Pregnancy Weight Gain Calculator

recommended gain after week 13 = total range × (week − 13) ÷ 27, plus 0–2 kg across the first trimester

pre-pregnancy weight

Weight before pregnancy, in kilograms (kg)

height

Standing height, in metres (m)

week

Week of pregnancy, counted from the first day of the last menstrual period

total range

Total recommended gain for your pre-pregnancy BMI category, in kilograms (kg)

Use it to check that your weight is tracking inside the recommended weight gain band at the week you are in, and to see the target you are heading for at term. It applies to a single baby. If you are carrying twins, or if you started the pregnancy with a BMI of 30 or above, the totals are different and your antenatal team's own chart is the one to follow.

1. 28 weeks, 58 kg before pregnancy, 65 kg now

   1. Pre-pregnancy BMI: 58 ÷ 1.63² = 21.8, which is the normal-weight row
   2. Total recommended gain for that row: 11.5 to 16 kg
   3. Elapsed share of the 27 weeks after the first trimester: (28 − 13) ÷ 27 = 0.56
   4. Minimum: 11.5 × 0.56 = 6.4 kg; maximum: 2 + (16 − 2) × 0.56 = 9.8 kg
   5. You have gained 65 − 58 = 7 kg, inside that range

   Seven kilograms at 28 weeks sits mid-range. Reaching the top of the band by week 40 means gaining 9 kg over the last twelve weeks, about 0.75 kg a week.

2. 36 weeks, 52 kg before pregnancy, 60 kg now

   1. Pre-pregnancy BMI: 52 ÷ 1.65² = 19.1, which is the normal-weight row
   2. Total recommended gain for that row: 11.5 to 16 kg
   3. Elapsed share of the 27 weeks after the first trimester: (36 − 13) ÷ 27 = 0.85
   4. Minimum: 11.5 × 0.85 = 9.8 kg; maximum: 2 + (16 − 2) × 0.85 = 13.9 kg
   5. You have gained 8 kg, about 1.8 kg below the bottom of the range

   The same 8 kg that reads as inside the range at 28 weeks reads as below it at 36, because the band keeps rising and the weight has not.

The published guidelines give a total range for the whole pregnancy and an average weekly rate for the second and third trimesters, not a target for every single week. This calculator interpolates linearly between two anchors — the 2 kg first-trimester ceiling at week 13 and the published total at week 40 — so what it prints is a band that contains the published weekly rate rather than the rate itself. It covers a single baby only. Twins, higher-order multiples, a pre-pregnancy BMI above 30, and any pregnancy with gestational diabetes or hypertension need individual targets from your antenatal team. Weight gain is one measurement among several; it is not a diagnosis.

How much weight to gain in pregnancy is normal?

The total depends on the BMI you started at. On the Institute of Medicine figures, a woman who began at a normal BMI is advised to gain 11.5 to 16 kg across the pregnancy; starting underweight raises that to 12.5 to 18 kg, and starting in the overweight or obese range lowers it to 7 to 11.5 kg or 5 to 9 kg. Almost none of it arrives in the first trimester: the first 13 weeks account for roughly 0.5 to 2 kg.

What does first trimester weight gain look like?

Very little. The Institute of Medicine puts the whole first trimester at about 0.5 to 2 kg, and nausea can mean no gain at all, or a small loss, without anything being wrong. The rate picks up from about week 14, which is why every recommended weight gain by week chart is flat until then and slopes afterwards.

How is the week-by-week range worked out?

Two published anchors and a straight line between them. The first anchor is the 2 kg ceiling at the end of week 13; the second is the total recommended gain for your BMI category at week 40. The calculator takes the fraction of those 27 weeks that has passed and applies it to the total. It is an interpolation, not a figure printed in the guidelines — the guidelines give the total and an average weekly rate.

Do the iom guidelines apply if I am carrying twins?

No, and the difference is large. For twins the 2009 report recommends roughly 17 to 25 kg for a normal pre-pregnancy BMI, well above the 11.5 to 16 kg for a single baby. This calculator covers a single baby only, and it does not ask whether you are carrying more than one.

Why does my pre-pregnancy bmi matter more than my current weight?

Because it selects the row of the table. Two women at the same weight and the same week can have different targets if they started at different weights — that is the whole reason the guidelines are indexed by pre-pregnancy BMI and not by weight. Use the weight you carried in the three months before conception; early-pregnancy weight is an acceptable stand-in when that is not known.

What if my gain is above the range?

It is common, and it is not on its own a complication. Above-range gain is associated with higher birth weight and with weight retained after delivery, so it is worth raising with your midwife or obstetrician — but the target is a population recommendation, and individual pregnancies are tracked on their own curve. Do not cut calories during pregnancy to hit a number.

• Weight Gain During Pregnancy: Reexamining the Guidelines — Institute of Medicine and National Research Council, 2009
• Obesity: preventing and managing the global epidemic (WHO Technical Report Series 894) — World Health Organization