Where the ranges come from
The targets used here are the Institute of Medicine (IOM, 2009) guidelines — the standard reference for pregnancy weight gain in the US and widely used elsewhere. They start from one insight: the healthy amount to gain depends on where you started, measured by your pre-pregnancy BMI. A lower starting BMI supports a higher gain; a higher starting BMI, a lower one.
How the week-by-week band works
Weight gain isn't linear. A typical first trimester adds only 0.5–2 kg (1–4.5 lb) in total; the steady climb happens in the second and third trimesters. This calculator anchors those two facts and interpolates a healthy band for your exact week, so a week-16 reading isn't judged against a week-40 target.
Reading your result
Within the range means your trajectory matches the guideline band — carry on. Below or above is a conversation starter, not a verdict: fluid shifts, morning sickness, multiple pregnancies and pre-existing conditions all move the number, and one weigh-in matters far less than the trend across weeks. Save results to your history and the trend builds itself.
Twins
Expecting two changes the arithmetic — the IOM's provisional twin ranges are substantially higher and appear automatically when you tick the twins box. For underweight mothers of twins no official range exists, so the calculator says so rather than inventing one.
What weight gain is made of
At term, a typical 13 kg gain is mostly not fat: roughly 3.5 kg of baby, 1.5 kg of placenta and amniotic fluid, 1 kg of uterus and breast tissue, 1.5 kg of blood, 1.5–2 kg of extra fluid — with about 3–4 kg of maternal energy stores for breastfeeding.
ListCalc health tools are educational estimates, not medical advice. Dates and ranges here are population averages — your midwife, OB-GYN or doctor confirms what applies to you. See our disclaimer.