Last updated on Feb 17th, 2021 at 03:03 pm

A blood clot or deep vein thrombosis (DVT) occurs in the upper or lower veins of your legs, explains gynaecologist and obstetrician Dr Bronwyn Moore. “The higher it’s situated, the more dangerous it is because pieces of the clot can shoot off as an embolus and travel to your lungs. The resulting pulmonary embolism (PE) is potentially life-threatening depending on its size, so requires immediate attention. Anyone can develop a blood clot under the right set of circumstances,” she says. “While your age, your weight, being a smoker and taking hormones (the Pill or hormone therapy) all play a role, pregnancy is an independent factor that puts you at a far higher risk. However, it’s the weeks after delivery that are the most problematic,” she adds. “Postpartum collapse from pulmonary embolism is one of the major causes of maternal death in the UK.”

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Why does it happen?

According to Virchow’s triad, a system that describes the causes of thrombosis, there are three general factors that promote the forming of a clot:

  • Impaired/slow blood flow in the vessel (venous stasis). If you’re bed-ridden with a fracture, immobile during a long-distance flight or road trip, or on bed rest post-operatively or post-delivery.
  • Viscosity, or stickiness, of the blood. In pregnancy, your blood-clotting factor levels are increased – particularly after delivery. When the placenta comes away, you’re left with a huge raw area, so there’s a surge of clotting factor to prevent you from losing too much blood. Ironically, your body’s natural response to protect your uterus puts you at risk elsewhere.
  • Vessel wall damage, usually due to trauma. For example, if you break your ankle, the resultant damage to the veins and the swelling set up the perfect conditions for a clot.

Who is at risk?

Certain families carry genetic abnormalities that increase the risk of clotting. It’s vital to inform your healthcare provider about any family history of, including incidences of DVTs, PEs or strokes caused by a clot, as this may put you at a far higher baseline risk when you’re pregnant. “The more risk factors you have, the more likely you are to have a DVT,” says Bronwyn. “So if you have inherited a specific abnormality and are overweight and pregnant, the likelihood of this occurring becomes significant. That’s why it’s so essential for every pregnant woman to be thoroughly assessed in order to determine her thrombosis risk.”

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Bronwyn advises you to look out for the following:

DVT symptoms:

  • One leg swelling more than the other
  • Sore or painful calf
  • A leg that feels warmer or looks redder than the other one.

Remember, most women experience some swelling of the lower legs during pregnancy.

PE symptoms:

  • Shortness of breath often sudden in onset and present at rest
  • Cough without other symptoms of a cold, which may be associated with a blood-stained sputum (mucous and saliva that has been coughed up)
  • Chest pain
  • A raised heart rate.

A DVT and PE can be life-threatening if they’re not diagnosed or managed timeously, says Bronwyn, so always seek medical help in the presence of any of the above symptoms. A DVT is difficult to exclude through
a clinical exam, so it’s essential you have an urgent Doppler ultrasound (which measures the amount of blood flow through your arteries and veins) if one of your legs is swollen.


Blood thinners are the mainstay of treatment for clotting issues. Although there are oral medications available, one is not registered for pregnant women and the other crosses the placenta, explains Bronwyn, which is why the injectable blood thinner is the preferred form of treatment. “Injections are safe to use throughout your pregnancy as they don’t transfer to your baby while doing their job of preventing any existing clots from expanding and allowing your body to break down the existing clot. Initially, you’ll need to inject yourself twice a day if you have a DV as a preventative or precautionary measure. So, if you had a DVT five years ago and are now pregnant, you’ll require this injection daily.”

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Before you fall pregnant:

  • Know your risks by finding out your family history and speak to your healthcare provider about whether testing is indicated.
  • Exercise regularly, maintain a normal BMI, and give up smoking.
  • Take blood thinners – especially if you’ve had a previous DVT, or have other diseases that increase your risk, such as antiphospholipid syndrome.

After delivery (you’re now at the highest risk):

  • Chat to your doctor about completing a standard risk assessment form to determine whether you need to start injecting blood thinners.
  • Get moving as soon as possible.
  • Avoid dehydration − especially if you have a family history as this can lead to clotting.
  • Report any abnormal symptoms described above and get them checked out.