Your BMI at booking is the number your care is built on

NICE guideline NG201 on antenatal care says that at the first face-to-face antenatal appointment you should be offered height and weight measurement and a BMI calculation. That single figure, recorded once, is what triggers most of what follows, and it is not recalculated later in pregnancy. If you think it was recorded wrongly, say so early.

The care that follows is additive. You get more screening, more scans and more planning. What you do not get, and should not accept, is a smaller set of choices about how you give birth.

The risk figures, with the body that published them and the comparison group

These come from RCOG's patient information on being overweight in pregnancy and after birth. Every one compares women with a BMI of 30 or above against a stated comparison group, which is exactly the information usually stripped out when these numbers circulate.

Gestational diabetes: "If your BMI is 30 or above, you are three times more likely to develop gestational diabetes compared with women with a BMI under 25."
Pre-eclampsia: "If you have a BMI of 30 or above, your risk of pre-eclampsia is 2-4 times higher compared with those with a BMI under 25."
Miscarriage: "The overall likelihood of a miscarriage in early pregnancy is 1 in 5 (20%), but if you have a BMI of 30 or above, your risk increases to 1 in 4 (25%)."
Neural tube defects: around 1 in 1000 babies in the UK, rising to "nearly doubled (2 in 1000)" with a BMI of 30 or above.
A baby over 4 kg: risk "doubled from 7 in 100 to 14 in 100 compared with women with a BMI of between 20 and 30".
Stillbirth: "The overall likelihood of stillbirth in the UK is 1 in every 200 births. If you have a BMI of 30 or above, this risk increases to 1 in every 100 births."

Notice what the honest denominators do. A doubled stillbirth risk sounds catastrophic; 1 in 100 rather than 1 in 200 means 99 in 100 of these pregnancies do not end in stillbirth. Both statements are true and only one of them is usually printed.

What gets added to your care

RCOG's leaflet describes the practical consequences. You are offered a test for gestational diabetes between 24 and 28 weeks. Blood pressure and urine are checked at every appointment. Where there are additional pre-eclampsia risk factors, low-dose aspirin may be recommended to reduce that risk; this page names it without a dose because it is a prescribed medicine.

Folic acid is prescribed at a higher strength than the over-the-counter pregnancy dose for a BMI of 30 or above, ideally starting before conception and continuing into the early second trimester. It is a prescription-only strength, which is precisely why people miss it: you cannot buy it off a shelf. Again, no dose here; ask your GP or midwife for the prescription.

Extra ultrasound is common. RCOG notes that above a BMI of 35 it may be difficult to measure the uterus accurately with a tape measure, so growth may be monitored by scan instead, and it is honest that the 20-week anomaly scan "is less accurate at picking up problems if your BMI is raised". That is a limitation of the physics, not of the sonographer.

The NICE thresholds for labour and birth

NICE guideline NG121 sets out several distinct thresholds, and they are different numbers doing different jobs.

BMI over 30: consider ultrasound at the start of established labour if the baby's presentation is uncertain, "particularly those with a BMI over 35 kg/m2" (recommendation 1.9.1).
BMI over 30: carry out a risk assessment in the third trimester covering preference, mobility, comorbidities and current weight when planning the birth (1.9.3), and consider advising the lateral position in the second stage where mobility is reduced (1.9.4).
All obstetric units: should have birthing beds able to take a safe working load of 250 kg (1.9.6), and should risk-assess that size-appropriate equipment is available, including blood pressure cuffs, theatre tables, transfer equipment and anti-embolism stockings (1.9.7).
BMI over 50: offer referral to an obstetric unit with suitable equipment and expertise "as early as possible in pregnancy" if that is not available in your current unit (1.9.8).

NG121 recommendation 1.9.2 is worth reading carefully because it is the opposite of what many people expect: for a BMI over 30 with no medical complications, intrapartum fetal monitoring should be based on preference and obstetric indications, not automatically escalated. A raised BMI alone is not a reason for continuous monitoring.

Clots, anaesthetics and the practical planning

Clot risk rises with BMI and rises again after birth. RCOG's leaflet says that with a BMI of 40 or above you may be offered blood-thinning injections for at least 10 days after birth, and that this may need to continue for 6 weeks. Those are durations, not doses, and they are the part most often left off a discharge summary, so ask for it in writing.

Ask for an antenatal anaesthetic appointment. RCOG notes that epidurals can be more difficult with a raised BMI and that cannula insertion in early labour is sometimes offered for the same reason. Both are easier to arrange as a plan than to solve at short notice.

Weight in pregnancy

RCOG is direct that "trying to lose weight by dieting during pregnancy is not recommended", while also saying that healthy dietary changes may mean you gain nothing or lose a small amount, and "this is not harmful". The UK has no national gestational weight gain targets, unlike the US, so if you have been given a target range check whose it is. The NHS pages on overweight and obesity in adults cover the general principles that apply outside pregnancy.

ACOG's Practice Bulletin No. 230 makes the point that obesity in pregnancy is often "unrecognized, overlooked, or ignored because of the lack of specific evidence-based treatment options", and that management "should begin before pregnancy and continue through the postpartum period". That framing, that this is a care-planning problem rather than a patient failing, is the useful one to hold.

Raised BMI in Pregnancy