Lucan · Conditions

Obesity in pregnancy

Australian guidance: RANZCOG C-Obs 49 Management of Obesity in Pregnancy (2022)

1. Definition and epidemiology

Obesity in pregnancy is defined as a body mass index (BMI) of 30 kg/m2 or greater, calculated using height and weight measured at the first antenatal consultation. Approximately 45 per cent of women who become pregnant in Australia are either overweight (BMI 25 to 30) or obese (BMI greater than 30), making excess weight one of the most important challenges in contemporary obstetric care. The WHO classification recognises three classes of obesity: Class I (BMI 30 to 34.99), Class II (BMI 35 to 39.99), and Class III (BMI 40 or above), with lower BMI cutoffs applied for Asian women.

2. Pathophysiology

Obesity produces a chronic low-grade inflammatory state driven by dysfunctional adipose tissue, particularly visceral fat. Hypertrophied adipocytes secrete pro-inflammatory cytokines including tumour necrosis factor-alpha and interleukin-6, which promote insulin resistance via impaired insulin receptor signalling and reduced GLUT-4 translocation. Adiponectin, which normally sensitises tissues to insulin, is paradoxically reduced in obesity. In pregnancy, this baseline insulin resistance is compounded by the anti-insulin effects of placental hormones (human placental lactogen, cortisol, oestrogen, and progesterone), greatly amplifying the risk of gestational diabetes mellitus. Leptin resistance in the hypothalamus impairs satiety signalling and contributes to ongoing weight gain. Excess adipose tissue also promotes a hypercoagulable state through raised levels of plasminogen activator inhibitor-1 and fibrinogen, elevating venous thromboembolism risk. In the foetus, maternal hyperglycaemia drives foetal hyperinsulinaemia, promoting macrosomia and organomegaly (Robbins Ch 22). Maternal and placental oxidative stress from lipid peroxidation contributes to endothelial dysfunction, which underlies the increased risk of pre-eclampsia.

3. Clinical presentation and diagnostic criteria

Most women with obesity in pregnancy present to antenatal care before symptoms of complications arise. History should capture pre-pregnancy weight and BMI, prior gestational diabetes, hypertension, obstructive sleep apnoea, polycystic ovarian syndrome, prior caesarean section, and any bariatric surgery. On examination, blood pressure may already be elevated and waist circumference is an additional marker of cardiometabolic risk beyond BMI alone. The diagnostic threshold for obesity in pregnancy is BMI 30 kg/m2 or above at the first antenatal visit, ideally before 10 to 12 weeks gestation. Clinicians should also assess for features of obstructive sleep apnoea, depression, and prior adverse pregnancy outcomes, as these frequently co-exist and influence management planning.

4. Investigations and AU-specific cutoffs

At first antenatal visit: height, weight, and BMI documented; blood pressure; full blood count (iron deficiency is common); serum vitamin D; thyroid function if indicated; urine microscopy and culture.

Early glucose tolerance testing (GTT) is recommended for all women with obesity, with a repeat GTT at 28 weeks if the early test is normal. Women who have undergone gastric bypass cannot tolerate the standard 75 g oral glucose load due to dumping syndrome; BSL monitoring over 4 to 7 days between 24 and 28 weeks is an accepted alternative.

Non-invasive prenatal testing (NIPT) carries a higher no-result rate due to reduced foetal fraction: approximately 5 per cent in Class I obesity rising to 10 per cent in Class II and higher in Class III. Patients must be counselled about this limitation before testing.

Serial foetal growth ultrasounds should be offered in the third trimester. Anatomy ultrasound at the routine morphology scan has reduced structural detection rates in women with obesity. Continuous electronic foetal monitoring in labour is recommended for BMI 40 or above.

Anaesthetic referral is recommended antenatally for all women with obesity, with particular priority for those with BMI above 40.

5. Management

Pre-conception: Primary care providers should measure and record BMI at pre-conception appointments, discuss the risks of obesity on fertility and pregnancy outcomes, and refer to a dietitian or metabolic weight loss clinic. Depression screening is important given its bidirectional relationship with obesity. Bariatric surgery should be discussed for Class III obesity but conception should be avoided for 12 to 24 months post-surgery during the rapid weight loss phase. Women who have had bariatric surgery require intensified nutritional supplementation and should avoid pregnancy during times of rapid weight loss.

Antenatal: Care should be provided within a multidisciplinary framework including a specialist obstetrician, midwife, dietitian, and anaesthetist. Dietitian referral is recommended for all women with obesity for tailored advice on gestational weight gain targets. Per Institute of Medicine guidelines, women with obesity should gain 5 to 9 kg total in a singleton pregnancy, at a rate of 0.22 kg per week in the second and third trimesters. Influenza and COVID-19 vaccination are strongly recommended given the substantially elevated risk of severe infection.

For pre-eclampsia prevention, low-dose aspirin 100 to 150 mg daily should be commenced between 12 and 16 weeks and continued to 36 weeks, combined with elemental calcium 1500 to 2000 mg daily in divided doses.

Timing and mode of birth: Women with BMI 50 or above should be offered delivery prior to their due date. Women with Class III obesity should be counselled about the approximately 40 per cent risk of emergency caesarean section following spontaneous onset of labour. Intrapartum preparation must include plans for managing shoulder dystocia and postpartum haemorrhage. Women with BMI above 40 kg/m2 should have intravenous access established on admission to the labour ward.

6. Pharmacology and AU PBS notes

Folic acid: High-dose folic acid 5 mg daily is recommended pre-conception and in the first trimester for all women with BMI above 30, given the elevated risk of neural tube defects. Standard dose is 0.4 to 0.5 mg; the higher dose is not PBS subsidised as a standard item but is available as a compounded or over-the-counter preparation.

Aspirin: Low-dose aspirin 100 to 150 mg nocte for pre-eclampsia prophylaxis. In vitro evidence suggests that 60 mg aspirin is ineffective in women with obesity, so doses below 100 mg should not be used.

Enoxaparin (venous thromboembolism prophylaxis): Weight-adjusted dosing applies. Standard dose is 40 mg daily for 50 to 90 kg; 60 mg daily for 90 to 130 kg; 80 mg daily for above 130 kg. PBS subsidised for VTE prophylaxis in obstetric patients meeting clinical criteria.

Prophylactic antibiotics at caesarean section: Weight-based dosing is recommended. Cefazolin 2 g for maternal weight 80 kg or above; 3 g for 120 kg or above.

Anti-obesity pharmacotherapy (orlistat, GLP-1 receptor agonists) is contraindicated during pregnancy and at the time of conception due to safety concerns.

7. Complications and monitoring

Antenatal complications include miscarriage, recurrent miscarriage, neural tube defects, gestational diabetes mellitus, pre-eclampsia, thromboembolism, obstructive sleep apnoea, preterm birth, stillbirth, foetal growth restriction, and macrosomia. The risk of stillbirth is elevated even in Class I obesity.

Intrapartum complications include prolonged labour, failed induction (up to 37.5 per cent in Class III obesity), shoulder dystocia, increased rates of instrumental delivery, caesarean section, and postpartum haemorrhage. Foetal heart rate monitoring is technically more difficult in women with obesity.

Post-partum complications include delayed wound healing and infection, thromboembolic disease, difficulties establishing breastfeeding, and postnatal depression. Long-term neonatal consequences include increased adiposity, excess infant weight gain, and offspring obesity.

Monitoring schedule: BMI and weight at first visit; weight reassessment at least once per trimester; early GTT then repeat at 28 weeks; serial growth ultrasounds in the third trimester; anaesthetic review antenatally; blood pressure at every visit; postpartum VTE risk reassessment before discharge.

Written for Australian practice and grounded in the guidance named above. Study material only — not medical advice. Always check against current guidelines.