Pregnancy Weight Gain Calculator
lbs
in
Week 20
Recommended Gain So Far12.5 - 17.5 lbs
Total Recommended Range25 - 35 lbs
Pre-Pregnancy BMI CategoryNormal
Healthy weight gain in pregnancy is not a single target but a range that depends on where you started. Following the Institute of Medicine guidelines, this calculator uses your pre-pregnancy BMI to set a recommended total gain for the whole pregnancy, then prorates it to your current week so you can see roughly how much you should have gained so far. Lower starting BMIs are advised to gain more, and higher starting BMIs less.
Formula
gainSoFar = totalRange × (min(week, 40) ÷ 40)
- totalRange
- Recommended full-term gain band set by pre-pregnancy BMI
- week
- Current week of pregnancy (capped at 40)
- 40
- Weeks in a full-term pregnancy used to prorate the gain
How it works
- Enter your pre-pregnancy weight, your height, and how many weeks pregnant you currently are.
- Your pre-pregnancy BMI is computed and matched to a total recommended gain: Underweight 28–40 lb, Normal 25–35 lb, Overweight 15–25 lb, and Obese 11–20 lb.
- That total range is multiplied by your progress through pregnancy (week ÷ 40, capped at 40 weeks) to estimate the gain you should be at by now.
Worked example
A person was 140 lb at 5 ft 5 in (65 in) before pregnancy and is now at week 20.
- Pre-pregnancy BMI ≈ 23.3 → Normal category → total recommended gain 25–35 lb.
- Progress fraction: 20 ÷ 40 = 0.5.
- Gain so far: 25 × 0.5 = 12.5 lb to 35 × 0.5 = 17.5 lb.
A normal-BMI start suggests 25–35 lb total, or roughly 12.5–17.5 lb gained by week 20.
Frequently asked questions
- Why does recommended weight gain depend on starting BMI?
- The Institute of Medicine guidelines tie total gain to pre-pregnancy BMI because people who start at a lower weight benefit from gaining more, while those who start heavier are advised to gain less to reduce pregnancy risks.
- Is weight gain supposed to be even across pregnancy?
- Not exactly. Gain is typically slow in the first trimester and steadier afterward, so this calculator’s even week-by-week prorating is a simplified guide rather than a strict schedule for any single week.
- What if I am carrying twins?
- Twin and multiple pregnancies have higher recommended gain ranges that this single-baby calculator does not cover. If you are expecting more than one baby, follow the targets your healthcare provider sets.
- Should I worry if I am outside the suggested range?
- These are general targets, and being a little above or below them is common. Discuss any significant deviation with your provider, who can account for your individual health and circumstances.