How pregnancy weight gain calculator works
Healthy pregnancy weight gain depends on your pre-pregnancy BMI and whether you are carrying one baby or twins. The US Institute of Medicine (IOM) guidelines give a recommended total gain range: for a singleton, underweight (BMI <18.5) aims for 28-40 lb, normal weight (18.5-24.9) for 25-35 lb, overweight (25-29.9) for 15-25 lb, and obese (30+) for 11-20 lb.
Gain is not spread evenly. The first trimester typically adds only 0.5-2 kg (1-4 lb) total. From the second trimester on, gain becomes a steady weekly rate — about 1 lb/week for a normal-BMI singleton, less for higher BMI categories. This calculator turns those rates into a running target so you can see how much gain is expected at your current week.
Enter your pre-pregnancy weight and height (so the tool can compute your BMI), your current gestational week, and whether it is a singleton or twin pregnancy. The tool reports your BMI category, the recommended total gain, the weekly rate, the first-trimester baseline, and your expected gain so far — in both kilograms and pounds. This is general guidance, not medical advice; your provider may set a personalised target.
Frequently asked questions
How much weight should I gain during pregnancy?
It depends on your pre-pregnancy BMI. For a singleton, the IOM recommends 28-40 lb if underweight, 25-35 lb if normal weight, 15-25 lb if overweight, and 11-20 lb if obese. For twins, the ranges are higher (e.g. 37-54 lb for a normal BMI). Enter your details to get your specific range.
Why does my pre-pregnancy BMI matter?
A higher starting BMI is linked to more pregnancy complications at higher gains, so the recommended range is lower. A lower starting BMI needs more gain to support the baby. The tool computes your BMI from your pre-pregnancy weight and height, then selects the matching IOM range.
How is gain distributed across trimesters?
The first trimester usually adds only 0.5-2 kg (1-4 lb) total — morning sickness can even cause weight loss. From the second trimester, gain becomes a steady weekly rate (about 0.4 kg or 1 lb per week for a normal-BMI singleton). The calculator shows your expected gain at your current week using this model.
What if I am having twins?
Twin pregnancies need more gain. The IOM recommends 37-54 lb for a normal BMI, 31-50 lb if overweight, and 25-42 lb if obese with twins. There is no published IOM range for underweight twin pregnancies, so the calculator flags that you should ask your obstetric provider for a personalised target.
Are these targets exact?
No — they are population guidelines, and individual needs vary with your health, the baby’s growth, and your provider’s judgement. Use them as a general reference and discuss your specific weight-gain goal with your obstetric provider, especially if you have any pregnancy complications.
