Pregnancy Weight Gain Calculator
The Institute of Medicine recommended gain for your pre-pregnancy BMI, week by week.
How much weight to gain in pregnancy depends almost entirely on where you started. The ranges below are the Institute of Medicine guidelines that maternity services across the world work to, converted into a target for the week you are in now.
How this is worked out
For a single baby the IOM recommends 12.5–18 kg if you started underweight (BMI under 18.5), 11.5–16 kg at a healthy weight, 7–11.5 kg if overweight and 5–9 kg with obesity. For twins the provisional ranges are 16.8–24.5 kg at a healthy weight, 14.1–22.7 kg if overweight and 11.3–19.1 kg with obesity.
Most of the gain belongs to the second and third trimesters. The guidance allows 0.5 to 2 kg in total across the first trimester and then a weekly rate — roughly 0.44 to 0.58 kg a week if you started underweight, 0.35 to 0.50 at a healthy weight, 0.23 to 0.33 if overweight and 0.17 to 0.27 with obesity. The week-by-week figure above is built from exactly that.
These are population ranges, and being outside them for a week or two is common and usually unremarkable. Your midwife or obstetrician is tracking this alongside your scans and blood pressure, and their advice for your pregnancy overrides any chart.
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Common questions
Do I need to eat for two?
No. The extra energy need is roughly 340 kcal a day in the second trimester and 450 in the third — a substantial snack, not a second dinner. The first trimester needs no increase at all.
I have gained more than the range. What now?
Do not try to lose weight while pregnant unless your doctor has specifically advised it. The usual approach is to slow the rate of gain through food quality and gentle activity, and it is a conversation to have at your next appointment.