Pregnancy Weight Gain Calculator Formula
Understand the math behind the pregnancy weight gain calculator. Each variable explained with a worked example.
Formulas Used
Recommended Gain (Low)
recommended_low = total_gain_lowRecommended Gain (High)
recommended_high = total_gain_highExpected Gain by Now (Low)
expected_so_far_low = weeks_pregnant <= 13 ? total_gain_low * weeks_pregnant / 40 : total_gain_low * 0.25 + (total_gain_low * 0.75) * (weeks_pregnant - 13) / 27Pre-Pregnancy BMI
pre_bmi = bmi_preVariables
| Variable | Description | Default |
|---|---|---|
pre_weight_lbs | Pre-Pregnancy Weight(lbs) | 140 |
height_inches | Height(inches) | 65 |
weeks_pregnant | Weeks Pregnant(weeks) | 20 |
bmi_pre | Derived value= (pre_weight_lbs * 703) / (height_inches * height_inches) | calculated |
total_gain_low | Derived value= bmi_pre < 18.5 ? 28 : (bmi_pre < 25 ? 25 : (bmi_pre < 30 ? 15 : 11)) | calculated |
total_gain_high | Derived value= bmi_pre < 18.5 ? 40 : (bmi_pre < 25 ? 35 : (bmi_pre < 30 ? 25 : 20)) | calculated |
How It Works
IOM Pregnancy Weight Gain Guidelines
| Pre-Pregnancy BMI | Recommended Gain | |---|---| | Underweight (<18.5) | 28-40 lbs | | Normal (18.5-24.9) | 25-35 lbs | | Overweight (25-29.9) | 15-25 lbs | | Obese (30+) | 11-20 lbs |
Most weight gain occurs in the second and third trimesters, with about 1-2 lbs per week being typical for normal-weight women.
Worked Example
A 140 lb, 65-inch tall woman at 20 weeks pregnant.
pre_weight_lbs = 140height_inches = 65weeks_pregnant = 20
- 01Pre-pregnancy BMI = (140 x 703) / (65 x 65) = 23.3 (Normal)
- 02Recommended total gain = 25-35 lbs
- 03Expected by week 20 = 25 x 0.25 + (25 x 0.75) x 7/27 = 6.25 + 4.9 = 11.1 lbs (low end)
Ready to run the numbers?
Open Pregnancy Weight Gain Calculator