Pregnancy Weight Gain Calculator
See the weight gain the Institute of Medicine guidelines recommend for your pre-pregnancy BMI — total range, where you should be at your current week, the weekly rate after week 13, and twin targets.
How you compare
Why the target depends on your starting BMI
The Institute of Medicine (now the National Academy of Medicine) found the healthiest gain range shrinks as pre-pregnancy BMI rises: 28–40 lb starting underweight, 25–35 lb at a normal BMI, 15–25 lb overweight, and 11–20 lb at a BMI of 30+. Staying inside the band is linked to fewer C-sections, fewer very large or very small babies, and less postpartum weight retention. Pattern matters as much as total: only 1–4½ lb should arrive in the whole first trimester, the rest at a steady weekly rate after week 13.
How it’s calculated
Pre-pregnancy BMI = 703 × lb ÷ in² (or kg ÷ m²) selects your IOM category and total range. First trimester: gain scales linearly to 1.1–4.4 lb (0.5–2 kg) by week 13. Weeks 14–40: the remaining gain is spread evenly, so the target at week w is the trimester-1 band plus (total − trimester-1) × (w − 13)/27. Weekly rate = (total − trimester-1 gain) ÷ 27 weeks, which reproduces the IOM’s published ~1, ~0.6, and ~0.5 lb/week rates. Twin targets use the IOM provisional twin ranges.
Informational only, not medical advice — targets shift with your history and how the pregnancy is going, so confirm your range with your provider.
Week-by-week target gain
IOM/NAM 2009 guideline ranges interpolated by week; twin ranges are provisional.
Worked example
Pre-pregnancy 5′6″ and 140 lb is a BMI of 22.6 — the normal-weight category, so the IOM total is 25–35 lb. At week 20 the on-track band is 7.3–12.3 lb gained (147.3–152.3 lb on the scale), and the target rate from here is 0.9–1.1 lb per week. Weighing 150 lb today means 10 lb gained — inside the band.
Common mistakes
- Using today’s BMI instead of pre-pregnancy BMI — the categories are defined from your starting weight.
- Weighing daily and reacting to water-weight noise; weekly trends are what the guideline describes.
- Applying singleton ranges to a twin pregnancy (twin targets are roughly 10–20 lb higher).
- Trying to “catch down” by dieting mid-pregnancy — weight loss during pregnancy is not recommended at any BMI.
Where it is used
- Prenatal visits — the same bands your OB or midwife plots your weight against.
- Meal planning: turning “eat for two” into a concrete ~340–450 extra calories a day.
- Twin pregnancies, where the target band is easy to misjudge.
- Postpartum planning — fat stores of 6–8 lb are part of the design, not overshoot.
Frequently asked questions
What are the IOM recommended ranges?
For a single baby, by pre-pregnancy BMI: underweight (BMI under 18.5) 28–40 lb; normal weight (18.5–24.9) 25–35 lb; overweight (25–29.9) 15–25 lb; obese (30+) 11–20 lb. For twins the provisional ranges are 37–54 lb (normal), 31–50 lb (overweight), and 25–42 lb (obese). Source: IOM/NAM 2009 guidelines, endorsed by ACOG.
How much extra should I eat?
ACOG’s rule of thumb is no extra calories in the first trimester, roughly 340 extra calories a day in the second, and about 450 extra a day in the third — a sandwich and a glass of milk, not double portions. Calorie needs vary with activity and starting weight, so treat these as starting points.
What if I’m expecting twins?
Switch the twins toggle to Yes and the calculator applies the IOM provisional twin ranges. There is no published twin range for an underweight pre-pregnancy BMI — the evidence was too thin — so in that case the tool tells you to set a target with your provider.
I’m outside the range — should I diet?
No. Weight loss during pregnancy is not recommended even at higher BMIs; restricting can shortchange the baby’s growth. If you are trending above or below your band, bring the chart to your next prenatal visit — providers adjust targets for your history, not the average.
Where does the weight actually go?
Roughly (per Mayo Clinic): baby 7–8 lb, placenta about 1.5 lb, amniotic fluid about 2 lb, uterus 2 lb, breast tissue 1–3 lb, extra blood 3–4 lb, extra fluid 2–3 lb, and fat stores for breastfeeding 6–8 lb. Most of the gain is infrastructure, not fat.