How the recommended range is determined
Pre-pregnancy BMI sorts you into a weight category, and the IOM's 2009 guidelines assign each category a total weight-gain target for the full pregnancy.
- weight — pre-pregnancy body weight (kg)
- height — height in meters (cm ÷ 100)
- BMI — pre-pregnancy Body Mass Index (sets the category)
More detail
What the IOM recommended range means
In 2009 the Institute of Medicine set total pregnancy weight-gain targets by pre-pregnancy BMI category. Lower-BMI mothers are advised to gain more, higher-BMI mothers less, balancing healthy fetal growth against maternal health risks.
Why twins has no underweight range
The 2009 IOM guidelines provide targets for normal, overweight, and obese twin pregnancies, but not for underweight — the evidence base was judged insufficient. If you're underweight and expecting twins, work with your obstetrician on an individualized target.
Practical tip. Gain isn't evenly spread across pregnancy — roughly 1–2 kg in the first trimester, then about 0.3–0.5 kg per week (singleton) in the second and third. The range above is a total-pregnancy target, not a weekly pace.
Frequently asked questions
What's the recommended weight gain for a pre-pregnancy weight of 55kg at 160cm?
BMI works out to 55/1.6²≈21.5, which falls in the normal category (18.5–24.9) — the IOM's recommended singleton range is 11.5–16kg total. Enter your own weight and height above to see your exact category.
I'm underweight and expecting twins — what range should I target?
The 2009 IOM guidelines don't provide a recommended range for underweight twin pregnancies due to insufficient evidence. Work with your obstetrician to set an individualized target instead of relying on a standard range.
Why is the obese-category target only 5–9kg?
Extra weight gain on top of higher pre-pregnancy body fat raises the risk of complications like gestational diabetes and hypertension, so the IOM sets the smallest recommended range — 5–9kg — for the obese (BMI ≥30) category.
Is this range for the whole pregnancy or a weekly rate?
It's a total-pregnancy target from conception to delivery, not a weekly pace — this calculator doesn't break down gain by trimester or week.
Where do these numbers come from?
From the National Academies Press report "Weight Gain During Pregnancy: Reexamining the Guidelines" (2009), by the Institute of Medicine and National Research Council Committee to Reexamine IOM Pregnancy Weight Guidelines. This calculator reproduces its BMI-category table directly.