Pregnancy Weight Gain Calculator - IOM Guidelines by Week

The guideline: IOM 2009, based on pre-pregnancy BMI

US recommendations for weight gain in pregnancy come from the Institute of Medicine (now the National Academy of Medicine) 2009 report, which ACOG and the CDC both use. The target depends on your BMI before pregnancy, calculated as weight in kg ÷ height in meters squared (or lb ÷ in² × 703).

For one baby: underweight (BMI below 18.5) 28-40 lb; normal weight (18.5-24.9) 25-35 lb; overweight (25-29.9) 15-25 lb; obese (30 or higher) 11-20 lb. The IOM applies the same 11-20 lb range to all classes of obesity. For the second and third trimesters, the typical rates are about 1-1.3 lb per week (underweight), 0.8-1 lb (normal), 0.5-0.7 lb (overweight), and 0.4-0.6 lb (obese).

For twins, the IOM gives totals only: 37-54 lb (normal), 31-50 lb (overweight), and 25-42 lb (obese). It found too little data to set a twin range for underweight women; the CDC lists 50-62 lb for that group. The IOM set no ranges for triplets or more.

How the week-by-week range is built

Most weight in the first trimester is small: the IOM assumes about 1.1-4.4 lb (0.5-2 kg) total by the end of week 13. After that, gain is roughly linear. The calculator scales the first-trimester range across weeks 0-13, then draws a straight line from there to your category's total at week 40. That keeps the range consistent with both the first-trimester assumption and the IOM total, and it closely tracks the published weekly rates.

Twin pregnancies often deliver around 36-37 weeks and the IOM published no week-by-week twin curve, so the calculator shows the total only for twins.

Worked example

Height 5'5" (65 in), pre-pregnancy weight 140 lb. BMI = 140 ÷ 65² × 703 = 23.3, normal weight, so the target is 25-35 lb. At 24 weeks: low end = 1.1 + (25 − 1.1) × 11 ÷ 27 = 10.8 lb; high end = 4.4 + (35 − 4.4) × 11 ÷ 27 = 16.9 lb. If she now weighs 154 lb, she has gained 14 lb, which is within range.

Height 5'4" (64 in), pre-pregnancy 165 lb. BMI = 28.3, overweight, target 15-25 lb. At 20 weeks the range is about 4.7-9.7 lb.

Where the weight goes

A typical 30 lb gain is not mostly fat. Roughly: baby 7-8 lb, placenta 1-2 lb, amniotic fluid about 2 lb, larger uterus about 2 lb, breast tissue 1-3 lb, extra blood about 4 lb, other fluid about 4 lb, and stored fat for breastfeeding 6-8 lb. That is why the target is higher for people who start lighter and lower for people who start with more reserves.

Common mistakes

Using current weight to calculate BMI. The category comes from your weight before pregnancy. If you are unsure, use the weight recorded at your first prenatal visit.

Dieting to stay "on track". Pregnancy is not the time for weight-loss diets. If you are above the range, talk with your provider about food quality and activity rather than cutting calories hard.

Panicking over one weigh-in. Weight moves 2-3 lb day to day with fluid, meals, and bathroom timing. Weigh at the same time, same scale, same clothing, and look at the trend over a few weeks.

Eating for two. Extra calories are modest: none in the first trimester for most people, then about 340 a day in the second trimester and 450 in the third, per the CDC.

Ignoring first-trimester loss. Nausea and vomiting can cause early weight loss. Mild loss is common; ongoing loss or inability to keep fluids down needs medical attention.

When to call your provider

Call promptly for sudden weight gain (more than about 4-5 lb in a week), swelling of the face or hands, a severe headache, vision changes, or pain under the ribs. These can be signs of preeclampsia, which needs urgent evaluation. Also call if you are losing weight after the first trimester, cannot keep food or fluids down, or feel your eating is out of control.

Your OB or midwife may set a different target based on your health, history, or ultrasound findings. Their plan beats any population guideline. For dates and trimester milestones, the due date calculator pairs with this one.