What the pregnancy weight gain calculator does
The pregnancy weight gain calculator shows the healthy weight gain range for your pregnancy, based on the widely used Institute of Medicine (IOM, now NASEM) 2009 guidelines. It first works out your pre-pregnancy BMI from your starting weight and height, then matches that to the recommended total gain for a single baby or twins. If you enter your current week, it also estimates how much gain is expected by now, so you can see whether you are tracking within the recommended band.
The IOM guidelines by BMI category
For a single baby, the recommended total gain is:
- Underweight (BMI under 18.5): 12.5–18 kg (28–40 lb)
- Normal weight (18.5–24.9): 11.5–16 kg (25–35 lb)
- Overweight (25–29.9): 7–11.5 kg (15–25 lb)
- Obese (30+): 5–9 kg (11–20 lb)
For twins the ranges are higher — for example 16.8–24.5 kg (37–54 lb) at a normal starting BMI. The guidelines are tailored to BMI because the healthiest amount of gain genuinely differs depending on where you start.
How the weight is distributed
Most pregnancy weight gain happens after the first trimester. The IOM suggests only about 0.5–2 kg (1–4.4 lb) in the first trimester, followed by a steadier weekly rate through the second and third — roughly 0.35–0.5 kg per week for someone starting at a normal weight, and a little less for those starting heavier. That is why the "recommended by this week" figure rises slowly early on and then climbs more steadily.
Where the weight actually goes
It is a common surprise that the baby is a minority of the total. A typical 12.5 kg gain includes around 3–3.5 kg of baby, plus the placenta, amniotic fluid, a much larger blood and fluid volume, an enlarged uterus and breasts, and some fat stores that help support breastfeeding. So gaining the recommended amount is supporting a whole system, not just the baby's growth.
If you are outside the range
Gaining more or less than recommended is associated with higher risks, but the right response always depends on your individual situation — which is a conversation for your provider, not a calculator. Critically, pregnancy is never the time for a restrictive diet: the goal is steady, healthy gain with good nutrition, not weight loss. Use these numbers as a general guide and let your midwife or doctor interpret them for you.
Use with the other tools
To find your starting BMI category on its own, use the BMI calculator. To track your pregnancy timeline, see the pregnancy calculator, and for your delivery date, the due date calculator.