Pregnancy Weight Gain Calculator
Recommended pregnancy weight gain for your pre-pregnancy BMI (IOM 2009), for one baby or twins: the total range, how much by your current week, your target weight range now, a week-by-week chart and table, and a check of your current weight against the range.
How it works
pre-pregnancy BMI = weight (kg) ÷ height (m)² · IOM 2009 total gain, one baby: underweight 28–40 lb (12.5–18 kg), normal 25–35 lb (11.5–16 kg), overweight 15–25 lb (7–11.5 kg), obesity 11–20 lb (5–9 kg) · twins: normal 37–54 lb (17–25 kg), overweight 31–50 lb (14–23 kg), obesity 25–42 lb (11–19 kg) · expected gain by week w: first trimester ≈ 1.1–4.4 lb (0.5–2 kg) × w ÷ 13; after week 13, first-trimester gain + (total − first-trimester gain) × (w − 13) ÷ (40 − 13), or ÷ (37 − 13) for twins
The Institute of Medicine (now the National Academy of Medicine) 2009 guidelines base recommended pregnancy weight gain on your pre-pregnancy BMI: the higher the starting BMI, the smaller the recommended gain. Most of that gain is the baby, placenta, amniotic fluid, extra blood and fluid, a larger uterus and breasts, and some fat and protein stores. Little is gained in the first trimester (about 0.5–2 kg, or 1–4 lb), then the weekly rate is fairly steady through the second and third trimesters. The calculator turns your BMI range into a schedule: how much you might have gained by your current week, the weight range that corresponds to, and a week-by-week table and chart to term. Choose Twins for the IOM’s provisional twin ranges, which are larger and are reached by about 37 weeks. Switch on the current-weight comparison to see your gain so far, whether it is below, within or above the range for this week, and how much is left to gain. The ranges are for adults carrying to term; your provider sets your personal target.
Worked example
Someone 5′5″ (1.65 m) weighing 140 lb (63.5 kg) before pregnancy has a BMI of 63.5 ÷ 1.651² = 23.3 — normal weight — so the IOM range is 25–35 lb (11.5–16 kg). By week 20 the schedule gives 1.1 + (25 − 1.1) × 7 ÷ 27 ≈ 7 lb at the low end and 4.4 + (35 − 4.4) × 7 ÷ 27 ≈ 12 lb at the high end, a target weight of 147–152 lb. Weighing 150 lb at that point (10 lb gained) is within the range, with 15–25 lb still to gain over the remaining 20 weeks. With twins the range is 37–54 lb by about 37 weeks.
Sources: Institute of Medicine (2009) — Weight Gain During Pregnancy: Reexamining the Guidelines · CDC — Weight gain during pregnancy · ACOG Committee Opinion 548 — Weight Gain During Pregnancy
Frequently asked questions
Is it bad to gain outside the range?
The ranges are guidance, not strict limits, and one weigh-in isn’t the whole picture. A steady trend matters more than an exact number. Discuss any concerns — too little or too much — with your provider, who tracks the pattern over time.
How much should I gain carrying twins?
More. The IOM suggests roughly 37–54 lb (17–25 kg) for a normal-weight twin pregnancy, 31–50 lb (14–23 kg) for overweight and 25–42 lb (11–19 kg) for the obesity range. The IOM has no figure for underweight; the CDC lists 50–62 lb from a 2003 study. Choose Twins above to see the week-by-week schedule.
I’m barely gaining in the first trimester — is that normal?
Often, yes. First-trimester gain is usually small (about 1–4 lb), and nausea can even cause a slight loss. Most weight is gained later. If you’re worried, check with your provider.
How is the week-by-week figure worked out?
It assumes 0.5–2 kg (1–4 lb) by the end of week 13, then spreads the rest of your total range evenly to week 40 (week 37 for twins). The IOM’s own weekly rates for the second and third trimesters are shown alongside; they give a similar pace.
How many extra calories do I need?
For one baby, typically none in the first trimester, about 340 kcal a day in the second and about 450 in the third. Needs vary with activity and starting weight, and are higher with twins — ask your provider or a dietitian.
Related calculators
Estimates based on standard formulas and average cycles — every pregnancy is different. This is not medical advice; your doctor or midwife, with an ultrasound, gives the accurate dates and guidance.