Pregnancy weight gain guidelines: how much is healthy?

How much weight to gain in pregnancy depends mainly on your weight before pregnancy. The most widely used guidance, published by the US Institute of Medicine (now the National Academy of Medicine) in 2009, recommends a total gain of 11.5 to 16 kg (25 to 35 lb) if you started at a healthy weight, more if you were underweight and less if you were overweight. The pregnancy weight gain calculator works out your range from your pre-pregnancy BMI and shows where you are week by week. This guide explains the numbers and their limits.

The ranges by BMI

The guidelines start from body mass index before pregnancy, worked out from your pre-pregnancy weight and height, the same sum as the BMI calculator. For one baby:

• Underweight (BMI below 18.5): 12.5 to 18 kg (28 to 40 lb).
• Healthy weight (18.5 to 24.9): 11.5 to 16 kg (25 to 35 lb).
• Overweight (25 to 29.9): 7 to 11.5 kg (15 to 25 lb).
• Obese (30 or more): 5 to 9 kg (11 to 20 lb).

The logic is that people with larger energy reserves need to add less, while those starting lighter benefit from gaining more. Gaining well above the range is linked with a higher chance of gestational diabetes, high blood pressure, a large baby and weight that stays on after birth; gaining too little is linked with a small baby and preterm birth. These are patterns across large groups, not predictions for any one pregnancy. BMI itself is a blunt measure, as Is BMI accurate? explains.

Twins

For twins the guidelines give provisional ranges: 17 to 25 kg (37 to 54 lb) for a healthy pre-pregnancy weight, 14 to 23 kg (31 to 50 lb) for overweight and 11 to 19 kg (25 to 42 lb) for obese. There was not enough evidence to set a range for people who were underweight, so in that case the calculator asks you to get a personal target from your midwife or doctor instead of guessing.

How gain is spread over time

Gain in the first trimester is usually small, typically 0.5 to 2 kg in total, and some people lose a little because of nausea. Most of the gain comes in the second and third trimesters, when the guidelines suggest an average of about 0.4 kg (roughly 1 lb) a week for a healthy starting weight, about 0.5 kg a week if underweight, about 0.3 kg if overweight and about 0.2 kg if obese. The calculator's week-by-week band follows this pattern: 0.5 to 2 kg by the end of week 13, then a straight line to your total range at week 40. Real weight does not move in a straight line. Fluid shifts can make it jump or stall for a week or two, as Why your weight goes up and down every day explains, so look at the trend over several weeks rather than any single weigh-in. The weight tracker can hold the readings and draw the trend.

Where the weight goes

A gain of around 12 kg can seem a lot when a newborn weighs about 3.5 kg, but the baby is only part of it. The rest is the placenta, the amniotic fluid, the enlarged uterus and breasts, a large increase in blood volume, extra fluid in the body's tissues, and fat stores that supply energy for late pregnancy and breastfeeding. Much of this is lost in the weeks after birth, though fat stores usually take longer. It is also why dieting to lose weight during pregnancy is not advised without medical supervision: most of the gain is working tissue and fluid, not simply fat.

Why advice differs between countries

The Institute of Medicine ranges were developed from US data and are used in many countries, but not everywhere. In the UK, NICE does not set numerical targets for weight gain in pregnancy in the same way; the emphasis is on a healthy diet and staying active. Other countries publish their own versions. The BMI cut-offs used to sort people into categories may also be less suitable for some ethnic groups, who can face higher health risks at a lower BMI. So if your midwife or doctor gives you different advice, follow theirs: they know your health history.

When to ask for advice

Being a little above or below the band in a given week is common and not a diagnosis. Talk to your midwife or doctor if you are gaining much faster or slower than the range over several weeks, or if you cannot keep food down. Get urgent advice for sudden swelling of the face, hands or feet, a severe headache, problems with your vision or pain just below the ribs, which can be signs of pre-eclampsia. Never start a weight-loss diet in pregnancy without medical advice. If you need your current week for the weight gain calculator, the due date calculator works it out. This guide is general information, not medical advice.