Pregnancy VTE Risk Score Calculator (RCOG Green-top 37a, 2015)

Understanding the pregnancy VTE risk score calculator — concept and principle

Pregnancy and the weeks after birth raise the risk of venous thromboembolism (VTE: blood clots in the legs or lungs) several-fold. This calculator adds up the risk factors in the Royal College of Obstetricians and Gynaecologists (RCOG) Green-top Guideline 37a risk assessment and shows what the score suggests about preventive treatment.

How it is calculated

Score = pre-existing factors + obstetric factors + transient factors

• Previous VTE 4 (3 if a single clot after major surgery); high-risk thrombophilia 3; serious medical condition 3.
• 1 each: family history, low-risk thrombophilia, age over 35, BMI 30 or more (2 if 40 or more), three or more previous births, smoking, gross varicose veins.
• Obstetric, 1 each: pre-eclampsia, IVF (antenatal), twins or more, mid-cavity or rotational delivery, labour over 24 h, bleeding over 1 L or transfusion, preterm birth, stillbirth; caesarean in labour 2, planned caesarean 1 (postnatal).
• Transient: surgery in pregnancy or after birth 3, hyperemesis 3, ovarian hyperstimulation 4 (first trimester), infection 1, immobility or dehydration 1.

Antenatal: 4 or more → consider prophylaxis from the first trimester; 3 → from 28 weeks. Postnatal: 2 or more → consider for at least 10 days.

The principle

Pregnancy makes blood clot more easily, the growing womb slows blood return from the legs, and birth can damage vessels. Each listed factor adds to this baseline. The RCOG gave points according to how much each factor raises risk, so several modest factors can add up to the same concern as one strong factor.

Worked example

Default inputs: antenatal assessment, age over 35, BMI 30 to under 40, smoker; everything else no.

1. Pre-existing = age 1 + BMI 1 + smoking 1 = 3.
2. Obstetric = 0; transient = 0.
3. Total = 3 → 3 — thromboprophylaxis considered from 28 weeks.

How to read the result

The score is a prompt for discussion with the maternity team, not a prescription. Any hospital admission and a stay of 3 days or more after birth are also reasons to consider prevention. The score should be repeated when circumstances change, for example after an admission or at delivery.

Common mistakes

• Scoring only once in early pregnancy.
• Counting birth-related factors before delivery.
• Forgetting temporary factors such as infection or dehydration.

Frequently asked questions

What are the signs of a clot in pregnancy?

Pain or swelling in one leg, chest pain or sudden breathlessness; seek urgent care if they occur.

What is thromboprophylaxis?

Preventive treatment, usually daily low-molecular-weight heparin injections, sometimes with compression stockings.

Does a high score mean I will get a clot?

No. It means the risk is high enough that prevention is worth considering.

Sources: Royal College of Obstetricians and Gynaecologists, Green-top Guideline No. 37a, "Reducing the risk of venous thromboembolism during pregnancy and the puerperium" (2015)