Pregnancy Weight Gain Calculator (IOM guidelines by BMI and week)

Understanding the pregnancy weight gain calculator — concept and principle

This calculator compares your weight gain so far in pregnancy with the 2009 US Institute of Medicine (IOM) guidelines, which set a recommended total gain and weekly rate based on your BMI before pregnancy. It shows whether you are within the range for your current week.

Gain vs IOM range: with the default inputs the result is 2.9–5.5 kg (5 kg within range).

How it is calculated

Pre-pregnancy BMI = weight before pregnancy (kg) ÷ height (m)²

Recommended total gain (one baby): underweight 12.5–18 kg, normal 11.5–16, overweight 7–11.5, obesity 5–9

Range for this week:

Up to week 13: 0.5 × week ÷ 13 to 2 × week ÷ 13 kg
After week 13: 0.5 + (week − 13) × low weekly rate to 2 + (week − 13) × high weekly rate

Weekly rates after week 13: underweight 0.44–0.58, normal 0.35–0.50, overweight 0.23–0.33, obesity 0.17–0.27 kg.

For twins the provisional totals are 17–25, 14–23 and 11–19 kg, with no weekly ranges.

The principle

Weight gain in pregnancy includes the baby, placenta, fluid, larger womb and breasts, extra blood and some fat stores. Little is gained in the first trimester, then gain is fairly steady. Women who start with less body fat benefit from gaining more, and those with obesity need to gain less, because outcomes such as birth weight, caesarean birth and weight retained after birth were best within these ranges.

Worked example

Default inputs: height 163 cm, 58 kg before, 63 kg now, 20 weeks, one baby.

BMI = 58 ÷ 1.63² = 21.8 → Normal weight; total gain 11.5–16 kg.
Gain so far = 63 − 58 = 5 kg.
Range at week 20: low = 0.5 + 7 × 0.35 = 2.95, shown as 2.9 kg; high = 2 + 7 × 0.50 = 5.5 kg.
5 kg is between them → Within the range; weekly gain 0.35–0.50 kg.

Each point is the calculator re-run with only “Weight now (kg)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Weight now (kg)	Gain so far (kg)
15.8	-42.3
31.5	-26.5
47.2	-10.8
63	5
78.8	20.8
94.5	36.5
126	68

How to read the result

Being slightly outside the weekly range is common and not a cause for alarm. A consistent trend well above or below deserves a conversation with your midwife or doctor. Asian BMI cut-offs are lower, so Korean clinics may use different categories.

Common mistakes

Using weight from early pregnancy instead of before pregnancy.
Comparing weights on different scales or at different times of day.
Dieting to lose weight in pregnancy without medical advice.

Keep in mind: This is general guidance, not a diagnosis. Sudden weight gain with swelling or headache can be a sign of pre-eclampsia and needs prompt care.

Frequently asked questions

How much weight should I gain in pregnancy?

For a normal pre-pregnancy BMI, 11.5–16 kg in total for one baby.

Why is little gain expected early?

The baby and placenta are small in the first trimester; most gain comes later.

What about twins?

The IOM gives higher provisional total ranges but no weekly rates.

Sources: Institute of Medicine and National Research Council, Weight Gain During Pregnancy: Reexamining the Guidelines, National Academies Press, 2009 · ACOG Committee Opinion No. 548, "Weight Gain During Pregnancy" (2013)

Checked 9 Nov 2026. Institute of Medicine and National Research Council, “Weight Gain During Pregnancy: Reexamining the Guidelines” (National Academies Press, 2009), Table S-1 (total and rate of weight gain by pre-pregnancy BMI) and the provisional twin ranges; reaffirmed in ACOG Committee Opinion No. 548 (2013).

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