Gaining weight during pregnancy is normal and necessary, but how much is healthy depends on where you started. Too little gain is linked to a baby who is born small, and too much to a larger baby, a higher chance of caesarean delivery and weight that is harder to lose afterwards. The most widely used guidance in the United States comes from the Institute of Medicine (now part of the National Academies), which in 2009 published ranges by pre-pregnancy BMI. This calculator applies those ranges to your height and weight, shows the pace expected at your stage, and compares your gain so far.
Everyone’s pregnancy is different. Use this as a guide for a conversation with your midwife or doctor, not as a target to meet at all costs.
How to use the pregnancy weight gain calculator
- Enter your weight before pregnancy and your height. These give your BMI category, which sets your range.
- Enter your current weight and the week you are in.
- Choose one baby or twins. Twin ranges are provisional.
- Read your recommended total gain, the approximate gain to date, and the comparison with your own gain.
Pregnancy weight gain formula
Gain so far = current weight − pre-pregnancy weight
Approximate gain by week w (single baby, w > 13) = 0.5 + low rate × (w − 13) to 2 + high rate × (w − 13)
The first trimester allows about 0.5 to 2 kg in total (1.1–4.4 lb). After that the Institute of Medicine gives a weekly rate for each BMI group. The range by week is an approximation built from those figures, not an official chart, and individual patterns vary widely. The source is Institute of Medicine and National Research Council, Weight Gain During Pregnancy: Reexamining the Guidelines (National Academies Press, 2009), which the American College of Obstetricians and Gynecologists endorses (Committee Opinion 548, 2013).
| Pre-pregnancy category | BMI | Total gain (single baby) | Weekly gain after week 13 | Twins (provisional) |
|---|---|---|---|---|
| Underweight | below 18.5 | 12.5 – 18 kg (28 – 40 lb) | 0.44 – 0.58 kg (1.0 – 1.3 lb) | Not enough evidence |
| Normal weight | 18.5 – 24.9 | 11.5 – 16 kg (25 – 35 lb) | 0.35 – 0.50 kg (0.8 – 1.1 lb) | 16.8 – 24.5 kg |
| Overweight | 25.0 – 29.9 | 7 – 11.5 kg (15 – 25 lb) | 0.23 – 0.33 kg (0.5 – 0.7 lb) | 14.1 – 22.7 kg |
| Obese | 30.0 and above | 5 – 9 kg (11 – 20 lb) | 0.17 – 0.27 kg (0.4 – 0.6 lb) | 11.3 – 19.1 kg |
Worked example
A woman was 60 kg before pregnancy, is 165 cm tall, and weighs 66 kg at 28 weeks.
- BMI = 60 ÷ 1.65² = 22.0, which is normal weight (18.5–24.9).
- Recommended total: 11.5 – 16 kg for one baby.
- Gain so far = 66 − 60 = 6 kg.
- Approximate range by week 28 = 0.5 + 0.35 × 15 to 2 + 0.50 × 15 = 5.8 – 9.5 kg, so she is within the pace.
Approximate gain by week for a normal-weight start
| Week of pregnancy | Approximate gain to date, normal-weight start |
|---|---|
| 13 | 0.5 – 2.0 kg |
| 20 | 2.9 – 5.5 kg |
| 28 | 5.8 – 9.5 kg |
| 36 | 8.5 – 13.5 kg |
| 40 | 9.9 – 15.5 kg |
Where the weight goes
A gain of about 12 kg at term typically includes the baby (around 3.5 kg), the placenta, amniotic fluid, growth of the uterus and breasts, extra blood and body fluid, and fat stores for breastfeeding. The exact split differs from person to person, which is one reason scale weight alone cannot show how a pregnancy is going.
Healthy habits during pregnancy
- Eat for health, not for two. Extra energy needs are small early on and rise to a few hundred calories a day in later pregnancy; ask your care team.
- Keep moving. Unless told otherwise, regular moderate activity is recommended.
- Do not diet. Intentional weight loss during pregnancy is not recommended without medical guidance.
- Talk about it. Sudden rapid gain, especially with swelling or headaches, can be a sign of a problem and needs prompt medical attention.
To check your BMI before pregnancy use the BMI Calculator. After birth, your baby’s growth can be checked with the Baby Weight Percentile Calculator, and the Healthy Weight Range Calculator helps with later goals.
This page is for information only and is not medical or dietary advice. Speak to a doctor or registered dietitian about your own needs.
Sources
- Institute of Medicine and National Research Council. Weight Gain During Pregnancy: Reexamining the Guidelines. National Academies Press, Washington DC, 2009.
- American College of Obstetricians and Gynecologists. Committee Opinion No. 548: Weight Gain During Pregnancy. Obstet Gynecol 2013;121:210–212.
- World Health Organization. Obesity: preventing and managing the global epidemic. WHO Technical Report Series 894, 2000 (BMI categories).
Frequently asked questions
How much weight should I gain during pregnancy?
It depends on your weight before pregnancy. The US Institute of Medicine (2009) recommends 12.5 to 18 kg (28–40 lb) if underweight, 11.5 to 16 kg (25–35 lb) at a normal weight, 7 to 11.5 kg (15–25 lb) if overweight and 5 to 9 kg (11–20 lb) if obese, for a single baby.
How much should I gain each week?
After a first-trimester gain of about 0.5 to 2 kg, the Institute of Medicine suggests roughly 0.44–0.58 kg a week if underweight, 0.35–0.50 kg at a normal weight, 0.23–0.33 kg if overweight and 0.17–0.27 kg if obese. Weekly changes vary, so look at the trend over several weeks.
What about twins?
The Institute of Medicine gives provisional ranges for twins: 16.8–24.5 kg (37–54 lb) at normal weight, 14.1–22.7 kg (31–50 lb) if overweight and 11.3–19.1 kg (25–42 lb) if obese. There was not enough evidence for an underweight range, so ask your care team.
What if I am gaining more or less than the range?
Do not try to correct it yourself. Weight gain is only one part of a healthy pregnancy, and the right plan depends on your health, your baby’s growth and your care team’s assessment. Raise it at your next appointment.
Does this include the baby, placenta and fluid?
Yes. Total gain includes the baby, placenta, amniotic fluid, larger breasts and uterus, extra blood and fluid, and maternal fat stores. Only a fraction is the baby’s weight, which is why gain is much more than newborn weight.