How the calculator works
The calculator uses the guidelines of the Institute of Medicine (IOM), now part of the US National Academies, published in 2009. They are the most widely used recommendations for weight gain in pregnancy and are endorsed by the American College of Obstetricians and Gynecologists.
The recommended gain depends on your body mass index (BMI) before pregnancy: your weight in kilograms divided by your height in metres squared. The calculator works out your BMI, puts it into one of the four standard categories, and shows the matching range for one baby or for twins.
| BMI before pregnancy | One baby | Twins (provisional) |
|---|---|---|
| Underweight: under 18.5 | 12.5 to 18 kg | No IOM range |
| Healthy weight: 18.5 to 24.9 | 11.5 to 16 kg | 17 to 25 kg |
| Overweight: 25 to 29.9 | 7 to 11.5 kg | 14 to 23 kg |
| Obese: 30 or more | 5 to 9 kg | 11 to 19 kg |
If you enter your current week, the calculator also shows what is typical so far. The IOM assumes a gain of about 0.5 to 2 kg in the first trimester (up to week 13). After that it gives a typical weekly gain for each category: about 0.44 to 0.58 kg a week if you were underweight, 0.35 to 0.50 kg at a healthy weight, 0.23 to 0.33 kg if overweight and 0.17 to 0.27 kg if obese. The calculator adds the weekly gain for each week after week 13 to the first-trimester range. For twins, the IOM gives no week-by-week rates.
A worked example
Say you weighed 65 kg before the pregnancy and you are 165 cm tall. Your BMI is 65 divided by 1.65 squared, which is 23.9: a healthy weight. For one baby, the recommended total gain is 11.5 to 16 kg.
At 24 weeks, you are 11 weeks past the first trimester. The low end is 0.5 kg plus 11 weeks at 0.35 kg, about 4.5 kg. The high end is 2 kg plus 11 weeks at 0.5 kg, 7.5 kg. So a gain of roughly 4.5 to 7.5 kg by week 24 is typical.
If you were expecting twins at the same BMI, the provisional range would be 17 to 25 kg.
Where the weight goes
Only about a quarter of the weight you gain is the baby. For a total gain of about 12.5 kg, roughly:
- the baby: about 3.5 kg
- the placenta and amniotic fluid: about 1.5 kg
- a larger womb and breasts: about 1.5 kg
- extra blood and other body fluid: about 3 kg
- fat stores, which help the body prepare for breastfeeding: about 3 kg
That is why a gain within the range is healthy and expected, and why a lot of it goes again in the weeks after the birth.
What the result means
The ranges describe what is linked with the best outcomes for parent and baby in large studies. Gaining well above the range is associated with a higher chance of a large baby, a caesarean birth and keeping extra weight after pregnancy. Gaining well below it is associated with a higher chance of a small baby or an early birth. Within the range, there is plenty of room for normal variation: no two pregnancies follow the same curve.
The weekly figures are averages. Weight goes up and down with fluid, meals and the time of day, so a single weigh-in says little. If you want to keep an eye on it, weigh yourself at the same time of day, every week or two, and look at the trend.
Not every country uses these ranges. In the UK, for example, there are no official weight gain targets, and the NHS notes that most people gain between 10 and 12.5 kg, mostly after week 20. Your midwife may weigh you at your first appointment and then only if there is a reason to.
Limits of this calculator
BMI is a rough measure: it does not tell muscle from fat, and the same cut-offs are used for everyone even though health risks at a given BMI differ between people. The IOM ranges were drawn up mainly from studies in the United States, and the twin ranges are provisional. The calculator also cannot account for conditions such as diabetes, high blood pressure or a history of eating disorders, which change the advice.
Use the result as a starting point for a conversation, not as a goal. Your midwife or doctor knows your history and can tell you what a healthy gain looks like for you, and what to do if you are worried about gaining too much or too little.