How much weight to gain during pregnancy depends mainly on where you started. The 2009 report Weight Gain During Pregnancy from the Institute of Medicine (IOM, now the National Academy of Medicine) set ranges by pre-pregnancy body mass index, and they remain the US standard. This calculator finds your BMI category, shows the total range for one baby or twins, and estimates where you should be at your current week.
How to use the pregnancy weight gain calculator
- Choose US or Metric units and enter your height.
- Enter your weight before pregnancy — this sets your BMI category.
- Optionally add your current weight to compare your gain so far with the range for this week.
- Enter how many weeks pregnant you are and choose one baby or twins.
- Read your total range on the tape, then use the chart and week-by-week table to follow the expected path.
How the recommendation is calculated
First, pre-pregnancy BMI picks the guideline row:
The IOM gives a first-trimester allowance (1.1–4.4 lb, or 0.5–2 kg) and a total range at term. The calculator connects them with straight lines: gain rises evenly to the first-trimester range at week 13, then evenly to the total range at week 40.
The lower and upper limits are calculated separately, giving a band rather than a single line.
Worked example
A woman who is 5 ft 5 in and weighed 140 lb before pregnancy is now 20 weeks along and weighs 150 lb.
Pre-pregnancy BMI = 23.3 → healthy weight, so the total range for one baby is 25–35 lb.
At week 20: lower limit = 1.1 + (25 − 1.1) × 7 ÷ 27 ≈ 7.3 lb; upper limit = 4.4 + (35 − 4.4) × 7 ÷ 27 ≈ 12.3 lb.
She has gained 10 lb, which is within the 7.3–12.3 lb range. Her weight at 40 weeks would land between 165 and 175 lb.
IOM 2009 weight gain guidelines
| Pre-pregnancy BMI | One baby, total | Weekly rate, 2nd–3rd trimester | Twins, total |
|---|---|---|---|
| Underweight (below 18.5) | 28–40 lb (12.5–18 kg) | 1–1.3 lb (0.44–0.58 kg) | not set |
| Healthy (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 (25–29.9) | 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 and above) | 11–20 lb (5–9 kg) | 0.4–0.6 lb (0.17–0.27 kg) | 25–42 lb (11–19 kg) |
The weekly rates are the IOM’s averages for the second and third trimesters. Because the straight-line path is fitted to the total range, its slope can differ slightly from these published rates.
Where the weight goes
Only part of pregnancy weight is the baby. At term, a typical gain within the healthy range breaks down roughly like this: the baby about 7–8 lb, the placenta 1–2 lb, amniotic fluid about 2 lb, growth of the uterus about 2 lb, breast tissue 1–3 lb, extra blood volume 3–4 lb, other body fluids 2–3 lb, and stored fat and nutrients for breastfeeding about 6–8 lb. Much of this is lost at delivery or in the weeks after.
Why the ranges differ by BMI
The IOM committee chose ranges associated with better outcomes for both parent and baby. Gaining less than recommended has been linked with smaller babies and preterm birth, while gaining more is linked with larger babies, cesarean delivery and weight that stays on after pregnancy. People who start with more stored energy need less additional gain, which is why the ranges narrow as BMI rises. If your pre-pregnancy BMI is close to a category boundary, the BMI calculator for women shows exactly where you fall.
This calculator is informational only and is not medical advice. Weight gain targets can differ for individual pregnancies; follow the guidance of your obstetrician, midwife or other prenatal care provider.
Frequently asked questions
How much weight should I gain during pregnancy?
It depends on your BMI before pregnancy. For one baby, the 2009 Institute of Medicine guidelines suggest 28–40 lb if underweight, 25–35 lb at a healthy weight, 15–25 lb if overweight and 11–20 lb with obesity. Your healthcare professional may set a different target for you.
How much weight should I gain in the first trimester?
The IOM assumes a modest gain of about 1.1 to 4.4 lb (0.5 to 2 kg) over the whole first trimester. Many people gain little or even lose a bit early on because of nausea, then gain more steadily from the second trimester.
What if I'm gaining more or less than the range?
A single reading outside the range is common and not necessarily a problem; fluid shifts alone can move the scale a few pounds. The trend over several weeks matters more. Discuss sustained differences with your prenatal care provider rather than making major diet changes on your own.
How much weight should I gain with twins?
The IOM's provisional twin ranges are 37–54 lb at a healthy pre-pregnancy BMI, 31–50 lb if overweight and 25–42 lb with obesity. No range was set for an underweight BMI because the evidence was too limited.
Are the 2009 IOM guidelines still used?
Yes. The Institute of Medicine is now the National Academy of Medicine, and its 2009 report remains the basis for US recommendations, including those of ACOG and the CDC.