Obesity in pregnancy: risks, care and what helps

Body mass index is calculated from height and weight, and in maternity care it is usually the booking measurement, taken early in pregnancy, that is used. The NHS describes a BMI of 30 or above as obesity in this setting. Because BMI does not distinguish muscle from fat or take account of ethnic differences in risk, it is a starting point for a conversation rather than a verdict.

Some maternity units have a separate pathway for higher BMIs, and the thresholds that trigger extra care can differ locally. Your midwife can tell you which steps apply to you.

According to the NHS, obesity raises the chance of miscarriage, gestational diabetes, high blood pressure and pre-eclampsia, blood clots, shoulder dystocia in labour, heavy bleeding after the birth, and instrumental or emergency caesarean delivery. The NHS pre-eclampsia page lists a BMI of 35 or more before conception as an additional risk factor for pre-eclampsia.

The RCOG lists a BMI of 30 or higher as a risk factor for gestational diabetes, which is why testing is offered.

The NHS notes a higher chance of premature birth, stillbirth and neural tube defects such as spina bifida. Larger babies are more common, and ultrasound assessment of growth can be harder in a larger abdomen, which can affect how closely the baby's size is followed.

These are population-level risks and do not predict what will happen in any single pregnancy. Practical measures can reduce several of them, which is why preparation and early care matter.

It can seem logical to try to lose weight to reduce risk, but the NHS states that weight loss in pregnancy will not reduce the chance of complications and may not be safe. The NHS weight gain page also discourages dieting in pregnancy because of possible harm to development.

The usual weight gain figure given by the NHS is 10kg to 12.5kg, mostly after week 20. If your BMI is above 30, the NHS suggests discussing personalised advice with your midwife or GP rather than aiming for a general target. Weight loss is better tackled before conception or after the baby is born.

The advice is a healthy, balanced diet and some physical activity every day, with a referral to a dietitian or other professional for personalised guidance if it would help. Walking, swimming and low-impact classes are common starting points, and new activity should be agreed with the midwife or GP.

• Take folic acid as advised. The NHS says 400 micrograms is standard, and a higher 5mg dose, available on prescription, is recommended in some situations such as diabetes.

• Take vitamin D, 10 micrograms daily, from early October to late March, and year-round if you have little sun exposure.

• Avoid vitamin A supplements and cod liver oil.

• Ask whether low-dose aspirin is advised for you if you have other pre-eclampsia risk factors.

You should be offered a glucose test for gestational diabetes and an appointment with an anaesthetist, who will talk through epidural and anaesthetic options in advance. This matters because venous access, spinal or epidural placement and emergency anaesthesia can be harder to arrange in a hurry.

The team may recommend giving birth in a consultant-led unit rather than a midwifery-led one, depending on your BMI and other health factors. Blood-clot prevention, such as moving early after birth and sometimes a short course of heparin injections, is often considered after delivery.

Many women find it helpful to separate the practical questions of this pregnancy from any feelings about weight. Mention it if you feel you are being judged or are unsure what is being offered. After the birth, breastfeeding, gentle activity and a later, planned weight-management conversation can follow, and a weight reduction before a future pregnancy is likely to help.