Pregnancy Weight Gain Calculator
See your recommended pregnancy weight gain by pre-pregnancy BMI on the IOM 2009 guidelines — total range, weekly rate, and whether you're on track.
Gaining the right amount of weight in pregnancy supports your baby’s growth and your own health, and lowers the chance of complications for both of you. How much is “right” depends mainly on your weight before pregnancy. This calculator uses your pre-pregnancy BMI and the Institute of Medicine (IOM) 2009 guidelines to show your recommended total gain, how much you should have gained by now, and — if you enter your gain so far — whether you’re on track.
Recommended total gain by pre-pregnancy BMI
The guidelines set a total range for a single baby based on your BMI before pregnancy:
| Pre-pregnancy BMI | Category | Recommended total gain |
|---|---|---|
| Below 18.5 | Underweight | 12.5–18 kg (28–40 lb) |
| 18.5–24.9 | Normal weight | 11.5–16 kg (25–35 lb) |
| 25.0–29.9 | Overweight | 7–11.5 kg (15–25 lb) |
| 30.0 and above | Obese | 5–9 kg (11–20 lb) |
The calculator works out your BMI from your pre-pregnancy weight and height, then applies the matching range.
How gain is spread across pregnancy
Weight gain isn’t meant to be even from week one:
- First trimester (weeks 1–13): a small total gain of about 0.5–2 kg (1–4.5 lb) for all groups.
- Second and third trimesters: a steady weekly rate — roughly 0.35–0.5 kg (about 1 lb) per week at a normal BMI, more if you were underweight and less if you were overweight or had obesity.
The tool combines these into a “where you should be by week X” range so a single total doesn’t mislead you early on.
Twin pregnancies
Carrying twins requires more weight gain. The IOM provisional ranges are about 17–25 kg (37–54 lb) at a normal BMI, 14–23 kg (31–50 lb) if overweight, and 11–19 kg (25–42 lb) with obesity. There isn’t enough evidence for a separate underweight-twin range, so switch on “Twins” and the calculator uses the normal-weight twin range as a reasonable guide.
Why the range matters
- Gaining too little is linked to a higher risk of a small or premature baby.
- Gaining well above the range raises the chance of a large baby, gestational diabetes, high blood pressure, a caesarean birth, and weight that’s harder to lose afterward.
Being outside the range isn’t a verdict — it’s a prompt to check in. Steady, balanced gain matters more than any single week’s number.
Talk to your clinician
These are population guidelines, not a personal prescription. Your midwife or doctor tailors advice to your health, and this is especially important if you started underweight or with obesity, are carrying multiples, or have any pregnancy complication. Use this calculator for general guidance only.
Frequently asked questions
How much weight should I gain in pregnancy?
It depends on your pre-pregnancy BMI. The IOM 2009 guidelines recommend 12.5–18 kg (28–40 lb) if you were underweight, 11.5–16 kg (25–35 lb) at a normal weight, 7–11.5 kg (15–25 lb) if overweight, and 5–9 kg (11–20 lb) if you had obesity. This calculator gives your range and where you should be by your current week.
How much weight gain is normal in the first trimester?
Most of the gain comes later. In the first trimester, a total gain of about 0.5–2 kg (1–4.5 lb) is typical for all BMI groups. After that, a steady weekly rate through the second and third trimesters makes up the rest.
How much should I gain each week?
In the second and third trimesters the IOM suggests roughly 0.35–0.5 kg (about 1 lb) per week at a normal BMI, a little more if you were underweight and less if you were overweight or had obesity. The calculator shows your weekly rate.
How much weight gain is recommended for twins?
Twin pregnancies need more: about 17–25 kg (37–54 lb) at a normal BMI, 14–23 kg (31–50 lb) if overweight, and 11–19 kg (25–42 lb) with obesity. The IOM did not set an underweight-twin range, so this tool uses the normal-weight twin range as a guide there.