CalcVitalsPregnancy Weight Gain Calculator
Recommended gain depends on where you start: for a normal starting BMI (18.5–24.9), the Institute of Medicine range is 11.5–16 kg across the pregnancy; underweight starts gain more (12.5–18 kg), overweight less (7–11.5 kg), and obesity least (5–9 kg).
How it works
Pregnancy weight gain is one of the few numbers in maternity care with an official target range, set by the Institute of Medicine (now the National Academy of Medicine) after reviewing outcomes for mothers and babies. The logic is a balance: too little gain raises the risk of a small, early baby; too much raises the risks of a large baby, caesarean delivery, and weight retained long after the birth. The right band depends on your starting BMI, which is why the categories come first on this page.
The shape of gain matters as well as the total. The guidelines expect only 0.5–2 kg across the whole first trimester — the baby is tiny; most early change is fluid and tissue preparation — then a steadier weekly rate through the second and third trimesters, printed with your result as the expected range at your current week. A gain running consistently outside the corridor, in either direction, is worth raising at a check-up, not panicking over a single weigh-in.
What the gain consists of surprises people: at term, the baby is typically only about 3–3.5 kg of a 12 kg gain. The rest is placenta, amniotic fluid, a doubled blood volume, the uterus and breasts, and maternal stores — nearly all of it with a job to do, and most of it gone within weeks of delivery.
The method
Institute of Medicine (2009) gestational weight gain guidelines by pre-pregnancy BMI class: underweight 12.5–18 kg, normal 11.5–16 kg, overweight 7–11.5 kg, obese 5–9 kg (singleton). Expected-at-week range: 0.5–2 kg in trimester 1, then class-specific weekly rates of 0.17–0.58 kg/week.
Frequently asked questions
What if I have already gained more than the range?
Mention it at your next appointment rather than dieting — pregnancy is not the time for weight loss attempts. Care teams usually respond by watching the trend and reviewing diet quality and activity, not by restricting intake sharply.
I have gained almost nothing in the first trimester. Is that normal?
Very common — nausea and food aversions often hold first-trimester gain to near zero, and the guideline itself only expects 0.5–2 kg by week 13. Persistent vomiting or actual weight loss beyond a couple of kilograms is worth reporting, though.
Do the ranges apply to twins?
No — twins have their own higher ranges (normal BMI: 16.8–24.5 kg). This calculator's figures are for singleton pregnancies, as stated in its note.