How pregnancy weight-gain targets are set
The Institute of Medicine (now the National Academy of Medicine) published updated weight-gain recommendations in 2009. Those numbers are still the standard reference for obstetric practice in the US and most national guidelines worldwide. The ranges are calibrated to minimize three things: low birth weight, postpartum weight retention, and the risk of large-for-gestational-age babies.
What the ranges look like
- Underweight (BMI under 18.5): 28-40 lb (12.5-18 kg) total. Roughly 1-1.3 lb per week in the 2nd and 3rd trimesters.
- Healthy weight (BMI 18.5-24.9): 25-35 lb (11.5-16 kg) total. About 0.8-1 lb per week.
- Overweight (BMI 25-29.9): 15-25 lb (7-11.5 kg) total. About 0.5-0.7 lb per week.
- Obesity (BMI 30+): 11-20 lb (5-9 kg) total. About 0.4-0.6 lb per week.
The first trimester (weeks 1-13) is grouped: most people gain 1-4.5 lb total, and some lose a little from morning sickness. Neither is concerning on its own.
What if I'm outside the range?
Plenty of healthy pregnancies fall outside the IOM range. Trends matter more than any one week. Sustained over-gain raises risks for gestational diabetes, hypertension, larger babies, and harder postpartum recovery. Sustained under-gain raises risks for low birth weight and preterm delivery. Your prenatal provider checks this at every visit - the calculator is a sanity check between appointments, not a verdict.
Pair this with
BMI calculator if you need help confirming your pre-pregnancy category. Pregnancy week calculator if you're not sure how far along you are. Due date calculator for estimated delivery and milestones.
Frequently asked questions
How much weight should I gain in pregnancy?
It depends on your pre-pregnancy BMI. The Institute of Medicine (IOM) 2009 guidelines - still the standard referenced by ACOG - recommend 28-40 lb (12.5-18 kg) if you started underweight, 25-35 lb (11.5-16 kg) at a healthy weight, 15-25 lb (7-11.5 kg) overweight, and 11-20 lb (5-9 kg) with obesity. The calculator shows your specific range.
How fast should I gain weight each week?
First trimester (weeks 1-13): total gain of about 1-4.5 lb (0.5-2 kg) - many people gain little or even lose a small amount from morning sickness, and that's fine. Second and third trimesters: roughly 1 lb (0.5 kg) per week at a healthy BMI, slower at higher BMI categories, faster if underweight.
What is BMI and which one matters here?
Body Mass Index is your pre-pregnancy weight divided by your height squared. Pregnancy weight-gain targets are based on your weight BEFORE pregnancy, not your current weight. If you don't know your pre-pregnancy weight precisely, use your weight at your first prenatal visit as the closest proxy.
I'm gaining faster or slower than the recommended range. Should I worry?
Many pregnancies fall outside the IOM range without complications. Trends matter more than any single week. Persistent gain well above the range raises the risk of gestational diabetes, hypertension, and a larger baby. Persistent gain below the range raises the risk of low birth weight. Your prenatal provider tracks this at every visit and will flag if intervention is needed.
Does the calculator work for twins?
Not yet - the IOM 2009 has separate (provisional) ranges for twin pregnancies (37-54 lb at healthy BMI). This calculator uses singleton guidelines. If you're carrying twins, your provider will give you a personalized target.
Is this free?
Yes, completely free. No signup, no account. Numbers stay in your browser - we don't store anything you enter.
These calculators give estimates based on cycle averages and standard formulas. They are for general information only and are not medical advice. For anything concerning your health or pregnancy, talk to a qualified healthcare provider.