How to use it
- Enter your Height in centimetres, then your Weight before pregnancy in kilograms.
- Choose the Pregnancy that applies. Add a Pregnancy week and the Weight gained so far to compare your gain with the range for that week.
- Read Total gain for the whole pregnancy, then the week range and comparison if you entered them.
How the range is set
In 2009 the Institute of Medicine revised its guidelines for weight gain in pregnancy. The ranges depend on your body mass index (BMI) before pregnancy: your weight in kilograms divided by your height in metres squared. Table 7-3 of the report gives the total gain for four groups: below 18.5 (underweight), 18.5 to 24.9 (normal weight), 25.0 to 29.9 (overweight) and 30.0 and above (obese).
The totals are 12.5 to 18 kg for underweight women, 11.5 to 16 kg for normal weight, 7 to 11.5 kg for overweight and 5 to 9 kg for obese women. The same totals are 28 to 40, 25 to 35, 15 to 25 and 11 to 20 lb. The table also gives the rate of gain in the second and third trimesters.
For twins the report gives separate provisional totals at term: 17 to 25 kg for normal weight, 14 to 23 kg for overweight and 11 to 19 kg for obese women. It gives none for underweight women.
This is not medical advice. The report's ranges are guidance for a healthy pregnancy. Your doctor or midwife sets your own target and checks your weight through the pregnancy.
BMI = weight (kg) / height (m)²
Normal weight, BMI 18.5 to 24.9: total gain 11.5 to 16 kg (25 to 35 lb)
First trimester (footnote): 0.5 to 2 kg
From week 14, normal weight: 0.35 to 0.50 kg a weekHow the week range is calculated
The report does not say which weeks make up each trimester, and it does not print a gain for each week. This page takes weeks 1 to 13 as the first trimester and shares the footnote's 0.5 to 2 kg evenly over them. From week 14 each week adds the weekly rate of your group, so the range at week n is 0.5 + rate × (n − 13) at the low end and 2 + rate × (n − 13) at the high end.
These week figures are this page's calculation. At week 40 they can sit about 1.6 kg away from the table total, the largest difference being at the low end for normal weight. Use the table total as the report's figure for the whole pregnancy.
Weeks 1 to 13: 0.5 x n / 13 to 2 x n / 13 kg
Week n from 14 on: (0.5 + rate x (n − 13)) to (2 + rate x (n − 13)) kg
Example, normal weight, week 20: 0.5 + 0.35 x 7 = 2.95 kg to 2 + 0.50 x 7 = 5.5 kgWhat the range does not cover
- Twins: the figures for twins are provisional totals at term only, and the page shows no week-by-week figures for them.
- Your own care: the ranges are a guide for a healthy pregnancy. A clinician who knows your history sets your target, and BMI does not separate fat from muscle.
Worked example: 60 kg at 165 cm, week 20, 5 kg gained
BMI = 60 / (1.65 x 1.65) = 22.0, which is in the normal weight group (18.5 to 24.9). The total gain range is 11.5 to 16 kg, which is 25 to 35 lb. With 5 kg already gained, 11.5 - 5 = 6.5 kg and 16 - 5 = 11 kg are still to gain.
By week 20 the page's calculation gives 0.5 + 0.35 x 7 = 2.95 kg at the low end and 2 + 0.50 x 7 = 5.5 kg at the high end. With 5 kg gained, you are within the range for week 20.
| Weight group | Total gain (kg) | Total gain (lb) |
|---|---|---|
| Underweight: BMI below 18.5 | 12.5 - 18 | 28 - 40 |
| Normal weight: BMI 18.5 to 24.9 | 11.5 - 16 | 25 - 35 |
| Overweight: BMI 25.0 to 29.9 | 7 - 11.5 | 15 - 25 |
| Obese: BMI 30.0 and above | 5 - 9 | 11 - 20 |
Frequently asked questions
What does BMI before pregnancy mean here?
It is your weight in kilograms divided by your height in metres squared, measured before you became pregnant. The Institute of Medicine uses it to place you in one of four groups, each with its own gain range.
Is the top of the range the most I should gain?
The top of the range is a guide for a healthy pregnancy, not a rule. If your gain goes above or below the range, talk it through with your doctor or midwife.
Why is the total range lower for women who start at a higher BMI?
The table gives smaller totals for women who start at a higher BMI, so women who begin the pregnancy heavier are advised to gain less in total. The report sets these ranges by BMI group.
What does the week range show?
It shows what this page calculates as the expected gain by a given week, from the report's first-trimester assumption and weekly rates. The report does not print a figure for each week, so use the table total for the whole pregnancy.
Does this calculator work for twins?
Only for the total at term. The report gives provisional totals for twins for normal, overweight and obese women, and none for underweight women. It gives no week-by-week figures for twins.
Are my weight and height stored?
No. The calculation runs in your browser and nothing you enter is sent to us.
Sources
Figures and text last checked on Tuesday, November 10, 2026.