Your BMI at the start of pregnancy helps your midwife decide whether you need any extra care. Once you're pregnant, BMI stops being a useful measure, which is why our calculator isn't designed for it. According to the NHS, most women gain between 10 kg and 12.5 kg (22 lb to 28 lb), most of it after week 20.
Why your midwife checks your BMI
At your booking appointment, usually somewhere between weeks 8 and 12, your midwife will measure your height and weight and work out your BMI. You can check yours in advance on our BMI chart. It's not a judgement. It's one of several things, along with your age and medical history, that help them plan the right level of care for you.
That early number is the one that counts. From then on, your weight changes for reasons BMI was never built to handle: the baby, the placenta, amniotic fluid, extra blood and fluid, and breast tissue.
If your BMI is 30 or above
Most women with a higher BMI, including many with PCOS, have healthy pregnancies and healthy babies. It does raise the chance of some complications, though, so you'll be offered a bit more support. The NHS lists these increased risks for you:
- miscarriage
- gestational diabetes
- high blood pressure and pre-eclampsia
- blood clots
- heavier bleeding after the birth, and a higher chance of an assisted birth or caesarean
And for your baby, a higher chance of being born early, of stillbirth, and of neural tube conditions such as spina bifida.
In practice, this usually means you'll be offered a test for gestational diabetes. The most important thing the NHS says here is this: don't try to lose weight while you're pregnant. It won't reduce the chance of complications and may not be safe. Eating well and staying gently active is the aim instead.
How much weight to expect to gain
The NHS gives the typical range of 10 to 12.5 kg but doesn't set official targets based on BMI. In the absence of UK targets, the most widely quoted guidance comes from the US Institute of Medicine, which suggests different amounts depending on your BMI before pregnancy:
| BMI before pregnancy | Suggested gain |
|---|---|
| Under 18.5 | 12.5 – 18 kg (28 – 40 lb) |
| 18.5 – 24.9 | 11.5 – 16 kg (25 – 35 lb) |
| 25 – 29.9 | 7 – 11.5 kg (15 – 25 lb) |
| 30 and above | 5 – 9 kg (11 – 20 lb) |
Treat these as a rough guide rather than something to hit exactly. Your midwife is the best person to tell you whether your weight is on track.
The NHS is clear that pregnancy isn't the time to go on a diet. That doesn't mean "eating for two" either. A steady, balanced diet is what your midwife will be looking for.
Folic acid and vitamin D
The NHS advises everyone to take 400 micrograms of folic acid a day while trying to conceive and for the first 12 weeks of pregnancy, plus 10 micrograms of vitamin D a day, at least from October to March.
You may read online that a BMI of 30 or more means you need a higher 5 mg dose of folic acid. That advice has changed, and the current NHS guidance doesn't list BMI as a reason. Some conditions, including diabetes, do qualify, so ask your GP or midwife which dose is right for you.
If your BMI is under 18.5
A low BMI before pregnancy can also affect how things go, and it's one reason the suggested weight gain in the table above is higher for this group. If you've struggled to keep weight on, tell your midwife early so they can support you. Our guide to being underweight has some practical ideas for eating well.
After the birth
Most of the extra weight comes off naturally over the following months, though it takes longer than many people expect. Give yourself time. Once your body has had a chance to settle, usually a few months after the birth, BMI becomes a meaningful measure again and you can use the calculator as normal.
Planning a pregnancy? Your BMI now is a helpful starting point to discuss with your GP.
Calculate my BMISources
- NHS: Weight gain in pregnancy
- NHS: Overweight and pregnancy
- NHS: Pregnancy vitamins and supplements
- Institute of Medicine. Weight Gain During Pregnancy: Reexamining the Guidelines, 2009.