Obesity increases the risk of venous thromboembolism—a deep-vein thrombosis or pulmonary embolism—but it does not automatically mean daily heparin injections throughout pregnancy. A body mass index is one risk factor, not a complete prescription.
The useful question is: Does this woman’s combined risk justify low-molecular-weight heparin now, only during a temporary high-risk period, after birth, or not at all?
Obesity Raises Clot Risk—but It Is Not a Prescription
Pregnancy naturally makes the blood more likely to clot. Obesity adds to that risk, but most women with obesity do not develop a clot, and the absolute risk for an individual may still be low. Low-molecular-weight heparin—LMWH—is used to prevent venous clots in women whose overall risk is high enough to outweigh the inconvenience and bleeding risk of treatment. It should not be prescribed merely because the weighing scale crosses one number or because someone describes the blood as “thick.” [1,2]
The Whole Risk Profile Decides
The assessment includes the booking BMI, previous deep-vein thrombosis or pulmonary embolism, thrombophilia, a strong family history, age, smoking, reduced mobility, multiple pregnancy, medical illness and obstetric complications. A previous clot or certain thrombophilias can change the plan far more than obesity alone. Professional guidelines do not use one universal scoring system. RCOG guidance counts combinations of risk factors, while other organisations use different thresholds and institutional protocols. The correct conclusion is therefore not “my friend had injections, so I need them,” but “what is my complete antenatal and postpartum risk?” [1,3,4]
The Answer Can Change During Pregnancy
A woman who does not need LMWH at booking may need it later. Risk should be reassessed after hospital admission, severe vomiting with dehydration, prolonged immobility, significant infection, surgery, ovarian hyperstimulation, preeclampsia or another major complication. Sometimes LMWH is required only while the temporary risk remains. Active bleeding, a high likelihood of labour or another bleeding concern may delay or alter prophylaxis. This is why the decision should be reviewed rather than copied forward unchanged at every visit. [1]
After Birth May Be the Highest-Risk Window
The risk of thrombosis is highest after delivery, so the postpartum answer may be different from the antenatal answer. RCOG advises that women with class III obesity—a BMI of 40 kg/m² or more—be considered for weight-appropriate LMWH for 10 days after birth; longer treatment may be needed when major or persistent risk factors remain. [1,2]
Caesarean birth requires a separate assessment. SMFM recommends compression devices for all women having a caesarean and pharmacological prophylaxis for selected higher-risk women. When drug prophylaxis is already indicated in class III obesity, an intermediate enoxaparin regimen may be considered rather than assuming one standard dose fits everyone. [3]
Dose, Labour and Anaesthesia Need a Written Plan
LMWH doses are not interchangeable. The indication, body weight, kidney function, bleeding risk and whether the medicine is preventive or treating an existing clot all matter. More medicine is not automatically safer. A recent injection can affect whether and when spinal or epidural anaesthesia can be given. Women taking LMWH need written instructions for labour, vaginal bleeding, a planned caesarean, an unexpected emergency and restarting treatment after birth. The safe interval differs between lower preventive doses and higher treatment doses, so doses should never be stopped, doubled or restarted from internet advice. [1,5]
Heparin and Aspirin Do Different Jobs
LMWH mainly prevents venous thrombosis. Low-dose aspirin is an antiplatelet medicine used in selected women to reduce preeclampsia risk; it is not a substitute for LMWH when venous-clot prophylaxis is required. Obesity is a moderate risk factor in aspirin assessment, but aspirin is usually recommended because of a high-risk condition or a combination of moderate risks—not simply as treatment for a raised BMI. Neither medicine should be added merely to “improve placental blood flow” without a recognised indication. [1,6]
Seek Urgent Assessment
Seek urgent medical assessment for new swelling, pain, warmth or redness in one leg; sudden breathlessness; chest pain; coughing blood; collapse; or an unexplained rapid heartbeat. These symptoms require investigation even if LMWH is already being used. Heavy vaginal bleeding, bleeding that will not stop, or labour beginning soon after an injection also needs immediate contact with the maternity team. Do not take an extra injection because a clot is suspected, and do not omit future doses without urgent instructions from the treating team.
Dr Tania’s takeaway: A raised BMI is a risk factor, not a complete prescription. Heparin should be used when the woman’s combined antenatal or postpartum clot risk justifies it, with the right dose, duration and written plan for labour, caesarean birth and spinal or epidural anaesthesia.
Evidence Base
[1] Royal College of Obstetricians and Gynaecologists. Reducing the Risk of Venous Thromboembolism during Pregnancy and the Puerperium. Green-top Guideline No. 37a. April 2015; amended 2023.
https://www.rcog.org.uk/media/m4mbpjwi/gtg-no37a-2015_amended-2023.pdf
[2] Denison FC, Aedla NR, Keag O, et al. Care of Women with Obesity in Pregnancy. RCOG Green-top Guideline No. 72. BJOG. 2019;126:e62–e106. doi:10.1111/1471-0528.15386.
[3] Pacheco LD, Saade G, Metz TD. Society for Maternal-Fetal Medicine Consult Series #51: Thromboembolism prophylaxis for cesarean delivery. American Journal of Obstetrics and Gynecology. 2020;223:B11–B17. Reaffirmed 2024; endorsed by ACOG.
[4] Bates SM, Rajasekhar A, Middeldorp S, et al. American Society of Hematology 2018 guidelines for management of venous thromboembolism: venous thromboembolism in the context of pregnancy. Blood Advances. 2018;2(22):3317–3359. doi:10.1182/bloodadvances.2018024802.
[5] Kopp SL, Vandermeulen E, McBane RD, et al. Regional anesthesia in the patient receiving antithrombotic or thrombolytic therapy: American Society of Regional Anesthesia and Pain Medicine Evidence-Based Guidelines, fifth edition. Regional Anesthesia and Pain Medicine. Published online January 29, 2025. doi:10.1136/rapm-2024-105766.
[6] American College of Obstetricians and Gynecologists and Society for Maternal-Fetal Medicine. Low-Dose Aspirin Use for the Prevention of Preeclampsia and Related Morbidity and Mortality. Practice Advisory. December 2021; reaffirmed October 2022.
Medical Disclaimer
This Article provides general education and does not replace patient-specific obstetric, maternal–fetal-medicine, haematology or anaesthetic advice. Do not start, stop, change or double a heparin injection because of this page. Seek emergency care for one-sided leg swelling or pain, sudden breathlessness, chest pain, coughing blood, collapse, heavy bleeding or feeling seriously unwell.