How does the pregnancy weight gain work?
- We work out your pre-pregnancy BMI: weight in kg divided by height in metres squared.
- The Institute of Medicine (2009) gives a total range for each BMI group. For a BMI of 18.5 to 24.9 it's 11.5 to 16 kg (25 to 35 lb).
- In the first trimester, a gain of 0.5 to 2 kg is typical. After that, IOM gives a weekly rate for each group.
- For twins, IOM gives provisional ranges for most groups. There's no twin range for a BMI under 18.5.
A note for the UK
NICE doesn't set weight-gain targets, and you may not be weighed regularly in NHS care. The focus is eating well, staying active in ways that feel good, and not dieting in pregnancy. If eating or body image feels hard, tell your midwife: it's common and they can help. Our guide to body image is written kindly.
When to see a doctor
- You're worried about your weight, eating or body image in pregnancy: tell your midwife.
- Sudden swelling of the face, hands or feet, with headache or vision changes: call your maternity unit now.
- You're being sick so much you're losing weight.
In the UK you can call NHS 111 for advice, any time. In an emergency call 999 (UK) or 911 (US).
Questions people ask
How much weight should I gain in pregnancy?
US guidance (IOM 2009, used by ACOG) suggests 12.5 to 18 kg for a BMI under 18.5, 11.5 to 16 kg for 18.5 to 24.9, 7 to 11.5 kg for 25 to 29.9 and 5 to 9 kg for 30 or more. UK guidance doesn't set targets.
Where does the weight go?
Baby, placenta, amniotic fluid, a larger womb and breasts, extra blood and fluid, and some stored energy for feeding.
I've gained more or less than the range. Is that bad?
Ranges describe populations, not you. Your midwife or doctor can look at your whole picture. Never restrict eating in pregnancy without advice.
Why doesn't the NHS use these ranges?
NICE found there wasn't enough UK evidence to set targets, so care focuses on healthy eating and activity rather than numbers.
Is anything I enter saved or sent?
No. Everything runs in your browser. Nothing is stored or sent, and the page makes no network requests when you use it.
Sources
Written by the Splash team · Sources checked Oct 2026 · Updated
