How it works
BMI is weight in kilograms divided by height in metres squared. The IOM sorts it into four categories using World Health Organization cut-offs, then gives a total range for each [1]. For the second and third trimesters it also gives a weekly rate, with a first-trimester gain of 0.5 to 2 kg assumed.
BMI = kg ÷ m² So far (week w ≥ 14) = 0.5 + low rate × (w − 13) to 2 + high rate × (w − 13)
The totals and rates are the IOM’s own; the “so far” sum is worked from them.
| Pre-pregnancy BMI | Total, lb | Total, kg | Weekly, lb (2nd and 3rd trimester) | Weekly, kg |
|---|---|---|---|---|
| Underweight (under 18.5) | 28–40 | 12.5–18 | 1 (1–1.3) | 0.51 (0.44–0.58) |
| Normal weight (18.5 to 24.9) | 25–35 | 11.5–16 | 1 (0.8–1) | 0.42 (0.35–0.5) |
| Overweight (25.0 to 29.9) | 15–25 | 7–11.5 | 0.6 (0.5–0.7) | 0.28 (0.23–0.33) |
| Obese (30.0 and over) | 11–20 | 5–9 | 0.5 (0.4–0.6) | 0.22 (0.17–0.27) |
Twins (provisional, total): normal weight 37–54 lb (17–25 kg), overweight 31–50 lb (14–23 kg), obese 25–42 lb (11–19 kg). None is given for underweight.
Example
Height 165 cm, weight 60 kg, week 27, one baby.
- BMI: 60 ÷ 1.65² = 22.04: normal weight (18.5 to 24.9).
- Total recommended: 25–35 lb (11.5–16 kg).
- So far at week 27: low 0.5 + 0.35 × 14 = 5.4 kg; high 2 + 0.50 × 14 = 9 kg (11.9–19.8 lb).
These are ranges for groups, not advice for you. Ask your doctor or midwife what is right for your pregnancy.
Questions
Where do the numbers come from?
From the 2009 Institute of Medicine (National Academies) report, Table S-1, exactly as published [1]. Nothing is adjusted except the “so far” range, which is worked from the report’s weekly rates.
What if I am expecting twins?
The IOM gives only provisional ranges, for normal weight, overweight and obese women, and “insufficient information” to give one for underweight women [1]. It gives no weekly rate for twins, so no range for your week is shown.
Do these ranges apply to everyone?
The guidelines are intended for women in the United States, and the IOM says to use them with good clinical judgment and a discussion with your care provider [1]. Ask your doctor or midwife what is right for you.
How is the range for my week worked out?
The IOM assumes 0.5 to 2 kg (1.1 to 4.4 lb) gained in the first trimester, then publishes a weekly rate range for the second and third. After week 13 this page adds that rate for each week: low end 0.5 kg plus the low rate, high end 2 kg plus the high rate. It is a worked figure, not a table from the report.
Why is the BMI category based on one decimal?
The report states its categories to one decimal (24.9 and 25.0), so the BMI is rounded to one decimal first: 24.96 counts as 25.0.