How to use the Pregnancy Weight Gain
- Enter your pre-pregnancy weight and height so the calculator can set your starting BMI category.
- Enter the week of pregnancy you are currently in.
- Enter today's weight, ideally measured in the morning on the same scale each time.
- Compare your gain against the target band for this week, not just against the full-term total.
- Discuss any sustained deviation with your midwife or obstetrician rather than adjusting intake on your own.
How the calculation works
Targets come from the 2009 Institute of Medicine revision, which sets total gain by pre-pregnancy BMI category: 12.5–18 kg if underweight, 11.5–16 kg at normal weight, 7–11.5 kg if overweight and 5–9 kg if obese. The ranges narrow as BMI rises because women starting heavier already carry energy reserves the pregnancy can draw on.
Gain is not linear. The first trimester accounts for only 0.5–2 kg — the fetus weighs a few grams — and most weeks in that period bring nothing at all, especially with nausea. From week 14 onward the guidelines specify a weekly rate, roughly 0.35–0.5 kg for normal weight and less for higher BMI categories. This calculator builds the week-by-week band from that first-trimester allowance plus the weekly rate applied to the weeks since 13.
Where the weight goes is worth knowing: at term, roughly 3.3 kg is the baby, 0.9 kg placenta, 0.9 kg amniotic fluid, 1 kg uterus, 0.4 kg breast tissue, 1.2 kg extra blood volume and 2.6 kg fluid and fat stores. Gain outside the range is associated in the literature with higher rates of caesarean and large-for-gestational-age babies at the high end, and with preterm birth and low birth weight at the low end — associations, not certainties, and always for a clinician to interpret.
Target by week = first-trimester allowance + weekly rate × (week − 13); pre-pregnancy BMI = weight ÷ height²Source: Institute of Medicine and National Research Council, Weight Gain During Pregnancy: Reexamining the Guidelines (2009); ACOG Committee Opinion 548.
Worked example
A woman of 65 kg and 165 cm, now 24 weeks pregnant and weighing 72 kg.
- Pre-pregnancy BMI = 65 ÷ 1.65² = 23.9, the normal-weight category.
- Full-term target = 11.5–16 kg; first-trimester allowance = 2 kg.
- Weeks past 13 = 11, at 0.35–0.5 kg each = 3.9–5.5 kg.
- Target by week 24 = 5.9–7.5 kg; actual gain = 7 kg.
A 7 kg gain at 24 weeks sits inside the 5.9–7.5 kg target band.
Frequently asked questions
I gained nothing in the first trimester. Is that a problem?+
It is common, particularly with morning sickness, and the guidelines allow for it. Steady gain from the second trimester matters more.
Do the ranges change for twins?+
Yes. The IOM gives separate, higher provisional ranges for twin pregnancies — 17–25 kg at normal weight — which this calculator does not model.
Should I diet if I am gaining too fast?+
No. Active weight loss is not advised in pregnancy. Talk to your clinician about food quality and activity instead.
Why is the target smaller if I started heavier?+
Existing fat stores can supply part of the pregnancy's energy needs, so less additional gain is required for the same fetal growth.
Last reviewed September 1, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.