Pregnancy weight gain calculator

The disagreement nobody tells you about

Search for how much weight you should gain in pregnancy and you will be given a number within seconds. What you will almost never be told is where it came from, or that the country you live in may have decided not to publish one at all.

The United States publishes ranges. The Institute of Medicine, now the National Academy of Medicine, set them in 2009 in Weight Gain During Pregnancy: Reexamining the Guidelines, stratified by pre-pregnancy BMI. ACOG restates them. RANZCOG reproduces the same table for Australia and New Zealand. They are a real, carefully built, widely used guideline, and this page shows them.

The United Kingdom decided the evidence does not support one. NICE guideline NG247, published in January 2025, says it plainly: “There is a lack of evidence about what the optimal total weight change in pregnancy or weight change in each trimester should be.” And then, in the recommendation that surprises people most: “Do not routinely offer to weigh people throughout their pregnancy unless there is a clinical reason to do so.”

So if you are pregnant in the UK and nobody has put you on the scales since your booking appointment, nothing has been forgotten. That is the guideline working exactly as written.

What NICE does instead

NICE asks for height and weight once, at the first antenatal appointment, and calculates BMI — because BMI at booking genuinely changes your care, affecting risk assessment for things like venous thromboembolism and pre-eclampsia. After that, the advice is about food and movement rather than a number on a scale.

And then NICE does something worth knowing about. Recommendation 1.2.14: “If people are interested in monitoring their weight change during pregnancy, refer them to the estimated healthy total weight change in a singleton pregnancy according to pre-pregnancy BMI; see table 1 in the National Academy of Medicine’s report.” That is the American table. The UK position is not that the numbers are wrong. It is that there is not enough evidence to weigh everybody against them — but if you want them, here they are.

Which is, more or less, what this page does.

The number the NHS does publish

The NHS page on weight gain in pregnancy says: “Most pregnant women gain between 10kg and 12.5kg (22lb to 28lb), putting on most of the weight after week 20.” That is a description of what happens, not a BMI-stratified target, and it is worth reading it as such. It sits comfortably inside the IOM healthy-weight band of 11.5 to 16 kg, and below the underweight band — which is exactly what you would expect a population average to do.

Where the weight actually goes

It is not fat. A term baby is around 3.3 to 3.5 kg. Add the placenta, the amniotic fluid, a uterus that has grown enormously, breast tissue, and an increase in blood volume of something like 40 per cent, and most of the recommended range is accounted for before any maternal fat store is involved. NICE lists exactly these components as the reason optimal gain is so hard to pin down: they “vary between individuals and affect weight differently”.

This matters practically, because it explains why the scale moves in ways that have nothing to do with eating. Fluid shifts by a kilogram or more across a day. Constipation is real weight. Hyperemesis takes weight off regardless of intention. A big baby and a small baby differ by more than a kilogram between them. A single reading tells you very little; a trend over weeks tells you a bit more.

Gaining more, or less, than the range

Gaining outside the band is not a moral event and it is not a diagnosis. It is a prompt for a conversation, and the conversation is usually short.

What everybody agrees on, on all sides of the guideline disagreement, is that trying to lose weight while pregnant is not recommended. NICE says so directly. If your BMI is 30 or over, the guidance is a smaller recommended gain — 5 to 9 kg rather than 11.5 to 16 — which is not the same thing as a diet, and is a distinction worth holding onto when the internet blurs it.

If you are losing weight because you cannot keep food down, that is not weight management, it is a symptom. Persistent vomiting in pregnancy has a name, hyperemesis gravidarum, and a treatment, and it is one of the specific clinical reasons NICE lists for weighing someone repeatedly. Speak to your midwife.

Twins

The twin ranges here are provisional, and the IOM uses that word itself: “the evidence to support them was more limited compared to singletons.” They were built from the interquartile range of gain among women who delivered twins weighing at least 2,500 g between 37 and 42 weeks — a good-outcome group, observed rather than trialled.

There is no weekly rate of gain published for twins, which is why this tool gives a total and not a week-by-week band when you select twins. And there is no range at all for a twin pregnancy starting from a BMI under 18.5. This calculator says so instead of extrapolating. If that is your situation, a referral to a dietitian is a far better answer than a number on a screen.

What to do with the result

Take it as orientation, not instruction. If you are in the US, Australia or New Zealand, your clinician is probably working from exactly this table and it is useful to know where you sit on it. If you are in the UK, the honest framing is that your health service has looked at the same evidence and decided not to set you a target — and that not being weighed is not the same as not being cared for.

Either way: eat when you are hungry, move in the ways you can, and take anything that worries you — sudden gain with swelling or headaches, weight falling because of sickness — to your midwife rather than to a calculator.