Pregnancy Weight Gain Calculator

How much weight to gain, by your pre-pregnancy BMI and how many weeks you are — using the Institute of Medicine ranges that ACOG and the CDC follow.

Reference table

Pre-pregnancy weight	Recommended Total Gain	Pre-pregnancy BMI	Where You Should Be at This Week
30	12.5-18.0 kg	11.0 · Underweight	3.6-6.1 kg
40	12.5-18.0 kg	14.7 · Underweight	3.6-6.1 kg
50	12.5-18.0 kg	18.4 · Underweight	3.6-6.1 kg
60	11.5-16.0 kg	22.0 · Normal weight	2.9-5.5 kg
70	7.0-11.5 kg	25.7 · Overweight	2.1-4.3 kg
75	7.0-11.5 kg	27.5 · Overweight	2.1-4.3 kg
80	7.0-11.5 kg	29.4 · Overweight	2.1-4.3 kg
90	5.0-9.0 kg	33.1 · Obesity	1.7-3.9 kg
100	5.0-9.0 kg	36.7 · Obesity	1.7-3.9 kg
150	5.0-9.0 kg	55.1 · Obesity	1.7-3.9 kg
200	5.0-9.0 kg	73.5 · Obesity	1.7-3.9 kg

📊 IOM 2009 Total Gain, Single Pregnancy

Underweight (BMI under 18.5)12.5-18 kg · 28-40 lb
Normal weight (18.5-24.9)11.5-16 kg · 25-35 lb
Overweight (25-29.9)7-11.5 kg · 15-25 lb
Obesity (30 and over)5-9 kg · 11-20 lb

Reading Your Result Honestly

The range comes from your weight BEFORE pregnancy, not from what you weigh today. Changing today's weight moves where you stand, never the target.
The first trimester carries 0.5-2 kg in total for every BMI category. If you are under 13 weeks and have gained almost nothing, that is the expected picture.
Nausea in the first trimester makes some people lose weight. Losing a kilo or two early is common and is not, on its own, a problem.
A range is not a rule. One week off the line means nothing; a trend over a month is what a midwife looks at.

Why Pre-pregnancy BMI Decides the Range

The Institute of Medicine set four gain ranges in 2009, one for each pre-pregnancy BMI category, and they differ by more than a factor of two at the top end: someone starting underweight is advised to gain up to 18 kilograms, someone starting with obesity up to nine. The reasoning is that the gain has two jobs at once. Part of it is the baby, the placenta, the amniotic fluid and the extra blood volume, which cost roughly the same whoever is carrying them. The rest is maternal fat laid down as an energy reserve for the third trimester and for breastfeeding, and someone who already has ample reserves needs to add far less of it. That is why the calculator asks what you weighed before you conceived and treats today's weight only as a position on the line, never as the thing that sets the line.

The First Trimester Has No Weekly Rate

Most calculators divide the total gain by forty weeks and call the answer a weekly target. The IOM does not do that, and the difference matters. Its model assigns 0.5 to 2 kilograms to the whole first trimester — the same figures for all four BMI categories — and only then applies a weekly rate for weeks thirteen to forty. For a normal-BMI pregnancy that rate is 0.35 to 0.50 kilograms a week. Flattening the total across forty weeks would set a first-trimester target roughly three times too high, which alarms people who are living on crackers and ginger, and a third-trimester target too low, which is the stretch where the gain actually accelerates. This page follows the IOM's shape: a flat allowance until week thirteen, then a rate.

What the Range Is Actually Protecting Against

Gaining below the range raises the chance of a small-for-gestational-age baby and of preterm birth. It is not the safer side to err on.
Gaining above the range raises the chance of a large baby, caesarean delivery, and weight retained after the birth.
Roughly half of pregnancies in high-income countries fall outside the IOM range in one direction or the other, so being off it is common rather than alarming.
The ranges were written for singleton pregnancies with adequate nutrition. They are guidance for a population, not a prescription for one person.
Height enters only through BMI. Two people of the same BMI get the same range whether they are 1.55 or 1.85 metres tall.
If you did not weigh yourself before conceiving, a weight from the few months before is closer than a guess from memory.

Twins, and What Is Not Known

The IOM twin ranges are labelled provisional in the original report, which is unusual and worth knowing before you treat them as firm.
There is no IOM twin range for an underweight starting BMI: the evidence was judged insufficient. This page shows the normal-BMI twin range and says so.
Normal BMI carrying twins: 16.8 to 24.5 kg. Overweight: 14.1 to 22.7 kg. Obesity: 11.3 to 19.1 kg.
The IOM published no weekly rate for twins. The rate shown here is derived by subtracting the first-trimester allowance and spreading the rest over weeks 13 to 40.
Twin pregnancies more often end before 37 weeks, so a total pegged to a 40-week finish may never be reached.
Triplets and above have no IOM range at all. This calculator does not cover them.

Worked Examples

The same week, two different starting points

1.65 m, 62 kg before pregnancy, week 20

BMI = 62 ÷ 1.65² = 22.8 → normal weight
Total range = 11.5 to 16 kg
Week 20: 0.5-2 kg (T1) + 7 weeks × 0.35-0.50 = 2.95 to 5.5 kg

Expected 2.95-5.5 kg by week 20; 4 kg is on track

1.70 m, 90 kg before pregnancy, twins, week 28

BMI = 90 ÷ 1.70² = 31.1 → obesity
Twin range = 11.3 to 19.1 kg (provisional)
Derived rate = (11.3 − 0.5) ÷ 27 to (19.1 − 2) ÷ 27 = 0.40 to 0.63 kg/week

Expected 6.5-11.5 kg by week 28

Frequently Asked Questions

How much weight should I gain during pregnancy?

It depends on your BMI before you conceived. The Institute of Medicine ranges are 12.5-18 kg if you started underweight, 11.5-16 kg at a normal weight, 7-11.5 kg if overweight, and 5-9 kg with obesity. Your weight today does not change the range — it only tells you where you are on it.

How much should I have gained by 20 weeks?

For a normal pre-pregnancy BMI, roughly 3 to 5.5 kilograms. That is the 0.5-2 kg allowed for the whole first trimester plus seven weeks at 0.35-0.50 kg. Someone starting overweight would be nearer 2 to 4.3 kg by the same week.

I have gained nothing in the first trimester. Is that a problem?

Usually not. The IOM assigns only 0.5 to 2 kilograms to the entire first trimester, and nausea makes some people lose a little instead. Early loss of a kilogram or two is common. It is worth mentioning at your next appointment, but on its own it is not a warning sign.

What if I gain more than the recommended range?

It raises the chance of a larger baby, of a caesarean, and of retaining weight after the birth — but roughly half of pregnancies fall outside the range in one direction or the other. The response is not to diet: it is to talk to your midwife or doctor about the trend, since restricting food in pregnancy carries its own risks.

How much weight should I gain with twins?

The IOM gives provisional ranges: 16.8-24.5 kg at a normal starting BMI, 14.1-22.7 kg if overweight, and 11.3-19.1 kg with obesity. There is no published range for an underweight start, because the evidence was judged insufficient. Twin ranges are less firmly established than singleton ones.

I do not know what I weighed before I got pregnant. What do I do?

Use the most recent weight you have from the months before conceiving — a doctor's record, a gym check, anything measured rather than remembered. If your first prenatal appointment was early, that weight is often close enough. An estimate that is a few kilograms out will usually keep you in the same BMI category, which is all the range depends on.