Pregnancy Weight Gain Calculator

Pregnancy weight gain guidelines are based on pre-pregnancy Body Mass Index (BMI), as established by the Institute of Medicine (IOM) in their landmark 2009 guidelines.
Weight gain during pregnancy supports the growing baby, placenta, amniotic fluid, and necessary changes in the mother’s body.

Pre-pregnancy BMI determines the recommended range:

BMI = Weight (kg) / Height (m)²

Or in imperial: BMI = (Weight lbs / Height inches²) × 703

IOM recommended total weight gain by pre-pregnancy BMI:

The blank twin cell is not an omission. The IOM had too few twin pregnancies with an underweight starting BMI to derive a range, and declined to invent one. The single-baby figure is definitely too low in that situation, so it has to come from your provider.

The IOM printed both units itself, and the pounds are not an exact conversion of the kilograms (17 kg is 37.5 lb, 25 kg is 55.1). This calculator uses whichever set matches the units you pick, so a metric result lands on 11.5 to 16 kg rather than a converted 11.3 to 15.9. The twin ranges are labelled provisional in the report.

Typical weight gain pattern by trimester:

Only a small part of the total belongs to the first trimester, which is why gaining almost nothing in the first three months is normal and not a cause for concern.

The weekly rate is the IOM’s published one. The band on the chart rises a little faster than it, about 0.9 to 1.1 lb a week for a normal BMI, because it is drawn to land exactly on the published totals and the IOM rounded its rates and its totals separately. Neither figure is precise enough for the difference to matter at an appointment.

Where the pregnancy weight goes (at term, for a normal-weight woman gaining 30 lbs):

Worked example:

Pre-pregnancy weight: 145 lbs, Height: 5'5"
BMI = (145 / 65²) × 703 = (145 / 4225) × 703 = 24.1 → Normal weight category
Recommended total gain: 25–35 lbs

At week 20, the expected range is 7.2 to 12.0 lbs. That comes from 1–4 lbs by week 13, then 7 more weeks along the band, which rises 0.89–1.15 lb a week. Someone who has gained 8 lbs at week 20 is inside that range.

The curve is anchored on the published total, so at week 40 it lands on exactly 25 and 35 rather than near them.

The band is wide, and deliberately so. The lower edge assumes you gained the minimum early and are gaining at the slower end; the upper edge assumes the opposite of both. Landing anywhere between them is normal, and where you sit within the band matters much less than the trend from one appointment to the next.

When weight gain is outside the range:

• Too little weight gain is associated with premature birth and low birth weight
• Too much weight gain is associated with gestational diabetes, C-section delivery, and heavier birth weight
• Both extremes carry risk, which is why the trend gets checked at every appointment rather than once

Important: These are population-level guidelines. Every pregnancy is individual.
Always follow your OB, midwife, or healthcare provider’s personalized recommendations for your specific situation.