Pregnancy Weight Gain Calculator — IOM guidelines

How it works

The Institute of Medicine (now the National Academy of Medicine) set the widely used guidelines for gestational weight gain in 2009. They depend on your pre-pregnancy body mass index (BMI) — weight ÷ height² — which sorts into four categories, each with a recommended total gain for a single baby:

Most of the gain comes in the second and third trimesters — the guidelines assume only about 1–4.5 lb (0.5–2 kg) in the first trimester, then a steady weekly rate: about 1 lb/week for normal weight, a little more if underweight, and less if overweight or obese. Much of the weight isn't fat: it is the baby, placenta, amniotic fluid, extra blood and tissue.

Gaining within your range is linked to better outcomes for both mother and baby, but these are population averages, not personal prescriptions. Carrying twins, your health history and your provider's advice all change the target — and you should never try to lose weight or diet during pregnancy without medical guidance.

Worked example

A woman 1.65 m tall weighing 60 kg before pregnancy:

• BMI = 60 ÷ 1.65² = 60 ÷ 2.7225 ≈ 22.0 → normal weight
• Recommended total gain: 11.5–16 kg (25–35 lb)
• Second/third-trimester rate: about 0.42 kg (≈1 lb) per week

Frequently asked questions

How much weight should I gain during pregnancy?

It depends on your pre-pregnancy BMI. The IOM recommends 25–35 lb for normal weight, 28–40 lb if underweight, 15–25 lb if overweight and 11–20 lb if obese, for a single baby.

How is the recommendation determined?

By your pre-pregnancy BMI category (weight ÷ height²). Each of the four categories has its own total gain range and its own recommended weekly rate for the second and third trimesters.

What if I'm carrying twins?

The IOM gives higher ranges for twins — for example 37–54 lb for normal-weight women — and separate figures for overweight and obese. This calculator covers singleton pregnancies; ask your provider for a multiples target.

Is it bad to gain outside the recommended range?

Gaining well above or below the range is associated with higher risks for mother and baby, but the ranges are guidelines, not strict cutoffs. Discuss your own pattern with your prenatal care provider.

Why is so much of the weight not fat?

Much of pregnancy weight is the baby, placenta, amniotic fluid, enlarged uterus and breast tissue, and increased blood and fluid volume — not stored fat. That is why the healthy gain is substantial.

Can I diet to limit weight gain?

No. You should not try to lose weight or follow a restrictive diet during pregnancy. Focus on balanced nutrition and activity, and let your provider guide your weight gain.