Pregnancy Weight Gain Calculator NZ 2026
This calculator estimates a healthy range of weight gain during pregnancy, both the recommended total and roughly how much to have gained by your current week, based on your weight before pregnancy and your height. Weight gain in pregnancy is normal, necessary and healthy: it supports the growing baby, the placenta, extra blood and fluid, and the body's preparation for breastfeeding. But the right amount depends a lot on where you started, which is why the widely used Institute of Medicine guidelines base the recommended range on your pre-pregnancy body mass index. Someone who began at a lower weight is advised to gain more, while someone who began at a higher weight is advised to gain less, because both too little and too much gain carry risks for parent and baby. This tool applies those guidelines. You enter your pre-pregnancy weight, your height and how many weeks pregnant you are, and it returns your pre-pregnancy BMI and category, the recommended total weight-gain range for a single pregnancy, and an indicative range for how much you might reasonably have gained by your current week, since most gain happens steadily through the second and third trimesters after a small gain early on. Use it as a general reference to discuss with your midwife or doctor, who track your progress in the context of your whole health. It is important to understand that this is a guide for single pregnancies based on population averages; twins or more, medical conditions, and individual circumstances change the picture entirely. Pregnancy is not a time to diet, and your lead maternity carer is the right person to advise on your specific situation. This is general information, not medical advice.
Institute of Medicine guidelines for a single pregnancy. Twins, medical conditions and individuals differ. Pregnancy is not for dieting. Not medical advice.
How it works
Your pre-pregnancy BMI is your weight divided by your height in metres squared, which sets your category: underweight, healthy, overweight or obese. Each category has a recommended total gain range from the Institute of Medicine guidelines. The by-week estimate assumes a small gain in the first trimester, then a steady weekly rate through the second and third trimesters.
Worked example
Someone 65 kilograms and 165 centimetres has a pre-pregnancy BMI of about 23.9, in the healthy-weight range, with a recommended total gain of 11.5 to 16 kilograms. By 20 weeks, allowing a small early gain plus a steady weekly rate, a reasonable range to date is approximately 3.5 to 5.1 kilograms, to be confirmed with their midwife.
Related calculators
- BMI Calculator: body mass index.
- Due Date Calculator: estimate your due date.
- Conception Date: estimate conception.
- Cost of a Baby: budgeting for baby.