Medical disclaimer. This article is general information about pregnancy weight gain and is not medical advice. Your midwife or obstetrician may set a different target for you based on your health history, and their number comes first. Contact them about sudden weight changes, swelling in your hands or face, or any symptom that worries you.
There is no single right answer to pregnancy weight gain — there are four, and which one applies to you depends on your body mass index before you conceived. The Institute of Medicine's 2009 guidelines set a total-gain range for each pre-pregnancy BMI band: 12.5–18 kg if you started underweight, 11.5–16 kg at a normal BMI, 7–11.5 kg if you were overweight, and 5–9 kg at a BMI of 30 or above.
That table is what most people came for. The rest of this article is what it doesn't say: how the gain is meant to be paced, why twins get different numbers, and why landing outside your range is a conversation, not a grade.
Start with your pre-pregnancy BMI, not today's weight
Every range below is keyed to the BMI you had before you conceived — weight in kilograms divided by height in metres squared. If you don't know your pre-pregnancy weight, the weight taken at your first prenatal visit is the usual stand-in — ACOG asks providers to establish BMI at that visit — because very little gain has happened by then.
The four bands are the standard WHO cut-offs: under 18.5 is underweight, 18.5 to under 25 is normal, 25 to under 30 is overweight, and 30 and above is obesity. If you're not sure which band you were in, our BMI calculator will place you in a few seconds.
The boundary is a cut-off, not a cliff: a BMI of 24.8 and one of 25.2 are not different bodies. If you sit near a line, ask your midwife or OB which range they are working from.
The IOM 2009 total-gain ranges
| Pre-pregnancy BMI | Total gain (kg) | Total gain (lb) |
|---|---|---|
| Under 18.5 (underweight) | 12.5 – 18.0 | 28 – 40 |
| 18.5 – 24.9 (normal) | 11.5 – 16.0 | 25 – 35 |
| 25.0 – 29.9 (overweight) | 7.0 – 11.5 | 15 – 25 |
| 30.0 and above (obesity) | 5.0 – 9.0 | 11 – 20 |
These aren't ideals plucked from the air. ACOG describes them as an attempt to balance several risks against each other: large-for-gestational-age babies, small-for-gestational-age babies, preterm birth, and weight the parent keeps after the birth. The headline change from the 1990 version, in the IOM's own words, is that “the new guidelines include a specific, relatively narrow range of recommended gain for obese women.”
Notice how wide each range is — even the tightest spans 4 kg. They are population bands, not personal targets.
How the gain is meant to be paced
The total is only half the guidance. The other half is the rate.
First trimester: very little. The IOM's own calculations assume a first-trimester gain of 0.5–2 kg (1.1–4.4 lb), and plenty of women gain nothing or lose a little to nausea. That's expected. Almost all of the growing — placenta, blood volume, amniotic fluid, the baby's own last sprint — happens later.
Second and third trimester: a steady weekly rate. The IOM's rates for the rest of the pregnancy, each a range around a mean (0.51 / 0.42 / 0.28 / 0.22 kg/week):
| Pre-pregnancy BMI | Weekly rate (kg/week) | Weekly rate (lb/week) |
|---|---|---|
| Under 18.5 | 0.44 – 0.58 | 1 – 1.3 |
| 18.5 – 24.9 | 0.35 – 0.50 | 0.8 – 1 |
| 25.0 – 29.9 | 0.23 – 0.33 | 0.5 – 0.7 |
| 30.0 and above | 0.17 – 0.27 | 0.4 – 0.6 |
Read these as averages across many weeks, not a target for any given Tuesday. A single weigh-in moves with hydration, salt and the time of day, so any one point matters far less than the line through the points. Weighing roughly weekly, same scale, same time of day, is enough to see that line.
The pregnancy weight gain calculator turns your pre-pregnancy weight, height and current week into a total range, a target for the week you're in, and a weekly rate.
Twins: a provisional range, and one real gap
Twin pregnancies get their own numbers, with more caveats than the singleton table.
| Pre-pregnancy BMI | Total gain, twins (kg) | Total gain, twins (lb) |
|---|---|---|
| 18.5 – 24.9 (normal) | 16.8 – 24.5 | 37 – 54 |
| 25.0 – 29.9 (overweight) | 14.1 – 22.7 | 31 – 50 |
| 30.0 and above (obesity) | 11.3 – 19.1 | 25 – 42 |
Three things to know about that table.
It's provisional. The twin ranges are the 25th–75th percentiles of gain in a historical US cohort (1979–1999) of twins delivered at 37–42 weeks with a mean birthweight of at least 2,500 g — a description of what happened where babies came out a good size, not a trial of what works. The IOM calls them provisional itself: it could not test cause and effect.
There's no published weekly rate for twins. The IOM gives a total at term and stops. Any twin weekly rate you see is derived by spreading that total across the weeks you have, not quoted from the guideline — the ProbaBaby app spreads it to week 37 and says on screen that it did.
There is no range at all for an underweight twin pregnancy. The IOM's summary says the information available was insufficient to develop even a provisional guideline for underweight women carrying multiples, so it published none. Anywhere you see a number for BMI under 18.5 with twins, someone filled that gap themselves.
Twins also compress the schedule: a much larger total has to arrive in a pregnancy that usually ends earlier than 40 weeks. The ranges may not travel well either — a 2023 analysis of 2,562 twin pregnancies from the French JUMODA cohort, screened to the same standard the IOM used (born at 37 weeks or later, mean twin birthweight at least 2,500 g), still classified 46.5% of those women as having gained too little. Our twin pregnancy guide covers how the whole timeline shifts.
What the evidence says about being outside the range
Being outside your range is the norm, not the exception. In 2015 US birth records from 48 states and DC, 32% of women having full-term singleton babies gained within the recommended range; 48% gained more and 21% gained less.
A 2017 JAMA meta-analysis pooling 23 studies and more than 1.3 million pregnancies found the same split (47% above, 23% below) and quantified what it's associated with:
- Above the range: higher odds of a large-for-gestational-age baby (OR 1.85), macrosomia (OR 1.95) and caesarean birth (OR 1.30).
- Below the range: higher odds of a small-for-gestational-age baby (OR 1.53) and preterm birth (OR 1.70).
Those are real associations, but they come from observational cohorts, describe populations rather than people, and are mostly modest in size. Landing outside your band is a reason to look at the trend with your provider, not a result you failed.
Why the range is a guide, not a grade
A few reasons to hold the number loosely:
It's a US-derived population range. Not every country copies it. The NHS publishes no BMI-band targets at all — only that most pregnant women gain 10–12.5 kg (22–28 lb), most of it after week 20 — and sends you to your midwife for anything more specific.
For an overweight or obese parent, under-gaining isn't automatically a problem to fix. ACOG's Committee Opinion No. 548 asks for individualised care and clinical judgement when an overweight or obese woman is gaining — or wants to gain — less than recommended but her baby is growing appropriately. The growth is the thing being watched, not the number.
Diet and movement are what the trials actually test. The interventional evidence here is a Cochrane review of diet and exercise in pregnancy: high-quality evidence that they cut the risk of excessive gain by about 20% (risk ratio 0.80), with moderate-intensity exercise, mostly regular walking, called out as an important part of it. What was tested was diet quality and regular movement — not eating less.
A number that makes you restrict is the wrong number. If tracking is pushing you to skip meals or feel bad about your body, say so to your midwife or OB. That's clinical information, not an aside.
When to call your provider
- Sudden swelling of your face, hands or feet — sometimes what you notice first is a jump on the scale over a day or two — especially alongside a severe headache or vision changes. The NHS lists those as pre-eclampsia symptoms that need checking urgently.
- Weight loss after the first trimester, or several weeks of no gain in the second or third trimester.
- Vomiting that stops you keeping fluids down.
- Any week where you're worried. You don't need to justify the call.
How ProbaBaby helps
ProbaBaby's pregnancy Track tab logs your weight with a chart and KPI tiles, and plots your line against a recommended band built from the IOM 2009 weekly rates above for your pre-pregnancy BMI, on top of a 0.5–2.0 kg first-trimester ramp. For a twin pregnancy it switches to the IOM provisional twin totals, interpolates them to a twin term of 37 weeks and holds the band flat after that; the weight-gain tool's progress bar counts to 40 weeks for a singleton and 37 for twins. If you started underweight and are carrying twins, the app shows a "no established guideline" note instead of a range, because IOM 2009 has none to show. The Track tab names the two guidelines it works from on screen — IOM 2009 and ACOG Committee Opinion No. 548 — and it sits inside the wider week-by-week pregnancy tracking module, so the weight-gain tool can fill your current week straight from your pregnancy profile.