How much weight is healthy to gain while pregnant depends mostly on where you started. The guidance used in the US and by many health services elsewhere – the Institute of Medicine’s 2009 report – sets a range for each BMI group before pregnancy, and a weekly pace for the second and third trimesters. Enter your weight before pregnancy and your height; add the week you are in to see where you might be now.
Example: 62 kg (137 lb) before pregnancy, 165 cm (5 ft 5 in) tall: a BMI of 22.8, a normal weight. The recommended total is 11.5–16 kg (25–35 lb). At week 20 the calculator’s curve gives 3.4–5.6 kg (7–12 lb) gained so far.
The IOM 2009 ranges (one baby)
| BMI before pregnancy | Total gain | Weekly in the 2nd and 3rd trimesters |
|---|---|---|
| Underweight, below 18.5 | 28–40 lb (12.5–18 kg) | 1–1.3 lb (0.44–0.58 kg) |
| Normal, 18.5–24.9 | 25–35 lb (11.5–16 kg) | 0.8–1 lb (0.35–0.50 kg) |
| Overweight, 25–29.9 | 15–25 lb (7–11.5 kg) | 0.5–0.7 lb (0.23–0.33 kg) |
| Obese, 30 and above | 11–20 lb (5–9 kg) | 0.4–0.6 lb (0.17–0.27 kg) |
Twins (provisional): normal weight 37–54 lb (17–25 kg), overweight 31–50 lb (14–23 kg), obese 25–42 lb (11–19 kg).
Week by week, normal BMI
The first trimester is assumed to add 0.5–2 kg (1–4 lb). After week 13 the calculator draws a straight line to the total range at week 40, so the curve ends exactly on the recommendation; its slope (0.41–0.52 kg a week for a normal BMI) is close to the IOM weekly rates.
| Week | Gained so far (kg) | Gained so far (lb) |
|---|---|---|
| 8 | 0.3–1.2 | 1–3 |
| 13 | 0.5–2.0 | 1–4 |
| 16 | 1.7–3.6 | 4–8 |
| 20 | 3.4–5.6 | 7–12 |
| 24 | 5.0–7.7 | 11–17 |
| 28 | 6.6–9.8 | 15–22 |
| 32 | 8.2–11.9 | 18–26 |
| 36 | 9.9–13.9 | 22–31 |
| 40 | 11.5–16.0 | 25–35 |
Weeks are counted from the first day of your last period, as in prenatal care; the due date calculator tells you which week you are in.
Reading the result
- The range is a guide, not a target to hit each week. Gains come unevenly: a heavy week can be fluid, and a light one can follow nausea.
- Gaining well above the range is linked with a higher chance of gestational diabetes, a large baby and weight that stays after birth; well below, with a baby who is small for its age. Your care team watches both.
- Other countries may use their own advice. The UK’s NICE, for example, does not set gain targets but asks for weight to be measured at booking. Follow the guidance your doctor or midwife gives you.
Your weight and dates are calculated on your device and never sent anywhere. Once zet4u is saved there, the calculator works offline – in a waiting room with no reception, too.
Frequently asked questions
How much weight should I gain during pregnancy?
It depends on your BMI before pregnancy. The Institute of Medicine's 2009 guidance, used by ACOG and the CDC: 28–40 lb (12.5–18 kg) if you were underweight, 25–35 lb (11.5–16 kg) at a normal weight, 15–25 lb (7–11.5 kg) if overweight and 11–20 lb (5–9 kg) with obesity – for one baby.
How much weight should I gain in the first trimester?
Not much: the guidance assumes 1.1 to 4.4 lb (0.5 to 2 kg) in all over the first 13 weeks, whatever your BMI. Many women gain little or even lose some because of nausea. Most of the gain comes in the second and third trimesters.
How much weight should I gain with twins?
The IOM's provisional ranges for twins are 37–54 lb (17–25 kg) at a normal weight, 31–50 lb (14–23 kg) if overweight and 25–42 lb (11–19 kg) with obesity. No range was set for women who were underweight; your doctor or midwife will give you one.
Where does the weight go?
At term, roughly: the baby about 7.5 lb (3.4 kg), the placenta and amniotic fluid about 3–4 lb, a bigger uterus and breasts about 3–4 lb, extra blood and body fluid about 7–8 lb, and the rest is fat stored for breastfeeding. That is why the gain is far more than the baby's weight.
Sources
- Institute of Medicine and National Research Council: Weight Gain During Pregnancy – Reexamining the Guidelines (2009)
- US CDC: Weight gain during pregnancy
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