Manage obesity in pregnancy, including preconception care, risk-specific antenatal surveillance, planning for labour and birth, caesarean technique, and postnatal and interpregnancy care

In one line

Obesity is not one complication of pregnancy; it is a multiplier on almost every other one, and it is the only risk factor in this domain that can be reduced between pregnancies rather than during one. Most of the harm is prevented by planning ahead: the right cuff, the right scan, the right theatre table, the right person in the room, and a conversation about weight that happens before conception rather than after a stillbirth.

Where this sits. The metabolic and endocrine groundwork is in the Primary chapter on endocrine physiology. The complication obesity most reliably produces is covered at consultant level in the Finals chapter on diabetes in pregnancy.

Why fat tissue changes a pregnancy

Adipose tissue is an endocrine organ, and understanding what it secretes explains why obesity produces the particular pattern of complications it does, rather than a random increase in everything.

Chronic low-grade inflammation. Adipose tissue in obesity is infiltrated by macrophages and secretes TNF-alpha and interleukin-6. These cytokines phosphorylate insulin receptor substrate 1 on serine rather than tyrosine residues, which blocks insulin signalling downstream of the receptor. That is the direct molecular link to gestational diabetes: the woman starts pregnancy already insulin resistant, and the placental hormones then add a further 50% fall in insulin sensitivity onto a pancreas that was already compensating. See Diabetes in pregnancy.

Adiponectin falls. Adiponectin normally activates AMP-activated protein kinase in skeletal muscle and promotes insulin-independent glucose uptake. Hypoadiponectinaemia removes that route at exactly the moment pregnancy suppresses the insulin-dependent one.

Endothelial dysfunction and oxidative stress. The same inflammatory milieu impairs nitric-oxide-mediated vasodilatation and promotes an antiangiogenic state. That is the link to pre-eclampsia, and it is why obesity appears on every pre-eclampsia risk list. See Pre eclampsia and HELLP.

Venous stasis and immobility add to the pregnancy-induced hypercoagulability, which is the link to venous thromboembolism. See Venous thromboembolism in pregnancy.

Mechanical and technical effects. Ultrasound resolution falls with tissue depth. Palpation becomes unreliable. Venous access, regional anaesthesia and airway management all become harder. Wounds sit under a heavier, less well perfused panniculus.

So the complication list is not arbitrary. Metabolic, vascular, thrombotic and technical, each traceable to a mechanism.

What the numbers look like

The RCOG classification, used throughout Green-top Guideline 72, is by booking BMI:

Most recommendations begin at BMI 30, several specific ones at BMI 35, and the resource-intensive ones at BMI 40. Knowing which threshold triggers which action is most of the practical content of this chapter.

The risks, all raised and most of them dose-related to BMI:

• Maternal: miscarriage, gestational diabetes, pre-eclampsia and gestational hypertension, venous thromboembolism, dysfunctional or prolonged labour, caesarean section, anaesthetic complications, postpartum haemorrhage, wound infection and dehiscence, endometritis, pressure sores, difficulty breastfeeding, and mental health problems.
• Fetal and neonatal: congenital anomaly including neural tube defects, macrosomia and shoulder dystocia, stillbirth, preterm birth, neonatal unit admission, and long-term childhood obesity and metabolic disease through intrauterine programming.
• Technical: reduced accuracy of chromosomal and structural anomaly screening, unreliable symphysis-fundal height, difficult palpation, difficult venous access, difficult regional anaesthesia, difficult intubation.

Preconception, where the risk is actually modifiable

This is the highest-yield section and the one most often skipped, because the woman is not pregnant and therefore not in an antenatal clinic.

Primary care should ensure every woman of childbearing age has the opportunity to optimise her weight before pregnancy, with weight and lifestyle advice given at preconception counselling or contraceptive consultations, and weight and BMI actually measured. That last point matters: the contraception visit is the reliable contact point with women of reproductive age, and it is where this conversation belongs.

Tell her what weight loss buys. Weight loss between pregnancies reduces the risk of stillbirth, hypertensive complications and fetal macrosomia, and increases the chance of a successful vaginal birth after caesarean section. Those are concrete, motivating outcomes, and they are more useful than a general statement that obesity is unhealthy.

Folic acid 5 mg daily, the high dose, starting at least one month before conception and continuing through the first trimester, for any woman with a BMI of 30 or greater. The reason is the increased risk of neural tube defects, which is compounded by the fact that ultrasound detection of a neural tube defect is less reliable at high BMI.

Vitamin D. Women with obesity are at high risk of deficiency, and BMI is inversely associated with serum vitamin D. RCOG's position is careful and should be reproduced honestly: supplementation may make women vitamin D replete, but whether routine supplementation improves maternal and offspring outcomes remains uncertain. The standard antenatal intake applies; do not claim more than the evidence supports.

Anti-obesity and weight-loss drugs are not recommended in pregnancy. This now includes the GLP-1 receptor agonists, which a growing number of women are taking. They should be stopped before conception and the woman moved to lifestyle measures, or to an agent with pregnancy safety data if she also has diabetes.

Bariatric surgery. A woman who has had bariatric surgery should be advised to delay conception, conventionally 12 to 24 months, until weight has stabilised, and she needs micronutrient surveillance and supplementation in pregnancy: iron, vitamin B12, folate, calcium, vitamin D and the fat-soluble vitamins, depending on the procedure. She should not have an oral glucose tolerance test after a bypass procedure because of dumping syndrome; use fasting and postprandial glucose profiles instead. And an obstructive complication such as an internal hernia can present as abdominal pain and vomiting in pregnancy, so do not assume a surgical abdomen in a post-bariatric woman is obstetric.