Pregnancy Weight Gain Calculator
Tells you how much weight you should have gained by a given week of pregnancy, based on your pre-pregnancy BMI. Use it to track whether your weight gain is on pace with clinical guidelines.
About this calculator
Recommended gestational weight gain depends on your pre-pregnancy Body Mass Index (BMI = weight (kg) / height (m)²). The Institute of Medicine (IOM) guidelines set different weekly gain targets for each BMI category. This calculator uses those rates for the second and third trimesters (after an assumed ~1–2 kg gain in the first trimester): Underweight (BMI < 18.5): 0.5 kg/week; Normal weight (BMI 18.5–24.9): 0.4 kg/week; Overweight or obese (BMI ≥ 25): 0.25 kg/week. The formula applied here is: Expected Gain = weeks × rate, where the rate is selected by your BMI bracket. Total recommended gain over a full pregnancy ranges from roughly 11–16 kg for normal-weight women.
How to use
Example: Pre-pregnancy weight = 65 kg, height = 165 cm, current week = 20. Step 1: BMI = 65 / (1.65)² = 65 / 2.7225 ≈ 23.9 → Normal weight category. Step 2: Rate = 0.4 kg/week. Step 3: Expected gain = 20 × 0.4 = 8.0 kg by week 20. Enter 65 kg, 165 cm, and 20 weeks into the calculator to confirm you should have gained approximately 8 kg so far.
Frequently asked questions
How much weight should I gain during pregnancy based on my BMI?
According to IOM guidelines, underweight women (BMI < 18.5) should gain 12.5–18 kg total; normal-weight women (BMI 18.5–24.9) should gain 11.5–16 kg; overweight women (BMI 25–29.9) should gain 7–11.5 kg; and obese women (BMI ≥ 30) should gain 5–9 kg. These ranges account for the placenta, amniotic fluid, increased blood volume, and fetal growth. Gaining within your recommended range lowers risks of complications such as gestational diabetes and preterm birth. Always discuss your personal target with your healthcare provider.
Why does pre-pregnancy BMI affect how much weight you should gain during pregnancy?
BMI is a proxy for your body's energy reserves before conception. Women who start underweight need more stored energy to support fetal growth, so a higher gain is recommended. Women who start with excess body fat already have reserves that can contribute to fetal nourishment, so a lower gain target reduces risks like gestational hypertension and macrosomia. The weekly rate differences (0.25–0.5 kg/week) reflect these varying physiological needs across the pregnancy.
Is it safe to gain less weight than recommended during pregnancy?
Gaining below the recommended range — especially in the second and third trimesters — is linked to low birth weight, preterm birth, and poor fetal brain development. The fetus depends on maternal nutrition for organ formation and growth, particularly after week 20 when weight gain accelerates. If you're struggling to gain adequate weight due to nausea or food aversions, a registered dietitian can help adjust your diet safely. Regular monitoring at prenatal visits allows your care team to intervene early if gain is insufficient.