How to use the pregnancy weight gain calculator
- Choose Metric (cm, kg) or US units (ft, in, lb).
- Enter your height and your weight before pregnancy. The calculator works out your pre-pregnancy BMI from these.
- Enter the current week of pregnancy, in completed weeks.
- Choose one baby or twins.
- Optionally add your current weight to see how your gain so far compares with the range for your week.
How the recommended gain is worked out
The ranges come from the Institute of Medicine (now the National Academy of Medicine) report Weight Gain During Pregnancy: Reexamining the Guidelines (2009). They are still the guidelines used by ACOG and the CDC. Your category depends on your BMI before pregnancy:
The IOM report assumed a gain of 0.5 to 2 kg in the first trimester and a steady weekly rate after that. The calculator spreads the rest of the recommended total evenly over weeks 14 to 40, so the range for your week always ends at the full-term recommendation.
| Pre-pregnancy BMI | One baby | Twins (provisional) | Weekly, 2nd and 3rd trimesters (one baby) |
|---|---|---|---|
| Under 18.5 | 12.5 – 18 kg (28 – 40 lb) | no IOM guideline | 0.44 – 0.58 kg (1 – 1.3 lb) |
| 18.5 to 24.9 | 11.5 – 16 kg (25 – 35 lb) | 17 – 25 kg (37 – 54 lb) | 0.35 – 0.50 kg (0.8 – 1 lb) |
| 25 to 29.9 | 7 – 11.5 kg (15 – 25 lb) | 14 – 23 kg (31 – 50 lb) | 0.23 – 0.33 kg (0.5 – 0.7 lb) |
| 30 or more | 5 – 9 kg (11 – 20 lb) | 11 – 19 kg (25 – 42 lb) | 0.17 – 0.27 kg (0.4 – 0.6 lb) |
Worked example
The calculator's defaults are a height of 165 cm, a pre-pregnancy weight of 60 kg, week 20 and one baby.
- BMI: 60 ÷ 1.65² = 60 ÷ 2.7225 = 22.0, in the healthy weight range
- Recommended total gain: 11.5 – 16 kg
- Expected gain by week 20, low end: 0.5 + (11.5 − 0.5) × 7 ÷ 27 = 3.4 kg
- Expected gain by week 20, high end: 2 + (16 − 2) × 7 ÷ 27 = 5.6 kg
- Expected weight at week 20: 63.4 to 65.6 kg
In US units, 5 ft 5 in and 132 lb also gives a BMI of 22.0 and a total of 25 to 35 lb, with 7.3 to 12.3 lb expected by week 20.
Limits of the guidelines
The IOM ranges describe gains linked with the best outcomes for mother and baby across large groups of people. They are not a target you must hit, and they do not apply to teenagers in the same way. The twin ranges are marked provisional because there was less evidence behind them, and the IOM gave no twin guideline for an underweight BMI. The weekly split is an approximation too: real weight gain is rarely even. Many people gain little in the first trimester, especially with nausea, then more later.
A gain well outside the range for your week is worth mentioning at your next appointment, but it is not a diagnosis. Your doctor or midwife will look at your health, your baby's growth and your history before suggesting any change. Don't start a weight-loss diet while pregnant unless they recommend it.
Frequently asked questions
Why does the recommended gain depend on BMI?
People who start pregnancy with less body fat need more reserves for the baby and for breastfeeding, while people with a higher BMI already have reserves. Matching the gain to the starting BMI is linked with fewer complications such as a very small or very large baby.
Is it normal to gain little in the first trimester?
Yes. The IOM assumed only 0.5 to 2 kg (1.1 to 4.4 lb) in the first 13 weeks, and some people lose a little because of nausea. Gain usually picks up in the second trimester. If you cannot keep food down, talk to your doctor or midwife.
Where does the weight go?
Only part of it is the baby. The rest includes the placenta, amniotic fluid, a larger uterus and breasts, extra blood and fluid, and fat stores for breastfeeding. That is why the guideline for healthy weight is far more than the 3 to 4 kg a newborn weighs.
What if I am already above the range?
Don't panic or cut calories sharply. Bring it up with your doctor or midwife, who can check for fluid retention or other causes and help you plan healthy eating and activity for the rest of the pregnancy.
Related calculators
- Pregnancy Calculator — How many weeks pregnant you are, your due date, trimester and key milestones, from your last period, conception, ultrasound or IVF.
- BMI Calculator — Your body mass index, weight category and healthy weight range, in metric or US units.
- Due Date Calculator — Your estimated due date by Naegele's rule, adjusted for cycle length, or from a conception or IVF transfer date.
Results are estimates for general information and are not medical advice. Talk to a doctor or other qualified professional about your health. See our disclaimer.