Calculate your recommended pregnancy weight gain by pre-pregnancy BMI and week, see if you are on track, and get your target weight at delivery.
Pregnancy Weight Gain Formula
The calculator starts from your pre-pregnancy body mass index (BMI), then applies the Institute of Medicine (IOM) weight gain ranges that match that BMI group. Your BMI is:
BMI = weight(kg) / height(m)^2
Your BMI decides your category and the recommended total gain for the whole pregnancy. To estimate how much you should have gained by a given week, the pregnancy is split into the first trimester and the weeks after it:
Gain by week W = FT + (Total - FT) * (W - 13) / 27
The first-trimester gain (FT) is about 1.1 to 4.4 lb (0.5 to 2 kg). After week 13 the remaining gain is spread evenly across the 27 weeks up to week 40. Track mode also works out the pace you need for the weeks you have left so you finish inside your range:
Weekly pace = (Total - gain so far) / (40 - W)
Variables:
- BMI is your pre-pregnancy body mass index in kg/m^2
- Total is the recommended total weight gain for your BMI group, taken from the table below
- FT is the first-trimester gain, about 1.1 to 4.4 lb (0.5 to 2 kg)
- W is your current week of pregnancy, from 1 to 40
- gain so far is your current weight minus your pre-pregnancy weight
The tool runs in two modes. Plan mode uses only your height, pre-pregnancy weight, and whether you carry one baby or twins, and returns your BMI category, your recommended total gain, a target weight at delivery, and the weekly rate to aim for in the second and third trimesters. Track mode adds your current weight and week, compares your actual gain to the amount recommended by that week, labels it below, within, or above range, and shows the weekly pace that lands you at your recommended total by week 40. Choosing Twins swaps in the higher IOM twin ranges.
Recommended Weight Gain by Pre-Pregnancy BMI
These are the IOM ranges the calculator uses. The weekly rate applies to the second and third trimesters, after the smaller first-trimester gain.
| Category | Pre-pregnancy BMI | Total gain (one baby) | Weekly rate (2nd/3rd tri) | Total gain (twins) |
|---|---|---|---|---|
| Underweight | Below 18.5 | 28 to 40 lb | 1.0 to 1.3 lb | Not set* |
| Normal weight | 18.5 to 24.9 | 25 to 35 lb | 0.8 to 1.1 lb | 37 to 54 lb |
| Overweight | 25.0 to 29.9 | 15 to 25 lb | 0.5 to 0.7 lb | 31 to 50 lb |
| Obese | 30.0 and above | 11 to 20 lb | 0.4 to 0.6 lb | 25 to 42 lb |
*The IOM did not set a twin range for underweight women, so the calculator shows the normal-weight twin range as a guide in that case.
This next table is the part most calculators leave out: how much of that total you would have reached by each week if you stay on the recommended curve. Values are cumulative pounds for a singleton pregnancy, rounded to the nearest pound.
| Week | Underweight | Normal | Overweight | Obese |
|---|---|---|---|---|
| 16 | 4 to 8 | 4 to 8 | 3 to 7 | 2 to 6 |
| 20 | 8 to 14 | 7 to 12 | 5 to 10 | 4 to 8 |
| 24 | 12 to 19 | 11 to 17 | 7 to 13 | 5 to 11 |
| 28 | 16 to 24 | 14 to 21 | 9 to 16 | 7 to 13 |
| 32 | 20 to 29 | 18 to 26 | 11 to 19 | 8 to 15 |
| 36 | 24 to 35 | 21 to 30 | 13 to 22 | 10 to 18 |
| 40 | 28 to 40 | 25 to 35 | 15 to 25 | 11 to 20 |
Example Problems
Example 1: plan a target from your starting weight.
You are 5 ft 5 in (1.65 m) and weighed 140 lb before pregnancy, carrying one baby. In kilograms that is 63.5 kg, so your BMI is 63.5 / 1.65^2 = 23.3, which is normal weight. Your recommended total gain is 25 to 35 lb. That puts your target weight at delivery at 165 to 175 lb, with a second and third trimester pace of about 0.9 to 1.1 lb per week.
Example 2: check whether you are on track.
The same person is now at week 24 and weighs 152 lb. Gain so far is 152 – 140 = 12 lb. The amount recommended by week 24 for a normal weight is about 11 to 17 lb, so 12 lb is within range. With 40 – 24 = 16 weeks left, the pace to finish inside 25 to 35 lb is (25 – 12) / 16 to (35 – 12) / 16, or about 0.8 to 1.4 lb per week.
Frequently Asked Questions
How much weight should I gain during pregnancy?
It depends on your pre-pregnancy BMI. For one baby, the IOM ranges are 28 to 40 lb if you were underweight, 25 to 35 lb at a normal weight, 15 to 25 lb if you were overweight, and 11 to 20 lb if you had obesity. Twin pregnancies run higher, roughly 37 to 54 lb at a normal weight. Most of the gain happens in the second and third trimesters, at close to a pound a week for a normal weight.
Where does the pregnancy weight actually go?
Only part of it is the baby. A rough breakdown near term is about 7 to 8 lb for the baby, 1.5 lb for the placenta, 2 lb for amniotic fluid, 2 lb for the enlarged uterus, 1 to 3 lb for breast tissue, 3 to 4 lb from the extra blood your body makes, 2 to 3 lb from other fluid, and 6 to 8 lb of fat stored to support breastfeeding. That is why a healthy total sits well above the baby’s own weight.
What if I gain more or less than the range?
The ranges are guidelines, not pass or fail lines, and one slow or fast week does not decide the outcome. Over the whole pregnancy, gaining well under the range is linked to smaller babies and preterm birth, while gaining well over it is linked to a larger baby, a harder delivery, and weight that is slower to come off afterward. If your gain stays outside your range, bring the numbers to your provider, who can adjust the target for your health and how the pregnancy is progressing.