How much weight should I gain during pregnancy?
Recommended pregnancy weight gain depends mainly on your body mass index (BMI) before pregnancy. The most widely used guidance comes from the US Institute of Medicine (now the National Academy of Medicine), published in 2009:
| Pre-pregnancy BMI | Total gain, one baby | Per week, weeks 14–40 | Total gain, twins |
|---|---|---|---|
| Underweight (below 18.5) | 28–40 lb (12.5–18 kg) | 1–1.3 lb (0.44–0.58 kg) | No guideline |
| Normal weight (18.5–24.9) | 25–35 lb (11.5–16 kg) | 0.8–1 lb (0.35–0.50 kg) | 37–54 lb (17–25 kg) |
| Overweight (25.0–29.9) | 15–25 lb (7–11.5 kg) | 0.5–0.7 lb (0.23–0.33 kg) | 31–50 lb (14–23 kg) |
| Obesity (30 or higher) | 11–20 lb (5–9 kg) | 0.4–0.6 lb (0.17–0.27 kg) | 25–42 lb (11–19 kg) |
The calculator works out your BMI from your height and pre-pregnancy weight, finds your category and shows the matching range in the units you choose.
How the calculation works
- BMI = weight in kilograms ÷ (height in metres)². For example, 60 kg at 1.65 m gives 60 ÷ 2.7225 = 22.0, a normal BMI.
- Total range comes from the table above for your BMI category (one baby or twins).
- Range at your week: the guidelines assume about 0.5–2 kg (1.1–4.4 lb) in total during the first trimester, then the weekly rate for your category from week 14. At 20 weeks with a normal BMI, that is roughly 3–5.5 kg (6.7–11.4 lb) gained so far.
- Your gain so far (optional): enter your current weight and week to see whether it is below, within or above that typical range.
Weight gain by trimester
Weight gain is not spread evenly. Many people gain little in the first trimester — and some lose a little because of nausea — while most of the gain happens in the second and third trimesters, when the baby, placenta, amniotic fluid, blood volume and breast tissue all grow. That is why the guideline gives a small first-trimester total and a steady weekly rate afterwards.
Twins
For twin pregnancies, the guidelines give higher provisional totals for normal, overweight and obese BMI categories (see the table). There is not enough evidence to set a twin range for an underweight BMI, so the calculator asks you to get personal advice. Weekly rates for twins are not published, so the calculator shows the total only.
Using these ranges sensibly
These figures are population guidelines, not personal targets. Health conditions, previous pregnancies, eating difficulties and many other factors can change what is right for you, and some countries use different guidance. If you are concerned about gaining too much or too little, talk to your midwife or doctor rather than changing your diet on your own — especially in pregnancy, when dieting to lose weight is generally not recommended without medical advice. To check how far along you are, use the Pregnancy Calculator.