IOM 2009 Pregnancy Weight Gain Guidelines
| Pre-Pregnancy BMI | Category | Total Gain | 2nd-3rd Trim Rate (mean, range) |
|---|---|---|---|
| < 18.5 | Underweight | 12.5 - 18 kg (28-40 lb) | 0.51 (0.44-0.58) kg/wk, 1 (1-1.3) lb/wk |
| 18.5 - 24.9 | Normal weight | 11.5 - 16 kg (25-35 lb) | 0.42 (0.35-0.50) kg/wk, 1 (0.8-1) lb/wk |
| 25.0 - 29.9 | Overweight | 7 - 11.5 kg (15-25 lb) | 0.28 (0.23-0.33) kg/wk, 0.6 (0.5-0.7) lb/wk |
| ≥ 30.0 | Obese | 5 - 9 kg (11-20 lb) | 0.22 (0.17-0.27) kg/wk, 0.5 (0.4-0.6) lb/wk |
| Twins (provisional): Normal 17-25 kg (37-54 lb) · Overweight 14-23 kg (31-50 lb) · Obese 11-19 kg (25-42 lb) | |||
Source: Institute of Medicine (IOM) and National Research Council. Weight Gain During Pregnancy: Reexamining the Guidelines. 2009. The rates assume a gain of 0.5 to 2 kg (1.1 to 4.4 lb) in the first trimester. The IOM did not set a twin range for underweight women because of limited data; the CDC's twin table lists 50 to 62 lb (23 to 28 kg) for them and notes that this differs from the IOM guidelines.
How the week-by-week target is worked out
The IOM gives a total range and an average weekly rate, not a target for every week. We assume 0.5 to 2 kg in the first 13 weeks, as the IOM does, and spread the rest of the total range evenly over weeks 14 to 40. The result stays close to the IOM weekly rates.
Worked example. A woman 165 cm tall weighed 60 kg before pregnancy. BMI = 60 ÷ 1.65² = 60 ÷ 2.7225 = 22.0, normal weight, so her total range is 11.5 to 16 kg. At week 20 she is 7 weeks past the first trimester. Low end: 0.5 + (11.5 − 0.5) × 7 ÷ 27 = 0.5 + 2.85 = 3.4 kg. High end: 2 + (16 − 2) × 7 ÷ 27 = 2 + 3.63 = 5.6 kg. A gain of 5 kg by week 20 is on track.
Where Does the Weight Go?
MedlinePlus, from the US National Library of Medicine, breaks down a gain of about 16 kg (35 lb) like this:
| Component | Weight Added |
|---|---|
| Baby | about 3.5 kg (8 lb) |
| Placenta | 1 - 1.5 kg (2-3 lb) |
| Amniotic fluid | 1 - 1.5 kg (2-3 lb) |
| Breast tissue | 1 - 1.5 kg (2-3 lb) |
| Blood supply | 2 kg (4 lb) |
| Fat stores | 2.5 - 4 kg (5-9 lb) |
| Uterus growth | 1 - 2.5 kg (2-5 lb) |
Why the range matters
Gaining less than recommended is linked with a higher chance of a small baby and preterm birth. Gaining more is linked with a larger baby, cesarean birth and keeping weight after pregnancy. These links are the evidence behind the IOM ranges. In the US, the CDC reports that about 1 in 3 women gain the recommended amount, about 1 in 5 gain less and about half gain more. Most extra calories are needed only later in pregnancy, and there is no need to "eat for two" in the first trimester.
Talk to your care team if your gain is well above or below the band, if you gain weight suddenly, or if you have swelling of the face or hands with headaches or vision changes. Sudden gain and swelling can be signs of preeclampsia, which needs prompt care.