Pregnancy Weight Gain Calculator

Recommended weight gain in pregnancy depends on the weight you started at, not on a single universal figure. The Institute of Medicine ranges below were set in 2009 by looking at the gain associated with the best outcomes for both parent and baby, and they are ranges rather than targets: gaining within your range is the goal, and where you land inside it matters much less than staying broadly on track.

The formula

Pre-pregnancy BMI = weight (kg) / height (m) squared
Second and third trimester rate = (total recommended gain - first trimester gain) / 27 weeks

• BMI is calculated from weight before pregnancy, not current weight
• First trimester gain is small for every category, conventionally 0.5 to 2 kg (1 to 4.5 lb)

Institute of Medicine and National Research Council. Weight Gain During Pregnancy: Reexamining the Guidelines, 2009.

Recommended total gain by starting BMI

The ranges below are for a single pregnancy. They are based on pre-pregnancy BMI, so the first step is calculating that from your weight before conception rather than your weight now.

• Twin pregnancy ranges are higher and provisional: 16.8 to 24.5 kg for healthy weight, 14.1 to 22.7 kg for overweight, and 11.3 to 19.1 kg for obesity. No range was set for underweight twin pregnancies because the data were insufficient.
• First trimester gain is small in every category, and gaining nothing or losing a little in early pregnancy through nausea is common and not usually a problem.

Where the weight actually goes

A common source of anxiety is the assumption that gained weight is body fat. Most of it is not. At term, a typical distribution accounts for the baby, the placenta, amniotic fluid, an expanded blood volume of roughly 1.5 litres, increased breast and uterine tissue, and extra fluid in the tissues, with maternal fat stores making up a minority of the total.

When the pattern matters more than the total

A steady rate within your range is more informative than the running total, because it is deviations from the expected rate that flag problems. Gaining much faster than expected raises the possibility of excess fluid retention, and sudden gain over a few days, particularly with swelling of the face and hands, headache or visual changes, is a recognised warning sign of pre-eclampsia and warrants same-day assessment.

Gaining much less than expected, or losing weight after the first trimester, is also worth raising, because it can reflect inadequate intake, hyperemesis or restricted fetal growth. Neither situation is diagnosed by the scale alone; the number is a prompt for a conversation, not a conclusion.

• Sudden weight gain with facial or hand swelling, headache or visual disturbance: contact your maternity team the same day.
• Deliberate weight loss is not recommended during pregnancy, even when starting BMI is high; the recommendation is a smaller gain, not a loss.

Limitations

• The IOM ranges are population recommendations associated with better average outcomes. They do not predict the outcome of any individual pregnancy.
• The ranges apply to single pregnancies except where twin values are given, and the twin values are described as provisional because the underlying evidence is limited.
• They assume an accurate pre-pregnancy weight. A remembered or estimated starting weight shifts the whole calculation.
• Weight gain says nothing on its own about fetal growth, which is assessed by measurement and ultrasound.
• Adolescent pregnancy, short stature and some medical conditions may warrant individualised targets that differ from these ranges.
• This is general information. Weight gain in pregnancy should be discussed with the midwife or obstetrician who knows the specific pregnancy.

Frequently asked questions

How much weight should I gain during pregnancy?

It depends on your pre-pregnancy BMI: 12.5 to 18 kg if underweight, 11.5 to 16 kg at a healthy weight, 7 to 11.5 kg if overweight, and 5 to 9 kg with obesity, for a single pregnancy.

I have not gained any weight in the first trimester. Is that a problem?

Usually not. Recommended first trimester gain is small in every category, and gaining nothing or losing a little to nausea is common. What matters more is the rate of gain from the second trimester onward.

How much of pregnancy weight is the baby?

At term the baby is typically 3.0 to 3.6 kg out of a total gain of 11.5 to 16 kg for someone starting at a healthy weight. The rest is placenta, amniotic fluid, expanded blood volume, breast and uterine tissue, tissue fluid, and a minority as maternal fat stores.

Should I try to lose weight while pregnant if my BMI is high?

No. The recommendation for a higher starting BMI is a smaller gain, 5 to 9 kg, not weight loss. Deliberate weight loss is not advised during pregnancy and should only ever be considered under direct medical supervision.

When is sudden weight gain a warning sign?

Rapid gain over a few days, especially with swelling of the face and hands, headache or visual changes, can indicate pre-eclampsia and warrants same-day contact with your maternity team rather than waiting for the next appointment.

References

1. Institute of Medicine and National Research Council. Weight Gain During Pregnancy: Reexamining the Guidelines, 2009.
2. Institute of Medicine. Weight Gain During Pregnancy, NCBI Bookshelf edition.
3. American College of Obstetricians and Gynecologists. Nutrition During Pregnancy.
4. MedlinePlus. Managing your weight gain during pregnancy.