How the Pregnancy Weight Gain Calculator works
The calculator works out your body mass index (BMI) from your height and your weight before pregnancy, and then shows the weight gain range recommended for that BMI group in the 2009 guidelines from the Institute of Medicine (now the National Academy of Medicine), which are widely used. If you enter how many weeks pregnant you are, it also shows the typical range of gain by that week; add your current weight to see where you are within it.
Recommended weight gain by BMI
| Pre-pregnancy BMI | Total gain (one baby) | Per week, 2nd–3rd trimester | Total gain (twins) |
|---|---|---|---|
| Underweight (below 18.5) | 28–40 lb · 12.5–18 kg | 1–1.3 lb · 0.44–0.58 kg | No guideline |
| Healthy weight (18.5–24.9) | 25–35 lb · 11.5–16 kg | 0.8–1 lb · 0.35–0.5 kg | 37–54 lb · 17–25 kg |
| Overweight (25–29.9) | 15–25 lb · 7–11.5 kg | 0.5–0.7 lb · 0.23–0.33 kg | 31–50 lb · 14–23 kg |
| Obesity (30 or more) | 11–20 lb · 5–9 kg | 0.4–0.6 lb · 0.17–0.27 kg | 25–42 lb · 11–19 kg |
The guidelines assume a first-trimester gain of about 1.1–4.4 lb (0.5–2 kg) for one baby. Twin ranges are provisional and there is no published range for an underweight BMI with twins.
Where does the weight go?
Pregnancy weight is not just fat. At full term, a typical gain is shared between the baby (around 7–8 lb or 3.4 kg), the placenta, amniotic fluid, a larger uterus and breasts, extra blood and body fluid, and some stored energy for breastfeeding. That is why some weight gain is an expected and healthy part of pregnancy.
Why the range matters
Gaining within the recommended range is linked with better outcomes for both parent and baby, while gaining much more or much less is linked with some risks. But the ranges are guides for populations, not personal targets. Many things — nausea in early pregnancy, twins, health conditions, activity and appetite — affect weight gain, and weight can rise unevenly from week to week. If you are concerned about your weight or eating, talk to your doctor, midwife or a registered dietitian.
BMI itself has limits: it does not account for muscle, body shape or ethnicity, so your care team may look at more than BMI. To follow your pregnancy week by week, use the Pregnancy Timeline.