Recommended weight gain by BMI
These are the Institute of Medicine (IOM) guidelines from 2009, which the CDC uses. They're based on your body mass index (BMI) before pregnancy.
| BMI before pregnancy | Total gain, one baby | Per week, 2nd & 3rd trimester | Total gain, twins |
|---|---|---|---|
| Under 18.5 (underweight) | 28–40 lb (12.5–18 kg) | 1 lb (1–1.3 lb) | No IOM range |
| 18.5–24.9 (normal weight) | 25–35 lb (11.5–16 kg) | 1 lb (0.8–1 lb) | 37–54 lb (17–25 kg) |
| 25–29.9 (overweight) | 15–25 lb (7–11.5 kg) | 0.6 lb (0.5–0.7 lb) | 31–50 lb (14–23 kg) |
| 30 or more (obesity) | 11–20 lb (5–9 kg) | 0.5 lb (0.4–0.6 lb) | 25–42 lb (11–19 kg) |
The twin ranges are provisional: the IOM had less information to go on for twins, and none to set a range for a BMI under 18.5. In kilograms, the weekly rates are 0.51, 0.42, 0.28 and 0.22 kg for the four groups.
How the weekly range is worked out
The IOM guidelines give a total for the whole pregnancy and a weekly rate for the second and third trimesters. They assume a gain of 0.5 to 2 kg (1.1 to 4.4 lb) in the first trimester. To show a range for the week you're in, the calculator draws a straight line from no gain at the start to that first-trimester amount at 13 weeks, then from there to the total range at 40 weeks.
For a normal BMI, that puts the range at about 1.1 to 4.4 lb at 13 weeks, about 12.6 to 19.1 lb at 26 weeks and 25 to 35 lb at 40 weeks. Real weight gain isn't a straight line, so treat the weekly range as a guide to the trend rather than a target for any one week. The IOM gives no weekly rates for twins, so the calculator shows twins only the total.
How to work out your BMI
BMI is your weight in kilograms divided by your height in meters squared, or 703 times your weight in pounds divided by your height in inches squared. Someone 5 ft 5 in (65 in) tall who weighed 160 lb before pregnancy has a BMI of 703 × 160 ÷ 65² = 26.6, which falls in the overweight group, with a recommended gain of 15 to 25 lb. BMI is a rough measure, which is one reason your provider may set a different goal.
Where the weight goes
Pregnancy weight isn't only fat: it includes your baby, the placenta, amniotic fluid, a larger uterus and breasts, and more blood and fluid. Gaining within the recommended range is linked with healthier outcomes for both parent and baby, which is why the IOM set ranges by BMI.
You don't need to eat for two. The CDC says there's no need for extra calories in the first trimester, and that you typically add about 340 calories a day in the second trimester and about 450 in the third.