How much weight should I gain in pregnancy?

It depends on what you weighed before. The 2009 IOM guidance recommends 12.5 to 18 kg if you started underweight, 11.5 to 16 kg if you started at a normal BMI, 7 to 11.5 kg if overweight, and 5 to 9 kg if obese. These are population ranges, and your own care provider may set something different for good reasons.

Why is less weight gain recommended for a higher starting BMI?

Because the guidance assumes a woman starting with larger fat stores needs less additional gain to support the pregnancy and the baby's growth. The recommendation was derived from observational data on birth outcomes across starting BMI categories, and it narrows steadily as starting BMI rises.

When does most pregnancy weight gain happen?

In the second and third trimesters. Across every category the whole first trimester accounts for only about half a kilo to two kilos, and the rest accrues at a fairly steady weekly rate afterwards — roughly 0.42 kg a week for someone starting at a normal BMI.

What if I am gaining more or less than the range?

Raise it with your midwife or obstetrician rather than adjusting anything on your own. The ranges are population averages from observational data, individual pregnancies vary for many ordinary reasons, and nutrition changes during pregnancy are exactly the kind of decision that needs someone who knows your history.

All calculatorsPregnancy weight gain calculator

Pregnancy weight gain calculator

The 2009 IOM ranges, keyed to your pre-pregnancy BMI.

Use your weight before the pregnancy — the guidance is keyed to your starting BMI, not your current one.

Pre-pregnancy weight

Height

Current week (optional)

weeks

Enter a weight above zero.

The IOM ranges, and what they are based on

The ranges here come from the 2009 Institute of Medicine report Weight Gain During Pregnancy: Reexamining the Guidelines, which is still the standard reference in the US and widely used elsewhere. How much weight is recommended depends on what you weighed before you were pregnant — specifically on which BMI category you started in.

That is the part people find counterintuitive: the guidance recommends less gain the higher your starting BMI, from 12.5–18 kg for someone starting underweight down to 5–9 kg for someone starting obese. The reasoning is that a woman starting with more fat stores needs less additional gain to support the pregnancy.

Formulas

Pre-pregnancy BMI

BMI = kg ÷ m²

From your weight before pregnancy, not now.

Expected gain

first-trimester gain + weekly rate × weeks after week 13

Expected gain by week

Starting category	Week 13	Week 20	Week 26	Week 32	Week 40
Underweight	0.5–2 kg	3.6–6.1 kg	6.2–9.5 kg	8.9–13 kg	12.4–17.7 kg
Normal weight	0.5–2 kg	2.9–5.5 kg	5.1–8.5 kg	7.2–11.5 kg	10–15.5 kg
Overweight	0.5–2 kg	2.1–4.3 kg	3.5–6.3 kg	4.9–8.3 kg	6.7–10.9 kg
Obese	0.5–2 kg	1.7–3.9 kg	2.7–5.5 kg	3.7–7.1 kg	5.1–9.3 kg

Every category gains the same 0.5–2 kg through the first trimester; they diverge only afterwards.

IOM 2009 recommended total gain

Pre-pregnancy BMI	Category	Singleton	Rate, 2nd–3rd trimester	Twins
Below 18.5	Underweight	12.5–18 kg (28–40 lb)	0.51 kg/week	Insufficient evidence
18.5 – 25	Normal weight	11.5–16 kg (25–35 lb)	0.42 kg/week	16.8–24.5 kg
25 – 30	Overweight	7–11.5 kg (15–25 lb)	0.28 kg/week	14.1–22.7 kg
30 and above	Obese	5–9 kg (11–20 lb)	0.22 kg/week	11.3–19.1 kg

Twin guidance is provisional, and absent for women starting underweight — the report found the evidence insufficient rather than the recommendation zero.

Worth knowing

Almost none of the gain happens early. The whole first trimester accounts for 0.5 to 2 kg across every category; the rest accrues at a steady weekly rate through the second and third.
The IOM guidance is drawn from observational data on outcomes, not from trials of what is best for any one person. It describes what was associated with good outcomes in a population.
It does not apply to a pregnancy with a medical complication, and it is not adjusted for age, height beyond BMI, or ethnicity.
Twin guidance was published as provisional, and only for three of the four categories. For women starting underweight the report found the evidence insufficient — so there is no range, rather than a range of zero.

Frequently asked questions

How much weight should I gain in pregnancy?: It depends on what you weighed before. The 2009 IOM guidance recommends 12.5 to 18 kg if you started underweight, 11.5 to 16 kg if you started at a normal BMI, 7 to 11.5 kg if overweight, and 5 to 9 kg if obese. These are population ranges, and your own care provider may set something different for good reasons.
Why is less weight gain recommended for a higher starting BMI?: Because the guidance assumes a woman starting with larger fat stores needs less additional gain to support the pregnancy and the baby's growth. The recommendation was derived from observational data on birth outcomes across starting BMI categories, and it narrows steadily as starting BMI rises.
When does most pregnancy weight gain happen?: In the second and third trimesters. Across every category the whole first trimester accounts for only about half a kilo to two kilos, and the rest accrues at a fairly steady weekly rate afterwards — roughly 0.42 kg a week for someone starting at a normal BMI.
What if I am gaining more or less than the range?: Raise it with your midwife or obstetrician rather than adjusting anything on your own. The ranges are population averages from observational data, individual pregnancies vary for many ordinary reasons, and nutrition changes during pregnancy are exactly the kind of decision that needs someone who knows your history.