Managing obesity in pregnancy

It is vital that professionals know the risks associated with obesity in pregnancy, says Dr Polly Weston.

DVT: obese pregnant women should be assessed for VTE risk (Photograph: SPL)

There is an obesity epidemic in the UK today. The latest Health Survey for England data showed that in 2009, 61% of adults were overweight. Of these, 23% of adults were obese, with a BMI greater than 35kg/m2.1

Furthermore, approximately 5% of women giving birth in the UK each year have a BMI over 35kg/m2. This translates into approximately 38,500 maternities each year and is concentrated largely in areas of lower social class and those with high proportions of black and minority ethnic groups.2

As the prevalence of obesity increases it is vital that healthcare professionals understand the care issues posed by obese pregnant patients. Obese women have an increased risk of subfertility, miscarriage, fetal abnormality, pre-eclampsia, hypertension, gestational diabetes, stillbirth, venous thromboembolism (VTE) and complications at delivery.

Patient information
It has been recommended that women of childbearing age with a BMI over 30kg/m2 should receive information about the risks of obesity during pregnancy and should be supported to lose weight before conception. They should take 5mg of folic acid daily.3 This is a higher dose than usual because neural tube defects are more common in obese patients.

Weight should be recorded at the patient's first booking appointment to allow accurate calculation of BMI, because this dictates the different levels of care that may be needed around the time of delivery.

Some women will gain 30-40kg during pregnancy so it is vital to weigh them again in the third trimester, because their BMI can alter by 10-15 units.4 The myth of 'eating for two' should be dispelled, as the increased calorie requirement for pregnancy is 300 calories per day, necessary only in the last two trimesters.

The GP should refer the patient to the antenatal clinic as early as possible if she has a BMI of 35-40kg/m2 plus any additional risk factors, such as first ongoing pregnancy, previous pregnancy more than 10 years before, age over 40 years, multiple pregnancy and an underlying medical condition, such as asthma, previous VTE and Crohn's disease. Any patient with a BMI over 40kg/m2 should be referred.

An uncomplicated obese woman with a BMI below 40kg/m2 can be cared for by her community midwife and GP, provided she is seen often (every three weeks from 24 to 32 weeks' gestation, then fortnightly).5

Venous thromboembolism
By 12 weeks' gestation, the woman should have been assessed (either in the antenatal clinic or by the GP) for risk of VTE. The table (right) outlines the risk factors for VTE in pregnant women.

Those at risk should be prescribed low molecular weight heparin at a dose appropriate for their size for the duration of the pregnancy (see table below). Aspirin has no use in this context.6

Patient's weight Heparin dose
<90kg 40mg enoxaparin once daily
91-130kg 60mg enoxaparin once daily
131-170kg 80mg enoxaparin once daily
>170kg 0.6mg/kg/day enoxaparin

Further management
Once attending the local antenatal clinic, the obese pregnant woman will be counselled about her increased risk of fetal abnormalities, hypertension, pre-eclampsia, large babies and gestational diabetes.

These risks increase in proportion to increasing size. Obese women are approximately twice as likely to have a stillborn baby as those with a healthy BMI. Ultrasound is less accurate in screening for abnormalities and measuring fetal growth. Biochemical screening is still valid.

A glucose tolerance test should be done between 24 and 28 weeks' gestation for women with a BMI over 30kg/m2. This is performed as a 75g glucose challenge over two hours.

During labour
Any woman with a BMI over 40kg/m2 should be seen by an anaesthetist before delivery to establish any prospective difficulties with venous access, regional or general anaesthesia.7

Once in labour, the obese pregnant woman is more likely to have complications. Difficulties with monitoring the fetus make interventions, such as caesarean section or instrumental delivery, more likely.

The current UK caesarean section rate is 25%. In women with a BMI exceeding 50kg/m2, the rate is 46%. There is a four times greater risk of postpartum haemorrhage and wound or perineal infection.

These are also harder to manage, owing to the patient's size. Increased maternal size often leads to increased fetal size and this leads to greater incidence of shoulder dystocia at delivery.8 Impaction of the fetal shoulders increases birth injury and neonatal asphyxia with admission to the neonatal unit.

Postpartum, all women with a BMI exceeding 40kg/m2, or those with a BMI over 30kg/m2 plus an additional risk factor, such as caesarean section, comorbidity or smoking, should be discharged on low molecular weight heparin for six weeks.

They should also see their GP for a BP check and repeat glucose tolerance test at six weeks postpartum. These are modifiable risk factors for long-term health concerns, so this screening is vital.

Risk factors for venous thromboembolism6
  • Previous VTE.
  • Thrombophilia.
  • Antithrombin deficiency, protein C deficiency, protein S deficiency, factor V Leiden.
  • Acquired antiphospholipid syndrome.
  • Medical comorbidities (heart or lung disease, cancer, inflammatory conditions, nephrotic syndrome (proteinuria >3g/day), sickle cell disease, IV drug user).
  • Age >35 years.
  • Obesity either pre-pregnancy or in early pregnancy.
  • Parity ≥3.
  • Smoking.
  • Gross varicose veins (symptomatic or above knee or with associated phlebitis, oedema/skin changes).
  • Paraplegia.
  • Multiple pregnancy, assisted reproductive therapy.
  • Surgical procedure in pregnancy or puerperium.
  • Hyperemesis, dehydration.
  • Ovarian hyperstimulation syndrome.
  • Immobility (≥3 days' bed rest), for example, symphysis pubis dysfunction restricting mobility.
  • Systemic infection (requiring antibiotics or admission to hospital).
  • Long-distance travel (>4 hours).
  • Dr Weston is a senior registrar in obstetrics and gynaecology at Southmead Hospital, Bristol


1. DoH. Obesity.

2. Confidential Enquiry into Maternal and Child Health (CEMACH) report. Maternal obesity in the UK, 2010.

3. Rasmussen SA, Chu SY, Kim SY et al. Maternal obesity and the risk of neural tube defects; a meta analysis. Am J Obstet Gynecol 2008; 198: 611-19.

4. Heslehurst N, Ellis LJ, Simpson H et al. Trends in maternal obesity incidence rates and health inequalities in 36,821 women over a 15-year period. BJOG 2007; 114 (2); 187-94.

5. Centre for Maternal and Child Enquiries and Royal College of Obstetricians and Gynaecologists. Management of women with obesity in pregnancy, London 2010.

6. Royal College of Obstetricians and Gynaecologists. Green top guideline 37. Reducing the risk of thrombosis during pregnancy and the puerperium, London 2009.

7. Hood DD, Dewan DM. Anaesthetic and obstetric outcome in morbidly obese parturients. Anaesthesiology 1993: 79 (6); 1210-18.

8. Royal College of Obstetricians and Gynaecologists. Green top guideline 52. Prevention and management of postpartum haemorrhage, London 2009.

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