How the recommendation is set
In 2009 the Institute of Medicine (IOM, now the National Academy of Medicine) published guidelines for weight gain in pregnancy that are still the reference used by the CDC. The recommended gain depends on your body mass index (BMI) before pregnancy, because people who start at a lower weight are advised to gain more and people who start at a higher weight less.
| Pre-pregnancy BMI | Total gain, one baby | Weekly gain, 2nd–3rd trimester | Total gain, twins |
|---|---|---|---|
| Underweight (below 18.5) | 28–40 lb (12.5–18 kg) | 1–1.3 lb (0.44–0.58 kg) | No recommendation |
| Normal weight (18.5–24.9) | 25–35 lb (11.5–16 kg) | 0.8–1 lb (0.35–0.50 kg) | 37–54 lb (17–25 kg) |
| Overweight (25.0–29.9) | 15–25 lb (7–11.5 kg) | 0.5–0.7 lb (0.23–0.33 kg) | 31–50 lb (14–23 kg) |
| Obesity (30.0 and above) | 11–20 lb (5–9 kg) | 0.4–0.6 lb (0.17–0.27 kg) | 25–42 lb (11–19 kg) |
The IOM calls the twin figures provisional and found too little information to give a range for twins when the pre-pregnancy BMI is under 18.5, so the calculator says so instead of inventing one. The report publishes pounds and kilograms separately, each rounded on its own (25 lb is 11.3 kg, but the table says 11.5 kg), so the calculator shows the report’s own figures in the units you choose.
The calculator works out your BMI (as in the BMI calculator), picks the range, and — for one baby — the recommended gain up to your current week. If you enter your current weight, it shows your gain so far next to that range.
How the weekly range is worked out
The IOM gives a first-trimester gain and a weekly rate for the rest of pregnancy, not a week-by-week curve. Its calculations assume a gain of 0.5–2 kg (1.1–4.4 lb) in the first trimester. This calculator spreads that evenly over the first 13 weeks and adds the weekly rate after that:
Gain by week w = T × w ÷ 13
Gain by week w = T + R × (w − 13)
- w
- completed weeks of pregnancy (days ÷ 7 when a due date is used)
- T
- first-trimester gain: 1.1 lb (0.5 kg) for the lower end, 4.4 lb (2 kg) for the upper end
- R
- the weekly rate for your BMI class: lower end for the lower line, upper end for the upper line
The IOM set the weekly rates and the total ranges separately, so they don’t line up exactly: for normal weight, the weekly rates lead to 22.7–31.4 lb at 40 weeks, while the total recommendation is 25–35 lb. The total range is the recommendation for the whole pregnancy; the weekly figures are a guide to the pace along the way.
With a due date, the week comes from today’s date: gestational age = 280 days − days left until the due date, the same dating rule the due date calculator uses.
Worked example
Someone who is 5 ft 5 in tall, weighed 135 lb before pregnancy and is 20 weeks pregnant with one baby:
- Height: 65 in × 0.0254 = 1.651 m. Weight: 135 lb × 0.45359237 = 61.24 kg.
- BMI: 61.24 ÷ 1.651² = 22.5, normal weight.
- Recommended total gain: 25–35 lb, at 0.8–1 lb per week in the second and third trimesters.
- By week 20: 1.1 + 0.8 × (20 − 13) = 6.7 lb to 4.4 + 1 × 7 = 11.4 lb.
- With a current weight of 145 lb, the gain so far is 10 lb, within the range for week 20. At 148 lb (13 lb gained) it would be above it.
In metric units, 165 cm and 60 kg give a BMI of 22.0 and a recommended total of 11.5–16 kg.
Who these ranges are not for
- Triplets or more. The IOM guidelines cover one baby and, provisionally, twins only.
- Underweight with twins. There is no IOM range; a clinician sets the goal.
- Uncertain pre-pregnancy weight. The class depends on weight before pregnancy. A weight measured well into pregnancy already includes some pregnancy gain and can put you in a higher class.
- BMI’s blind spots. BMI does not distinguish muscle from fat and uses the same cut-offs for everyone, which may fit some bodies poorly.
- Medical conditions. Conditions such as gestational diabetes, severe nausea and vomiting, or a baby growing larger or smaller than expected can change what your care team recommends.
Being above or below a line in one week is common and is not a diagnosis. Weigh at the same time of day on the same scale, and discuss the trend with your prenatal care team rather than a single number. For energy needs, see the calorie calculator.
Frequently asked questions
How much weight should I gain during pregnancy?
It depends on your BMI before pregnancy. The IOM recommends 28–40 lb for underweight, 25–35 lb for normal weight, 15–25 lb for overweight and 11–20 lb for a BMI of 30 or more, for a pregnancy with one baby.
How much weight should I gain with twins?
The IOM’s provisional ranges for twins are 37–54 lb for normal weight, 31–50 lb for overweight and 25–42 lb for obesity. It gives no range for underweight women expecting twins because there was not enough information.
How much weight should I gain in the first trimester?
The IOM’s calculations assume about 1.1–4.4 lb (0.5–2 kg) over the first trimester for all BMI classes. Most of the gain comes in the second and third trimesters, at a steadier weekly rate.
Why doesn’t the week 40 value match the total range?
The IOM published the weekly rates and the total ranges separately, and they don’t add up exactly. For normal weight the weekly rates reach 22.7–31.4 lb by week 40, while the total recommendation is 25–35 lb. Use the total range as the overall goal.
What should I do if I am above or below the range?
Talk to your prenatal care team. A single reading outside the range is common and can reflect fluid, the time of day or a different scale; they can look at the trend and your health and advise on what fits you.
Sources
Last reviewed September 19, 2026