Written by:

Dr Etienne Ciantar


Dr Ciantar is a consultant obstetrician at the Leeds Teaching Hospitals NHS Trust. His specialist interests are obstetric haematology and medical education. He runs a weekly obstetric thrombosis and haemostasis clinic and a monthly obstetric haemoglobinopathy clinic.

Venous blood clots in pregnancy

In this article:

  • Background
  • Treatment
  • Prevention
  • Conclusion
  • Spotlight Issues

Venous blood clots in pregnancy


Blood in pregnancy undergoes significant changes making it more “pro-thrombotic”, which means that it is more likely to develop blood clots. This is a protective mechanism which helps reduce the risk of haemorrhage immediately after delivery. These changes occur from conception which therefore means that the risk of developing a blood clot in pregnancy is increased from the first trimester and peaks in the pueperium, i.e. the six weeks after delivery.

Pregnancy increases the risk of developing a blood clot by five times compared to the non-pregnant state. The absolute risk is approximately 1 in 1000. The commonest site to develop a blood clot is in the leg veins and this is known as a deep vein thrombosis (DVT) – 90% of DVTs develop in the left leg. The left preponderance is secondary to pressure on the left leg veins by one of the main arteries in the pelvis. A blood clot in the leg can detach itself and get stuck in the lung circulation. This is referred to as a pulmonary embolus (PE) and can be very serious. DVTs in pregnancy are more likely to occur in the leg veins above the knee as opposed to DVTs in the non-pregnant state which are more common below the knee. This is particularly relevant as above-knee DVTs are more likely to embolise to the lung.


DVTs typically present with a painful, red and swollen leg. This requires medical attention and an ultrasound of the leg is requested when a DVT is suspected. Ultrasound of the leg is very good at picking up DVTs above the knee.

PEs present with chest pain which is usually worse when breathing in, together with shortness of breath and occasionally haemoptysis (coughing up blood). This also requires urgent medical attention. When a PE is suspected the clinician will request chest imaging in the form of a chest X-ray followed by one of the following options – a ventilation perfusion (VQ) scan or a CTPA (computerised tomography pulmonary angiogram). Unfortunately both modalities have risks with the VQ scan potentially increasing the child’s risk of childhood cancer marginally, while a CTPA increases the woman’s risk of breast cancer by approximately 14%. However, these risks need to be balanced against the risk of not diagnosing (and therefore not treating) a PE, which may prove fatal to the woman.

The treatment of both DVTs and PEs is a blood thinning injection known as low molecular weight heparin (LMWH). These injections are given once or twice daily (depending on the brand used) and will need to be given for the duration of the pregnancy and for some time after delivery, the duration depending on when the blood clot occurred.

LMWH injections are safe and effective in the treatment of blood clots in pregnancy. It has been shown that the risk of developing another blood clot while on LMWH is only around 1%. Although they are blood thinning injections, they do not increase the risk of severe bleeding after delivery. The medication does not cross the placenta and is therefore safe for the baby.

The biggest issue with LMWH use in pregnancy is its effect on the use of regional anaesthesia (i.e. epidural and spinal anaesthesia). This is because 24 hours will have to pass from the last injection and the administration of an epidural for pain relief or a spinal anaesthetic (which is the anaesthetic routinely used for Caesarean sections). This means that other forms of pain relief will have to be offered if the woman has had her injection less than 24 hours before. In addition she will need to be put to sleep (which comes with its own risks) if she needs to be taken to theatre for an operation (e.g. C-section) and less than 24 hours have passed from her last injection.

For this reason women who are being treated for a blood clot may be offered an induction of labour by their obstetrician. This will allow stopping LMWH 24 hours before starting the induction process and the woman can have an epidural if she wanted one or a spinal anaesthetic if this is needed. It is also important for the woman to see an anaesthetist (the doctor who performs regional anaesthesia), ideally in the antenatal period, to discuss other pain relief options during labour if an epidural cannot be given.


Preventing blood clots in pregnancy is very important and all pregnant women in the United Kingdom are risk assessed for this when they see their community midwife and every time they are seen in hospital, including after delivery. There are certain risk factors which increase a woman’s risk of developing a blood clot in pregnancy. The biggest risk factor is a history of a blood clot in the past. This increases the risk of developing a blood clot in pregnancy by 25 times. Women who have previously had a blood clot will therefore require LMWH injections to prevent developing a blood clot from the beginning of pregnancy and for six weeks after delivery. The dose used is lower than that required to treat blood clots.

Other factors which increase the risk of developing a blood clot in pregnancy and the approximate increased risk are summarised in the following table:

Risk FactorIncreased risk of blood clot*
Age > 351.4
BMI > 305
Parity > 3**2.4
Varicose Veins2.4
IVF Pregnancy4.3

*when compared to a normal healthy pregnancy
**3 previous pregnancies delivered after 24 weeks

Certain risk factors are relevant after delivery, e.g. a postpartum haemorrhage and a blood transfusion increase the risk of a blood clot, as does delivery by C-section. The latter is associated with a fivefold increased risk of a blood clot when compared to a normal vaginal delivery.

The risk assessment for blood clot prevention used in UK hospitals is very thorough and evidence based. Midwives and doctors will decide on the need to start LMWH for clot prevention depending on the woman’s risk factors. LMWH may be started at the start of pregnancy or at 28 weeks depending on the number and severity of risk factors. Alternatively LMWH may only be needed postnatally for 10 days (e.g. after an emergency C-section in the absence of other risk factors) or six weeks if there are other risk factors.


Pregnancy increases every woman’s risk of developing a blood clot and some women are at a higher risk because of additional risk factors. Blood clots in pregnancy can be serious, especially when they occur in the lungs. Risk assessment for blood clot prevention is widely adopted in UK practice and is very useful in preventing blood clots. In addition there is effective and safe treatment for those women who develop a blood clot.


  1. Pavord & Hunt 2010. The Obstetric Haematology Manual. Cambridge University Press;
  2. Royal College of Obstetricians & Gynaecologists Green-top Guideline No. 37a, April 2015. Reducing the Risk of Venous Thromboembolism during Pregnancy and the Puerperium;
  3. Royal College of Obstetricians & Gynaecologists Green-top Guideline No. 37b, April 2015. Thromboembolic Disease in Pregnancy and the Puerperium: Acute Management.


Spotlight Issues

  • Pregnant women are five times more likely to develop blood clots during pregnancy than when not pregnant.
  • There is a 1 in 1,000 chance that a pregnant woman will develop a blood clot.
  • Deep vein thrombosis (DVT) in pregnancy is more likely to occur in leg veins above the knee, which can cause an embolism in the lung –Pulmonary embolus (PE).
  • A painful, red, swollen leg could be the sign of DVT.
  • Chest pain (particularly on an in-breath), shortness of breath and/or coughing up blood are signs of PE
  • Treatment of DVT and PE is by a blood thinning injection which is safe for mother and baby
  • All pregnant women in the UK will be assessed by their midwife for blood clots.
  • Immobile women have a higher risk of suffering blood clots in pregnancy than smokers.