait for your next appointment.

When it is an emergency

A clot that travels to the lungs is a pulmonary embolism. The RCOG lists the symptoms:

sudden unexplained difficulty in breathing
tightness in the chest or chest pain
coughing up blood
feeling very unwell or collapsing

The HSE adds pain when you breathe in, and pain in your upper back, and makes the point that matters most: "You can have a PE without symptoms of a DVT."

Call your local emergency number immediately — 999 in the UK and Ireland, 911 in the US and Canada, 000 in Australia, 111 in New Zealand — for any of those. The RCOG notes that dying from a PE "is very rare in women who are pregnant or who have just had a baby", and that diagnosing and treating a DVT reduces the risk of a PE developing. Both of those facts argue for calling early, not for waiting to see.

What raises your risk

The RCOG groups the risk factors by when they apply.

Before pregnancy: being over 35; having already had three or more babies; a previous venous thrombosis; a parent or sibling who has had one; a thrombophilia; medical conditions such as heart disease, lung disease or arthritis; severe varicose veins that are painful or above the knee with redness or swelling; being a wheelchair user.

Lifestyle: a BMI over 30, smoking, or intravenous drug use.

During pregnancy: being admitted to hospital; carrying more than one baby; becoming dehydrated or less mobile, for example through severe vomiting, a serious infection or ovarian hyperstimulation syndrome; being immobile for long periods, including travelling for 4 hours or longer by air, car or train; and pre-eclampsia.

After birth: a labour longer than 24 hours, a caesarean birth, losing a lot of blood, or receiving a blood transfusion.

The HSE's list overlaps closely and adds inflammatory bowel disease and autoimmune conditions such as lupus.

The risk assessment nobody explains

This is a formal, repeated process, not a one-off. The RCOG: "Your midwife should carry out a risk assessment at your first antenatal booking and at around 28 weeks of pregnancy. A risk assessment should also be carried out if your situation changes during your pregnancy and/or if you are admitted to hospital. After your baby is born a further risk assessment should be done."

The point of it is prevention. The RCOG says "most DVTs and PEs that occur during pregnancy and after birth are preventable", and that some risk factors — a previous thrombosis, for instance — are significant enough on their own to justify treatment, while others only count in combination.

Your risk is not fixed. The RCOG notes it can rise if you become unwell, develop severe varicose veins, travel for over 4 hours or have a complicated birth, and can fall if, for example, you stop smoking. That is why the assessment is repeated.

The underlying clinical framework in the UK is the RCOG's Green-top Guideline No. 37a on reducing the risk of thrombosis and embolism during pregnancy and the puerperium.

Preventive treatment

The RCOG lists the general measures: stay as active as you can, wear graduated elastic compression stockings if advised, and keep hydrated by drinking normal amounts of fluids.

Where preventive medication is indicated, it is heparin by injection rather than tablets. The RCOG explains why warfarin is generally avoided: it "can be harmful to your unborn baby", so people already taking it are usually advised to switch to heparin before pregnancy or as early as possible in it — though it notes that for some people warfarin remains the only option, which is a conversation to have before conceiving.

ACOG's patient information on preventing deep vein thrombosis covers the equivalent US approach.

If a clot is diagnosed

The RCOG's leaflet on diagnosis and treatment covers the tests used in pregnancy and the associated risks, treatment with heparin, what to do when labour starts, what happens after birth, and breastfeeding while on treatment. Two practical things are worth knowing in advance: you will be given instructions about stopping injections once labour begins, and treatment usually continues for a period after birth rather than stopping at delivery.

The HSE has a specific page on birth planning while being treated for blood clots, which is worth reading before you are asked to make decisions in labour.

The NHS's general DVT page covers what a clot is and how it is treated outside pregnancy, and is useful background — but the pregnancy-specific guidance above is what applies to you, because both the risk assessment and the drug choices differ.

Long flights and long journeys

Immobility for 4 hours or more is on the RCOG's own risk list, and it applies to cars and trains as well as planes. If you are planning long-distance travel in pregnancy, raise it at an antenatal appointment beforehand rather than after booking — whether you need stockings, or in some cases heparin cover, depends on the rest of your risk profile, and that is not something to judge for yourself.

Blood Clots and VTE in Pregnancy