Pregnancy Weight Gain Calculator

How pregnancy weight gain targets are set

Every recommended range on this page comes from one number you had before you were pregnant: your body mass index. The Institute of Medicine (now the National Academy of Medicine) published the current ranges in 2009, and they are what US obstetricians, UK midwives and most prenatal apps still use. The logic is simple — a woman who started underweight has fewer reserves and is asked to gain more, while a woman who started at a higher BMI already carries some of the energy a pregnancy needs, so her range is smaller. The ranges are not weight-loss advice in either direction. They are the band associated with the lowest combined risk of a small baby, a large baby, pre-eclampsia and a caesarean birth.

Worked example: 140 lb, 5 ft 5 in, week 24

Start with pre-pregnancy BMI. A 140 lb woman at 5 ft 5 in is 63.5 kg at 1.651 m, so BMI = 63.5 / (1.651 x 1.651) = 23.3 — normal weight. Her IOM total is 25 to 35 lb (11.3 to 15.9 kg) for the whole pregnancy.

Next, split that total by trimester. The IOM allows 1.1 to 4.4 lb (0.5 to 2 kg) in the first trimester, and the rest is spread across weeks 14 to 40 — 27 weeks. The low edge of her weekly rate is (25 − 1.1) / 27 = 0.9 lb per week; the high edge is (35 − 4.4) / 27 = 1.1 lb per week. That is the familiar "about a pound a week" advice, derived rather than memorised.

At week 24 she is 11 weeks past the first-trimester mark. Low edge: 1.1 + (0.885 x 11) = 10.8 lb. High edge: 4.4 + (1.133 x 11) = 16.9 lb. So an expected 10.8 to 16.9 lb (4.9 to 7.7 kg) so far. If her scale says 14 lb, she is comfortably mid-range; at 19 lb she is running ahead and worth a conversation, not a panic.

The IOM 2009 table, in words

Underweight (BMI under 18.5): 28 to 40 lb total, roughly 1.0 to 1.3 lb per week after the first trimester. Normal weight (18.5 to 24.9): 25 to 35 lb, about 0.9 to 1.1 lb per week. Overweight (25 to 29.9): 15 to 25 lb, about 0.5 to 0.8 lb per week. Obesity (30 and above): 11 to 20 lb, about 0.4 to 0.6 lb per week. For twins the IOM gives provisional ranges — 37 to 54 lb at normal BMI, 31 to 50 lb with overweight, 25 to 42 lb with obesity — and explicitly declines to set a range for underweight mothers because the evidence is too thin.

Why the pattern matters more than the total

Gain is not linear. The first trimester adds almost nothing, and many women lose a pound or two to nausea instead — that is normal and does not need making up. From week 14 the curve becomes close to a straight line, which is why prenatal charts look like a hockey stick. The last month often flattens again as the baby's growth outpaces maternal gain. If you are behind at week 16, you have 24 weeks to land inside the band; if you are 10 lb ahead at week 20, the aim is usually to slow the rate rather than reverse it, since deliberate weight loss is not advised in pregnancy without supervision.

After the birth

Delivery removes about 10 to 13 lb in one go: baby, placenta and amniotic fluid. Another 5 to 8 lb of extra blood volume and fluid leaves over the first two weeks, largely through urine and sweat. What remains is the fat stored in the second and third trimesters, and it comes off on a scale of months, not weeks. Women who gained inside their IOM range are far more likely to be back near their starting weight at six months, which is one of the strongest practical arguments for the guideline.

Limits of this calculator

It uses BMI, which does not know whether your pre-pregnancy weight was mostly muscle, and it cannot see your blood pressure, thyroid function, glucose tolerance, medication list or the baby's measured growth. It also assumes a single healthy pregnancy at a standard 40-week term. Hyperemesis, gestational diabetes, oedema and multiple births all shift the picture. Use the numbers to know roughly where you should be and to spot a trend that has drifted for a month or more, then bring that trend to your midwife or doctor. They are the ones who set your actual target.