Why does clot risk rise in pregnancy?
Venous thromboembolism (VTE) is a blood clot in the veins. It most often occurs in the deep veins of the leg (deep vein thrombosis); if part of the clot breaks off and travels to the lungs it can cause a serious condition called pulmonary embolism. In pregnancy the blood naturally clots more easily and the growing womb slows blood returning from the legs. The risk is therefore several times higher than in women who are not pregnant, and it is highest in the first weeks after birth.
How does this tool calculate?
The calculation is based on the risk assessment form in the Royal College of Obstetricians and Gynaecologists (RCOG) Green-top Guideline No. 37a (2015). Each risk factor carries points and the points are added up:
- 4 points: a previous clot (other than a single clot after major surgery); ovarian hyperstimulation syndrome after IVF (first trimester).
- 3 points: a previous clot after major surgery, a high-risk inherited clotting disorder, conditions such as cancer or heart failure, surgery during pregnancy or after birth, severe nausea and vomiting (hyperemesis).
- 2 points: caesarean section in labour; body mass index 40 or above.
- 1 point: age over 35, body mass index 30 or above, 3 or more previous births, smoking, marked varicose veins, family history of clots, low-risk thrombophilia, pre-eclampsia, IVF pregnancy, multiple pregnancy, planned caesarean, assisted birth, labour over 24 hours, blood loss over 1 litre, birth before 37 weeks, stillbirth, infection, immobility or dehydration.
Some factors count for one period only: IVF, ovarian hyperstimulation and hyperemesis during pregnancy; the mode of birth, bleeding and labour-related factors after birth.
How to read the result
During pregnancy the guideline suggests considering preventive blood-thinning treatment (low-molecular-weight heparin injections) from the first trimester when the total score is 4 or more, and from 28 weeks when it is 3. Women who have had a clot before are at high risk; prevention throughout pregnancy and care with an experienced team are recommended.
After birth at least 10 days of prevention is considered when the score is 2 or more. At least 6 weeks is recommended for women with a previous clot, those who needed blood thinners in pregnancy, those with a high-risk thrombophilia, and carriers of a low-risk thrombophilia with a family history of clots.
Limitations
The score reflects your situation at one point in time, and risk can change during pregnancy; it should be recalculated after a hospital admission, an operation or a new illness. The form does not assess bleeding risk. Blood thinners may be unsuitable with bleeding disorders, active bleeding or placenta praevia, so the treatment decision is always made by the doctor, taking all your health information into account. Thresholds can differ slightly between countries and guidelines.
When to seek help immediately
One-sided leg swelling, pain, redness or warmth, sudden breathlessness, chest pain, coughing up blood or fainting can be signs of a clot. In that case go to the nearest emergency department without delay. If your score is high or one of the conditions listed applies to you, discuss it with your doctor at your next visit.
Sources
- Royal College of Obstetricians and Gynaecologists. Reducing the Risk of Venous Thromboembolism during Pregnancy and the Puerperium. Green-top Guideline No. 37a. 2015 (Appendices I and III).