Pregnancy Weight Gain Calculator

Recommended weight-gain range during pregnancy by pre-pregnancy BMI category — Institute of Medicine 2009 guidelines.

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How to use this calculator

  • Enter your pre-pregnancy weight (best from a doctor visit shortly before conception, or first prenatal appointment).
  • Enter your height — fixed since adulthood.
  • Pick your current gestational week from your last menstrual period (LMP) or ultrasound dating.
  • Switch to twin if applicable — the recommended ranges shift up substantially.
  • Compare actual gain to the "expected at week N" line; consult your OB if you're notably above or below.

About this calculator

The Institute of Medicine (now National Academy of Medicine) issued updated pregnancy weight-gain guidelines in 2009, replacing the older 1990 ranges. The new ranges are stratified by pre-pregnancy BMI: underweight women should gain MORE (28-40 lb total) than obese women (11-20 lb), with normal-weight at 25-35 lb. The clinical reason: total maternal-fetal weight gain affects birthweight, gestational diabetes risk, and post-partum weight retention. The first trimester typically contributes just 1-4.5 lb of total gain; the second and third trimesters add ~1 lb/wk (less for higher BMI categories). Twin pregnancies have separate, larger ranges (37-54 lb for normal pre-pregnancy BMI). These guidelines are an evidence-based target, not a strict rule — actual gain depends on appetite, nausea, fluid retention, and complications. Discuss specifics with your OB or midwife.

Frequently asked

Why do underweight women need to gain MORE?+
Larger total gain in underweight women (BMI < 18.5) is associated with lower rates of small-for-gestational-age babies and preterm birth. The IOM 2009 report committee explicitly raised this lower bound vs the 1990 guidelines.
Why do obese women gain LESS?+
Higher-BMI mothers carry more energy reserves; large gestational gain in this group is linked to higher rates of gestational diabetes, hypertension, c-section, and post-partum weight retention. The 11-20 lb range is intentionally lower than the older blanket recommendation.
What about losing weight during pregnancy?+
Intentional weight loss during pregnancy is NOT recommended by ACOG, even for women with obesity. Some women lose weight in the first trimester due to nausea (hyperemesis gravidarum); this typically corrects in the second trimester. Discuss with your OB.
Why is the first trimester so light?+
The fetus + placenta + uterus + blood volume + amniotic fluid + breast tissue all weigh very little at weeks 1-13. Most of the maternal weight gain comes from the rapidly growing fetus and increased blood volume in trimesters 2-3.
How accurate are these ranges?+
They are population-average evidence-based ranges, not individual predictions. The IOM committee notes 25-35 lb covers ~80% of healthy singleton pregnancies but says "individual variability is high."
Twin source?+
IOM 2009 report Table S-1 (twin pregnancies). Underweight twin guidance is marked "insufficient evidence" in the original publication; this tool reports that explicitly.

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