Pregnancy Weight Gain Calculator
Based on the Institute of Medicine 2009 guidelines — the reference used by ACOG — this calculator gives your total recommended gain, the weekly rate for the second and third trimesters, how much you should have gained by your current week, and how much is left.
Your details
Gain above or below the band is common and usually correctable — discuss it with your midwife or OB before changing your diet, and never restrict calories in pregnancy without clinical guidance.
How much weight a pregnancy should add depends mostly on one number nobody asks for at the baby shower: your pre-pregnancy BMI. The Institute of Medicine's 2009 guidelines — still the reference standard used by ACOG — set total targets by BMI category: 12.5–18 kg for underweight, 11.5–16 kg for normal weight, 7–11.5 kg for overweight, and 5–9 kg for obesity, with a slower weekly pace for higher BMI categories.
The distribution matters as much as the total. Only about 2 kg accumulates in the first trimester; from week 13 onward, gain should track a steady weekly rate (about 0.35–0.5 kg per week for normal BMI, less for higher categories). This calculator allocates the total across your week count so "how much should I have gained by 24 weeks" gets a real answer with a range, not a shrug.
Two scope notes. Twins have their own IOM table (roughly 1.4× the singleton targets by category) — we offer an approximate twin mode and label it as such, but twin pregnancies are monitored closely by MFM specialists and this tool is no substitute. And the guidelines are population-level references: individual recommendations legitimately vary with edema, multiples, eating disorders history, and medical conditions, which is exactly the list of things to bring to your own appointments.
How the calculation works
BMI = pre-pregnancy weight in kg ÷ (height in metres)². The category selects the IOM total target range and the recommended second/third-trimester weekly rate: 0.44–0.58 kg/week underweight, 0.35–0.50 normal, 0.23–0.33 overweight, 0.17–0.27 obese.
Expected gain by week N: about 2 kg through week 12, then 2 kg plus weekly rate × (N − 12). The calculator shows both ends of the range, plus what remains to reach the total target.
Twin mode scales singleton targets by roughly 1.4 — a transparent approximation of the IOM twin table rather than its full values, flagged in the output so it cannot be mistaken for the guideline itself.
Frequently asked questions
How much weight should I gain during pregnancy?
By pre-pregnancy BMI, per the IOM/ACOG reference: underweight (BMI under 18.5) 12.5–18 kg; normal (18.5–24.9) 11.5–16 kg; overweight (25–29.9) 7–11.5 kg; obese (30 or more) 5–9 kg. Twin pregnancies have higher targets, roughly 17–25 kg for normal BMI. The right number for you individually is set with your care team.
When does pregnancy weight gain start?
Slowly — roughly 1–2.5 kg across the entire first trimester, with many people gaining nothing or even losing weight to nausea. The steady weekly gain belongs to the second and third trimesters: about 0.35–0.5 kg per week for normal BMI, at a declining rate for higher categories. If your first trimester gain was low, you have not "missed" anything.
Is it safe to lose weight during pregnancy?
Deliberate weight loss is generally not recommended in pregnancy, including for higher BMI categories — the IOM targets exist precisely so that gaining within the band is the goal at every starting weight. If your weight is tracking outside your range in either direction, the right move is a conversation with your clinician or a prenatal dietitian, not a diet change on your own.
What if I gain more than the recommended amount?
It happens in a substantial share of pregnancies and is usually manageable. Excess gain is associated with gestational diabetes, hypertension, and higher birth weight, which is why the guidelines exist — but the response is adjustment, not guilt. One useful framing: at the recommended weekly rate, each remaining week carries a specific target, and getting back into the band for the rest of the pregnancy carries most of the benefit.
Do the IOM guidelines apply outside the US?
The IOM 2009 report is the most widely adopted reference worldwide, but several countries issue their own ranges, and some differ modestly — often in the overweight and obese bands. If your national guideline differs, follow your own care team's numbers; the structure of this calculator (total target plus weekly rate) applies identically.
References & sources
- Institute of Medicine. Weight Gain During Pregnancy: Reexamining the Guidelines. National Academies Press. 2009.
- ACOG Committee Opinion 548: Weight Gain During Pregnancy (2013, reaffirmed).
- IOM twin-gestation weight gain recommendations (included in the 2009 report).