Pregnancy Weight Gain Calculator
Enter your height and pre-pregnancy weight to get the recommended total gain range from the 2009 Institute of Medicine guidelines, the weekly rate for the second and third trimesters, and where you stand right now if you add your current weight and week.
How the math works
The calculator does not invent a target from your weight alone. It first works out your pre-pregnancy BMI, sorts you into one of four bands, and then reads off the Institute of Medicine (2009) recommendation for that band — the same table used by the American College of Obstetricians and Gynecologists.
For a singleton pregnancy the totals are: 12.5–18 kg (28–40 lb) below BMI 18.5, 11.5–16 kg (25–35 lb) for BMI 18.5–24.9, 7–11.5 kg (15–25 lb) for BMI 25–29.9, and 5–9 kg (11–20 lb) at BMI 30 or above. For twins the ranges are 23–41 kg, 17–25 kg, 14–23 kg and 11–19 kg in the same four bands.
Where the weight actually goes
A round figure of 12.5 kg at term is not 12.5 kg of fat. Roughly 3.5 kg is the baby, 0.5–1 kg each is the placenta and amniotic fluid, about 1.5 kg is extra fluid volume, 1–1.5 kg is breast tissue, and 2.5–4 kg is stored fat that supplies the third trimester and breastfeeding. That is why the first trimester target is small — 0.5–2 kg (1–4.4 lb) in total — and why the gain then settles into a steady weekly rate: about 0.44–0.58 kg/week if you started underweight, 0.35–0.50 if normal weight, 0.23–0.33 if overweight, and 0.17–0.27 kg/week at BMI 30+.
Why the target is a range, not a number
Gaining inside the range is associated with lower risk of both a small-for-gestational-age baby and a large-for-gestational-age baby, fewer preterm births, and less retained weight after delivery. Gaining well above it raises the odds of gestational diabetes, high blood pressure, caesarean delivery, and keeping the weight afterwards; gaining well below it raises the odds of low birth weight. The band is wide on purpose — a person who gains 13 kg and another who gains 15 kg from the same starting point can both be squarely on target.
FAQ
- What if my BMI band feels wrong for me?
- BMI does not separate muscle from fat and the cut-offs were derived largely from white European and North American populations, so athletes, very short people, and some ethnic groups are misclassified often enough to matter. Treat the band as a starting point and ask your midwife or obstetrician to set an individual target.
- I have already gained more than the range. Should I lose weight now?
- Deliberately losing weight during pregnancy is not recommended. Show the number at your next appointment so the cause can be checked — sometimes it is fluid, sometimes it is a dating correction — and let your care team adjust the plan.
- Does the calculator work for triplets?
- No. Published evidence for triplet and higher-order pregnancies is too thin for a firm table, so the tool only offers singleton and twin ranges. Your specialist team will set the target.
- How often should I weigh myself?
- Once a week at roughly the same time of day is enough, and it smooths out normal daily swings from food, fluid and bowel habit. Daily weighing mostly produces noise and anxiety.
This tool provides general information only and is not medical advice. Weight gain targets in pregnancy should be set with your midwife or obstetrician, who knows your history, scanning results and any complications.