Obesity in Pregnancy

Introduction

Obesity is now the commonest medical problem in UK obstetric practice. Roughly one in five women booking for antenatal care has a body mass index of 30 kg/m2 or above, and the proportion with a BMI of 40 or above has risen steadily. It is worth treating as a medical condition of pregnancy in its own right, on a par with diabetes or hypertension, rather than as a background characteristic.¹

The reason is that it affects every stage: conception, miscarriage risk, the accuracy of screening, the development of gestational diabetes and pre-eclampsia, the reliability of clinical examination, the feasibility of regional anaesthesia, the mechanics of labour, and the risks of surgery and of thromboembolism afterwards. MBRRACE-UK reports have repeatedly found that a substantial majority of women who died during or shortly after pregnancy were overweight or obese.²

None of this justifies a judgemental tone in the consultation. Weight is a sensitive subject, and women frequently report feeling blamed, which drives late booking and disengagement from care - precisely the behaviours that increase risk. The clinical task is to explain what will be done differently and why, not to moralise.

Classification and measurement

Height and weight are measured at booking rather than being self-reported, and the BMI calculated. Weight should be re-measured in the third trimester in women with a BMI of 30 or above, both to guide thromboprophylaxis dosing, which is weight-banded, and to inform anaesthetic and manual handling planning.

Risks

Pre-conception and booking

The most effective intervention is weight loss before conception, which reduces the risk of essentially every complication listed above. Women of childbearing age with a BMI of 30 or above should be offered structured weight management support and clear information about the pregnancy-specific risks. Weight loss is not attempted during pregnancy: dieting in pregnancy has not been shown to improve outcomes and may compromise fetal growth. The realistic aim in pregnancy is to limit excess gestational weight gain, not to lose weight.

• Folic acid 5 mg daily, from before conception until 12 weeks, because of the increased risk of neural tube defects and reduced folate bioavailability
• Vitamin D 10 micrograms (400 units) daily throughout pregnancy and breastfeeding; deficiency is common in obesity because vitamin D is sequestered in adipose tissue
• Aspirin 75-150 mg daily from 12 weeks where BMI is 35 or above together with another moderate risk factor for pre-eclampsia, or where a high-risk factor is present
• Oral glucose tolerance test at 24-28 weeks for BMI above 30
• Documented VTE risk assessment at booking, on any admission, and postnatally
• Manual handling assessment and equipment planning for BMI 40 or above, including appropriate beds, theatre tables, hoists and trolleys
• Referral for antenatal anaesthetic review for BMI 40 or above, so that any difficulty with venous or neuraxial access is identified before labour

Thromboprophylaxis

Pregnancy is a hypercoagulable state and obesity compounds it through immobility, venous stasis and a chronic inflammatory state. Venous thromboembolism remains a leading cause of direct maternal death in the UK, and obesity features in a large proportion of those deaths.^2,4

• A total antenatal score of 4 or more means prophylactic low molecular weight heparin from the first trimester
• A score of 3 means prophylaxis from 28 weeks
• A postnatal score of 2 or more means at least 10 days of prophylaxis after birth
• BMI of 40 or above is itself an indication for 10 days of postnatal prophylaxis irrespective of other factors and of mode of birth
• Low molecular weight heparin dosing is weight-banded, so use the most recent measured weight rather than the booking weight
• Anti-embolic stockings, early mobilisation and adequate hydration are used alongside, not instead of, pharmacological prophylaxis

Antenatal surveillance

Symphysis-fundal height measurement becomes progressively less reliable as BMI rises, because subcutaneous fat obscures the fundus and the measurement is not corrected for it. Above a BMI of around 35, most units abandon serial SFH in favour of ultrasound growth assessment at intervals through the third trimester, typically at 28, 32 and 36 weeks.

Fetal movement counting remains important and should be actively encouraged, because it is one of the few forms of surveillance that is unaffected by maternal habitus. Women should be told that reduced movements warrant immediate assessment rather than waiting for the next appointment.

Consider screening for obstructive sleep apnoea where there is snoring with daytime somnolence, since it is common, under-recognised, associated with hypertension and pre-eclampsia, and treatable. Mental health should be asked about at each contact, as depression and anxiety are more common and are compounded by the experience of receiving care that repeatedly emphasises risk.¹

Intrapartum care

Women with a BMI of 35 or above are advised to give birth in an obstetric unit rather than at home or in a midwife-led unit, because of the higher rate of intrapartum complications and the practical difficulty of transferring a woman in an emergency.

• Inform the senior obstetrician and the anaesthetist when a woman with a BMI of 40 or above is admitted in labour
• Establish intravenous access early, since it becomes considerably harder in an emergency
• Continuous electronic fetal monitoring is often needed; external transducers may fail to obtain a reliable trace, and a fetal scalp electrode may be required
• Regional analgesia is recommended and should be sited early, both for analgesia and because it avoids a difficult emergency general anaesthetic later
• Anticipate a longer decision-to-delivery interval at caesarean section, and factor this into the timing of decisions
• Ensure appropriate equipment is available: an operating table with an adequate weight limit, long spinal needles, and additional staff for transfer and positioning
• Active management of the third stage, with a low threshold for an oxytocin infusion, because postpartum haemorrhage is more likely

Caesarean section

• Prophylactic antibiotics before skin incision, at a dose adjusted for weight
• Suture the subcutaneous tissue layer if it is more than 2 cm deep, which reduces wound dehiscence
• Consider negative pressure wound therapy dressings, which reduce surgical site infection in high-BMI women
• Expect a longer operating time and greater blood loss; ensure adequate assistance and retraction
• Weight-adjusted thromboprophylaxis and early mobilisation afterwards
• Careful wound review before discharge and community follow-up, as infection is substantially more common

Postnatal care

• Continue thromboprophylaxis for at least 10 days where indicated, and longer after caesarean section with additional risk factors
• Proactive breastfeeding support, since initiation and maintenance rates are lower; positioning is harder and lactogenesis may be delayed
• Wound care and infection surveillance
• Screen for postnatal depression at the routine contacts
• Postnatal glucose testing for women who had gestational diabetes, and annual HbA1c thereafter
• Contraception discussion: a BMI of 35 or above is UKMEC category 3 for combined hormonal contraception, meaning the risks generally outweigh the benefits, whereas progestogen-only methods, the implant and intrauterine methods are unrestricted
• Offer referral to a structured weight management programme, framed around the next pregnancy as well as long-term health

Inter-pregnancy weight change matters more than most women realise. Losing weight between pregnancies substantially reduces the risk of gestational diabetes, pre-eclampsia and macrosomia in the next pregnancy, while gaining weight increases them. The postnatal period is therefore a genuinely useful window for intervention, and referral at that point is more likely to help than advice given for the first time at a booking appointment.^1,6

Red flags

Prognosis

The great majority of women with a raised BMI have straightforward pregnancies and healthy babies. The risks described here are increases in relative risk applied to conditions that remain uncommon in absolute terms, and it is important to convey them proportionately: quoting a threefold increase in gestational diabetes without saying that most women will not develop it is not informed counselling.

What changes outcomes is anticipation rather than intervention after the fact: the right cuff, the right dose of folic acid, aspirin where indicated, an OGTT, a documented thrombosis score, growth scans instead of a tape measure, an anaesthetic plan made in clinic rather than in theatre, and honest, non-judgemental conversations about weight before the next pregnancy.¹