The formula
How it works
See how much weight gain is recommended during pregnancy, based on your pre-pregnancy BMI and how far along you are. Enter your starting weight, height and current week to get a healthy total range and a target for this point.
FAQ
Why does pre-pregnancy BMI matter?
The recommended gain depends on where you started. Someone underweight is advised to gain more, and someone overweight less, to support the baby while protecting the mother’s health. The calculator uses the Institute of Medicine ranges, which are set by BMI category.
Is a range normal rather than a single number?
Yes. Healthy pregnancy weight gain is always a range, not an exact figure, and it is gradual — little in the first trimester, then steadier through the second and third. Your midwife or doctor may adjust the target for twins or specific health needs, so treat this as guidance.
Why is so little gain expected in the first trimester?
Early pregnancy calls for only a small amount of weight gain — around 1 to 4 pounds in total — because the baby is still tiny and most maternal changes happen later. The calculator scales that small first-trimester amount by how far through week 13 you are.
Why does the weekly rate differ between BMI categories?
Someone who started underweight is guided toward a faster weekly gain to help reach a healthy overall total, while someone who started with a higher BMI is guided toward a slower rate. These weekly rates only apply from the second trimester onward, once gain typically becomes more consistent.
Does this apply to a twin pregnancy?
No — the ranges here follow general guidance for a single baby. Twin and multiple pregnancies have separate, higher recommended ranges, so talk to your midwife or doctor for guidance tailored to a multiple pregnancy.
Is weight loss ever recommended during pregnancy?
This calculator always shows a positive gain range, including for the obese category, because some weight gain is generally recommended to support the pregnancy even when the starting BMI is high. Any decision about weight loss during pregnancy should only be made with a doctor’s guidance, never based on a general calculator.
What if my actual gain is outside the suggested range?
Being a bit above or below the range is common and not automatically a cause for concern, since every pregnancy is different. Consistent or large deviations are worth mentioning at your next appointment so your midwife or doctor can check on you and the baby.
About the pregnancy weight gain calculator
This calculator shows the recommended weight gain during pregnancy, tailored to your pre-pregnancy BMI and the week you are in. Guidance from the Institute of Medicine sets a healthy total range for the whole pregnancy by BMI category, and a slower rate early on that steadies through the later trimesters. The calculator turns those guidelines into a total target and a figure for where you should be right now.
How to use it
Enter your pre-pregnancy weight, your height and your current week of pregnancy, in either metric or imperial units. The calculator works out your pre-pregnancy BMI, places you in a category, and shows the recommended total gain along with the gain expected by your current week. For example, a woman starting at a normal BMI is advised to gain about 25–35 pounds in total.
The formula
First the pre-pregnancy BMI is found as in metric units. That places you in one of four categories, each with its own total range and weekly rate. The expected gain by a given week adds a small first-trimester gain to the weekly rate applied across the second and third trimesters, so it rises gradually as the pregnancy progresses.
Where it is used
Expectant mothers use it to check that their weight gain is on a healthy track between appointments, and to understand why the recommended amount differs from someone else’s. It complements the advice of a midwife or doctor rather than replacing it, and pairs naturally with a due date calculator to see progress week by week. Staying within the range supports both the baby’s growth and the mother’s recovery.