How much weight should pregnancy add? The Institute of Medicine answered in 2009 with ranges keyed to pre pregnancy BMI, and those ranges still guide obstetric care worldwide. Pick your category for your target.
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Where the gained weight actually goes
Pregnancy weight gain is not a single blob of fat. At term in a typical normal weight pregnancy, the baby accounts for about three and a half kilograms, the placenta and amniotic fluid another two, expanded blood volume and breast tissue add three more, the growing uterus one, and the remaining four or so kilograms are maternal stores that fuel late pregnancy and breastfeeding. Understanding the accounting makes the ranges feel less arbitrary and the scale less personal.
Why the starting BMI sets the target
The 2009 Institute of Medicine report pooled outcomes across hundreds of thousands of pregnancies and found a U shaped curve. Gaining too little raises risks of small babies and preterm birth, while gaining too much raises cesarean rates, large babies, and weight that lingers for years after delivery. The sweet spot slides with starting BMI. Underweight mothers do best gaining 12.5 to 18 kilograms, normal weight mothers 11.5 to 16, overweight mothers 7 to 11.5, and mothers with obesity 5 to 9. Twin pregnancies carry their own higher schedules.
The rhythm matters as much as the total
First trimester barely moves the scale, a kilogram or two at most, since the baby weighs grams. The steady climb runs through the second and third trimesters at roughly half a kilogram weekly for normal weight mothers, slower for higher starting BMIs. A smooth trajectory along the right weekly slope beats a last minute scramble either direction.
Working with the number honestly
These are population targets, and individual pregnancies wander within them healthily under midwife or obstetric guidance. The pattern that earns attention is a sudden jump, which can signal fluid retention worth reporting, or complete stall in the second half. Pregnancy is not a dieting season in either direction, and any deliberate restriction belongs in a conversation with your care team, not a calorie app.