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Pregnancy Weight Gain Calculator
Enter your height, your weight before pregnancy and the current week to see the Institute of Medicine range for total gain with one baby, the usual weekly rate, and where you stand if you add your current weight.
Where the ranges come from
The ranges used here are the 2009 recommendations of the US Institute of Medicine (now the National Academy of Medicine), published in the report Weight Gain During Pregnancy: Reexamining the Guidelines. They apply to a single baby (a singleton pregnancy) and are chosen by the pre-pregnancy body mass index, calculated as weight in kilograms divided by height in metres squared.
| Pre-pregnancy BMI | Total gain (kg) | Weekly rate, 2nd and 3rd trimester (kg) |
|---|---|---|
| Under 18.5 | 12.5 to 18 | 0.44 to 0.58 |
| 18.5 to 24.9 | 11.5 to 16 | 0.35 to 0.50 |
| 25.0 to 29.9 | 7 to 11.5 | 0.23 to 0.33 |
| 30.0 and over | 5 to 9 | 0.17 to 0.27 |
For every group the report suggests a small first-trimester gain of roughly 0.5 to 2 kg in total, because early pregnancy needs few extra calories and nausea often limits eating.
How the week-by-week range is estimated
Before week 13 the calculator spreads the first-trimester allowance evenly: at week 6.5, for instance, you would expect to be about halfway between 0.25 and 1 kg above your starting weight. From week 14 onward it adds the weekly rate range to the first-trimester figures. For a normal-weight starting BMI at week 20 that means 0.5 + 0.35 × 7 = 2.95 kg at the low end and 2 + 0.50 × 7 = 5.5 kg at the high end.
This is an approximation built from published rates, not a growth chart. The report gives totals and rates as separate rounded figures, so the estimated range at week 40 will not match the total range exactly. Treat the week-specific range as a rough guide to the trend and the total range as the official target.
Reading your result
Staying inside the range is associated in the IOM review with fewer problems for both mother and baby than gaining far less or far more, but individual outcomes depend on many things the calculator cannot see. Weight in pregnancy is also made up of the baby, placenta, amniotic fluid, larger breasts and uterus, extra blood and fluid, and body stores of fat. Only part of it is fat.
- Below the range: mention it at your next appointment, especially if you also have vomiting, poor appetite or a low starting weight.
- Inside the range: no action needed; weight does not rise in a straight line, and short plateaus or jumps are normal.
- Above the range: a prompt to review food and activity with your care team. Sudden swelling of the face or hands with headache or vision changes should be reported urgently, since fluid can signal a blood pressure problem.
Limits and who should not rely on it
These figures are for one baby. The IOM gave only provisional ranges for twins and none for higher multiples, so anyone expecting more than one baby should follow personalised advice. Teenagers, short women, people with a history of eating disorders, diabetes, gestational diabetes, kidney disease or thyroid disease, and anyone told to gain weight on a specific plan should use the plan from their care team instead.
Use your weight from before pregnancy or from the first visit, whichever is a reliable measurement. Weighing on the same scale, at the same time of day and in similar clothing makes changes easier to read. This page is educational and is not medical advice; it cannot diagnose a problem or replace a visit with a qualified professional.
Worked example
A woman 165 cm tall weighed 60 kg before pregnancy, is now at week 20 and weighs 63.5 kg.
Inputs
| Units | Metric (cm, kg) |
|---|---|
| Height | 165 |
| Weight before pregnancy | 60 |
| Weeks pregnant | 20 |
| Current weight (optional) | 63.5 |
Result
Category from pre-pregnancy BMI: Normal weight. You have gained 3.5 kg so far.
| Pre-pregnancy BMI | 22.0 (Normal weight) |
|---|---|
| Recommended total gain | 11.5 kg to 16.0 kg |
| Weekly rate after week 13 | 0.4 kg to 0.5 kg (about 0.4 kg) |
| Gain in the first trimester | 0.5 kg to 2.0 kg in total |
| Approximate range by week 20 | 2.9 kg to 5.5 kg |
| Gain so far | 3.5 kg (inside the approximate range for this week) |
Medical disclaimer: This tool is for education and general planning. It is not medical advice and does not diagnose any condition. If you are pregnant, under 18, have an eating-disorder history or a medical condition, speak to a doctor or registered dietitian before changing your diet or exercise.
Frequently asked questions
How much weight should I gain in pregnancy?
It depends on your weight and height before pregnancy. For one baby the IOM range is 12.5 to 18 kg when underweight, 11.5 to 16 kg at a normal BMI, 7 to 11.5 kg when overweight and 5 to 9 kg with obesity.
How much should I gain in the first trimester?
The IOM suggests roughly 0.5 to 2 kg in total across the first 13 weeks for every starting BMI group. Many people gain nothing or even lose a little at first because of nausea, which is usually not a concern.
Does this work for twins?
No. The ranges here are for a singleton pregnancy only. Twin pregnancies have their own provisional guidance and need advice from your midwife or obstetrician.
What if I gain more or less than the range?
The range is a guide, not a pass mark. Gaining outside it is a reason to talk with your care team about food, activity and the baby growth checks, rather than a reason to diet during pregnancy.
Which weight counts as my pre-pregnancy weight?
Use the most reliable weight from just before conception or from your first prenatal visit. If you are unsure, ask the clinic to share the weight recorded at your booking appointment.
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References and sources
- Institute of Medicine and National Research Council (2009). Weight Gain During Pregnancy: Reexamining the Guidelines. Washington, DC: The National Academies Press.
- American College of Obstetricians and Gynecologists, clinical guidance on weight gain during pregnancy. Link