Pregnancy weight gain, explained
By Rick Campbell · Updated · Sourced to primary literature · Not medical advice
The weight gain ranges used in antenatal care almost everywhere come from one document: the 2009 report from the Institute of Medicine and the National Research Council, Weight Gain During Pregnancy: Reexamining the Guidelines. It sets four ranges, one for each pre-pregnancy BMI category, and those four ranges have shaped a generation of obstetric conversations.
They deserve to be understood rather than simply obeyed. The evidence behind them is observational, the associations they rest on are real but modest, a later and much larger analysis produced noticeably different optimal ranges with limited ability to predict outcomes, and for one group (women carrying twins who started underweight) the report declined to give a range at all. This guide sets out what the ranges are, what sits behind them, where the ground is soft, and what a calculator can honestly do here. It is not medical advice, and nothing on this page should change what your midwife or obstetrician has told you.
In brief
- The 2009 Institute of Medicine ranges are 12.5–18 kg for a pre-pregnancy BMI below 18.5, 11.5–16 kg for 18.5–24.9, 7–11.5 kg for 25–29.9, and 5–9 kg for 30 and above.
- Pre-pregnancy BMI is the only input that sets the range, which is why an accurate pre-pregnancy weight matters far more than the weight on the scale today.
- First-trimester gain is small and not rate-based: the report expects 0.5–2 kg in total, with the weekly rate applying from about week 14 onwards.
- The evidence is observational. Gaining outside the range is associated with different risks (below with small-for-gestational-age birth and preterm birth, above with large-for-gestational-age birth and caesarean delivery), but association is not causation, and the effects are modest in absolute terms.
- A 2019 analysis of 196,670 pregnancies produced optimal ranges that differ from the IOM's and noted that these ranges discriminate poorly between individual women who do and do not have adverse outcomes.
What the 2009 guidelines actually say
The report sets a total recommended gain across the whole pregnancy, and a rate of gain per week for the second and third trimesters. Both depend on one thing: the BMI category the woman was in before she became pregnant. The categories are the standard WHO adult ones, which is itself a decision worth noticing, since those categories were not derived with pregnancy in mind.
The ranges are wide by design. A span of 4.5 kilograms for a woman of healthy pre-pregnancy weight is not a target to hit but a band within which the committee found no clear signal of increased risk. That width is honest, and it is regularly lost in translation: a phone app that plots a single line through the middle of the band is inventing a precision the source document does not claim.
Note the obesity row in particular. The 2009 report set a single range of 5 to 9 kilograms for all pre-pregnancy BMI values of 30 and above, without subdividing by obesity class. The committee was explicit that the evidence did not support finer distinctions at the time. That remains one of the most-discussed gaps in the guidance, because a woman with a pre-pregnancy BMI of 31 and a woman with a BMI of 45 are being given the same number.
| Pre-pregnancy BMI | Category | Total gain, single baby | Rate in the 2nd and 3rd trimesters | Total gain, twins |
|---|---|---|---|---|
| Below 18.5 | Underweight | 12.5–18 kg (28–40 lb) | 0.44–0.58 kg per week | No range given: the evidence was judged insufficient |
| 18.5–24.9 | Healthy weight | 11.5–16 kg (25–35 lb) | 0.35–0.50 kg per week | 16.8–24.5 kg (37–54 lb) |
| 25.0–29.9 | Overweight | 7–11.5 kg (15–25 lb) | 0.23–0.33 kg per week | 14.1–22.7 kg (31–50 lb) |
| 30.0 and above | Obesity, all classes | 5–9 kg (11–20 lb) | 0.17–0.27 kg per week | 11.3–19.1 kg (25–42 lb) |
Why pre-pregnancy BMI is the input that matters
Every range on this page is selected by pre-pregnancy BMI and by nothing else. Not current weight, not height alone, not age, not how the pregnancy has gone so far. This is the single most common source of confusion, because the number people have to hand is today's weight and the number the guidance needs is the one from before conception.
The reasoning behind it is straightforward. The committee was looking for the amount of gain associated with the best outcomes, and it found that the answer depended on how much fat mass a woman already carried. A woman starting with more stored energy needs to gain less to support the pregnancy, because some of the additional energy demand can be met from existing reserves. A woman starting underweight has less to draw on and a correspondingly larger recommended gain.
Practically, this means a recalled or estimated pre-pregnancy weight carries the whole calculation. If the figure you enter is two or three kilograms out, you may be reading the wrong row entirely: the boundary between the healthy weight and overweight categories at a BMI of 25 sits at 68.1 kg for a woman of 165 cm, and crossing it moves the recommended range down by 4.5 kilograms at both ends. A weight recorded by a clinician before conception is worth far more than a recollection, and if you have neither, say so rather than guessing precisely.
There is one more wrinkle. Pre-pregnancy BMI is normally taken from weight at conception, but many women first attend antenatal care at eight to twelve weeks, by which point a little weight has already changed in either direction. Booking weight is a reasonable substitute and is what most services use, but it is a substitute, and it drifts in the direction of the early pregnancy experience: appetite, nausea, or neither.
First trimester and the rest are different
The guidance treats the first trimester separately, and for good reason: very little of the weight gained in pregnancy is gained early. The foetus is small, the placenta and blood volume expansion are still building, and total expected gain across the first thirteen weeks is only 0.5 to 2 kilograms. That is a total, not a rate, and there is no weekly figure attached to it.
From about week 14 the weekly rate takes over, and it is the rate rather than the running total that most usefully answers the question of whether things are on track at a given point. A woman of healthy pre-pregnancy weight at 24 weeks would be expected to have gained, on the low side, 0.5 kg in the first trimester plus eleven weeks at 0.35 kg (roughly 4.4 kg), and on the high side 2 kg plus eleven weeks at 0.50 kg, roughly 7.5 kg. A 3-kilogram spread at 24 weeks is the honest width of the expectation.
It is worth naming what is in the gain, because a surprising amount of it is not fat. By term, a typical pregnancy includes roughly 3 to 4 kilograms of baby, around half a kilogram of placenta, close to a kilogram of amniotic fluid, a kilogram or so of enlarged uterus, about a kilogram of breast tissue, and perhaps one to two kilograms of additional blood and extracellular fluid. That accounts for a large share of the recommended gain in every category before any fat storage is counted at all.
Early pregnancy also routinely defies the guidance in ways nobody should worry about. Significant nausea and vomiting can produce no gain or an outright loss in the first trimester, which is common and usually resolves. This is a reason to talk to a midwife about symptoms and hydration, not a reason to try to make up a shortfall.
What the evidence behind the ranges is
The 2009 committee worked from observational data: cohorts of pregnancies in which weight gain was recorded and outcomes were observed. There is no randomised trial that assigned women to gain particular amounts of weight, for reasons that are obvious once stated, so the entire evidence base is associative. That is a limitation, not a disqualification (much of obstetric practice rests on the same kind of evidence), but it should shape how confidently anyone speaks about it.
The largest test of the guidelines came in 2017, when Goldstein and colleagues pooled 23 studies covering 1,309,136 women. They found that gestational weight gain sat below the IOM recommendations in 23% of pregnancies and above them in 47%, so under half of all pregnancies fall inside the bands at all. Gaining below the recommendation was associated with a higher risk of small-for-gestational-age birth (odds ratio 1.53) and preterm birth (odds ratio 1.70), and a lower risk of large-for-gestational-age birth. Gaining above was associated with higher risks of large-for-gestational-age birth (odds ratio 1.85), macrosomia (1.95) and caesarean delivery (1.30), and lower risks of small-for-gestational-age and preterm birth.
Two features of that result deserve attention. First, the directions are exactly what you would expect if weight gain is partly a proxy for foetal growth, which means some of the association is close to definitional rather than causal. Second, the absolute risk differences were mostly in the range of two to six percentage points: real, but a long way from the deterministic language that sometimes surrounds the topic.
Where the evidence is weak
The most important challenge to the 2009 ranges came from the LifeCycle Project in 2019, an individual participant meta-analysis of 196,670 pregnancies drawn from 25 cohorts in Europe and North America, with a separate validation sample of 3,505. Rather than testing the IOM bands, it asked what ranges of gain were actually associated with the lowest risk of any adverse outcome, and derived them from the data.
The answers differed from the IOM's, sometimes considerably: 14.0 to under 16.0 kg for women categorised as underweight, 10.0 to under 18.0 kg for healthy weight, 2.0 to under 16.0 kg for overweight, 2.0 to under 6.0 kg for obesity grade 1, weight loss or a gain of 0 to under 4.0 kg for obesity grade 2, and 0 to under 6.0 kg for obesity grade 3. The overweight range in particular is enormously wider than the IOM's 7 to 11.5 kg, which tells you how flat the risk curve is across most of that span.
The finding that matters most, though, is the one the authors put in their own conclusion. The derived optimal ranges discriminated between women who did and did not have adverse outcomes with areas under the receiver operating characteristic curve of only 0.55 to 0.76, that is, from barely better than chance to moderate. Their stated conclusion was that the ranges may inform prenatal counselling but had limited predictive value for the outcomes assessed.
Several other soft spots are worth naming. The 2009 report did not subdivide obesity by class, which the LifeCycle data suggests matters. Most of the underlying cohorts are European and North American, so generalisation to other populations is an assumption rather than a finding. Gestational weight gain is measured inconsistently across studies: some use self-reported pre-pregnancy weight, some booking weight, some last recorded weight before delivery. And weight gain that arrives as oedema is indistinguishable on a scale from weight gain that arrives as tissue, while being clinically very different indeed.
| Pre-pregnancy category | IOM 2009 range | LifeCycle 2019 derived range | What the difference suggests |
|---|---|---|---|
| Underweight (BMI below 18.5) | 12.5–18 kg | 14.0 to under 16.0 kg | A narrower optimum, sitting inside the IOM band |
| Healthy weight (18.5–24.9) | 11.5–16 kg | 10.0 to under 18.0 kg | A wider optimum: the risk curve is flat across a broad span |
| Overweight (25.0–29.9) | 7–11.5 kg | 2.0 to under 16.0 kg | Very wide; little signal that the IOM band is uniquely favourable |
| Obesity grade 1 (30.0–34.9) | 5–9 kg (all obesity classes) | 2.0 to under 6.0 kg | Lower than the IOM band, and class-specific |
| Obesity grade 2 (35.0–39.9) | 5–9 kg (all obesity classes) | Loss, or 0 to under 4.0 kg | The IOM's single obesity row hides real differences by class |
| Obesity grade 3 (BMI 40 and above) | 5–9 kg (all obesity classes) | 0 to under 6.0 kg | Again lower, and again not distinguished in the 2009 guidance |
Twins, and the pregnancies the guidance does not cover
Twin pregnancies get their own ranges in the 2009 report, and they are labelled provisional in the source document rather than presented with the same confidence as the singleton figures. The committee gave 16.8 to 24.5 kg for a healthy pre-pregnancy BMI, 14.1 to 22.7 kg for overweight, and 11.3 to 19.1 kg for obesity. For women who started underweight, it gave nothing, because the available data was judged insufficient to support a recommendation.
That absence is worth respecting rather than filling in. Various figures for underweight twin pregnancies circulate online, and some official sources publish a range while noting explicitly that it does not come from the Institute of Medicine. Our pregnancy weight gain calculator returns no range for this combination and says why, which is less satisfying than a number and more honest than an invented one.
Triplets and higher-order multiples have no IOM range at all, in any pre-pregnancy category. Neither does any pregnancy complicated by the conditions that most change the arithmetic: significant oedema, hyperemesis, gestational diabetes under active management, or a pregnancy following bariatric surgery. In every one of those cases, weight gain is being managed as part of a clinical picture rather than read off a chart.
Adolescent pregnancy is a further case the adult categories handle poorly, since the mother is herself still growing and adult BMI categories do not apply cleanly before growth finishes. This is exactly the situation our child BMI guide describes from the other direction, and it is one where individualised clinical guidance replaces any published range.
What a calculator can and cannot do here
What a calculator can do is arithmetic, transparently. It can work out your pre-pregnancy BMI from a height and a pre-pregnancy weight, select the matching IOM row, apply the first-trimester allowance and the weekly rate to the week you are at now, and tell you whether the weight you have gained falls inside, below or above the range that published guidance expects at this point. All of that is reproducible, and all of it is stated in the source document.
What it cannot do is anything clinical. It does not know whether a gain above the range is tissue or fluid, whether a gain below it reflects nausea that has since resolved, whether your blood pressure has changed, how the baby is growing on ultrasound, or what your own history makes likely. Those are the questions that actually determine whether any of this matters in a given pregnancy, and none of them is answerable from three numbers.
There are two things this site will not do, and they are deliberate. It will not recommend a calorie deficit during pregnancy, because deliberate weight loss in pregnancy is a clinical decision taken with a clinician and never a default. And it will not present a single target line to track against, because the guidance is a band, the band is wide, and drawing a line through the middle of it converts uncertainty into a daily source of anxiety.
The right use of any of this is as preparation for a conversation. If your gain sits outside the expected range at your current week, that is a reasonable thing to raise at your next appointment, with the numbers in hand. It is not a reason to change what you eat before you have had that conversation. Midwives and obstetricians see the full distribution of real pregnancies every week, and most of what looks alarming on a chart turns out to be ordinary.
Frequently asked questions
How much weight should I gain during pregnancy?
It depends entirely on your pre-pregnancy BMI. The 2009 Institute of Medicine ranges are 12.5 to 18 kg if you started below a BMI of 18.5, 11.5 to 16 kg from 18.5 to 24.9, 7 to 11.5 kg from 25 to 29.9, and 5 to 9 kg at 30 and above. Those are totals across the whole pregnancy, of which only 0.5 to 2 kg is expected in the first trimester. The bands are deliberately wide because that is how wide the evidence is, and your midwife or obstetrician may reasonably suggest something different for your particular pregnancy.
What happens if I gain more than the recommended range?
It is extremely common (a meta-analysis of more than 1.3 million pregnancies found 47% gained above the recommendations), and it shifts some risks rather than guaranteeing any outcome. Gaining above the range was associated with higher odds of a large-for-gestational-age baby, macrosomia and caesarean delivery, and with lower odds of a small-for-gestational-age or preterm baby. The absolute risk differences were mostly two to six percentage points. A later analysis of 196,670 pregnancies found these ranges predict individual outcomes only weakly. Raise it at your next appointment rather than changing what you eat on your own.
Do the ranges change if I am carrying twins?
Yes, and the 2009 report labels the twin ranges provisional rather than presenting them with the confidence of the singleton figures. They are 16.8 to 24.5 kg for a healthy pre-pregnancy BMI, 14.1 to 22.7 kg for overweight, and 11.3 to 19.1 kg for obesity. For women who started underweight, the report gives no range at all, because the committee judged the available data insufficient. Some other sources publish a figure for that group while noting it does not come from the Institute of Medicine. Our calculator returns nothing for that combination and explains why, rather than inventing a number.
Should I use my current BMI or my pre-pregnancy BMI?
Pre-pregnancy, always. Every range in the guidance is selected by the BMI you had before conception, and current weight has no role in choosing which row applies; it is only used to see how much you have gained. Getting this wrong can put you in the wrong category entirely: for a woman of 165 cm, the boundary between the healthy weight and overweight rows sits at 68.1 kg, and crossing it moves the recommended total down by 4.5 kg at both ends. If you have a weight recorded by a clinician before conception, use that; if not, booking weight from your first antenatal appointment is the usual substitute.
Is it safe to try to lose weight during pregnancy?
That is a question for your midwife or obstetrician, and not one this site will answer with a number. Deliberate weight loss during pregnancy is not a default recommendation at any pre-pregnancy BMI, and the published guidance sets ranges of gain rather than loss for every singleton category, including obesity. The one place loss appears at all is in the 2019 LifeCycle analysis, which identified loss or a gain of up to 4 kg as associated with the lowest adverse outcome risk for women with a pre-pregnancy BMI of 35 to 39.9, a research finding about populations, not an instruction to any individual. Our calculators will not generate a pregnancy calorie deficit.
Put it into practice
Run your own numbers through the pregnancy gain calculator, the BMI calculator and the healthy weight range calculator. Related reading: Child BMI percentiles, BMI explained and 2025 obesity definition.
Sources
- Institute of Medicine and National Research Council. Weight Gain During Pregnancy: Reexamining the Guidelines. Washington DC: National Academies Press, 2009. doi.org/10.17226/12584
- American College of Obstetricians and Gynecologists. Committee Opinion No. 548: Weight gain during pregnancy. Obstet Gynecol 2013;121:210–12. doi.org/10.1097/01.AOG.0000425668.87506.4c
- Goldstein RF, Abell SK, Ranasinha S, et al. Association of gestational weight gain with maternal and infant outcomes: a systematic review and meta-analysis. JAMA 2017;317:2207–25. doi.org/10.1001/jama.2017.3635
- LifeCycle Project-Maternal Obesity and Childhood Outcomes Study Group. Association of gestational weight gain with adverse maternal and infant outcomes. JAMA 2019;321:1702–15. doi.org/10.1001/jama.2019.3820
- Champion ML, Harper LM. Gestational weight gain: update on outcomes and interventions. Curr Diab Rep 2020;20:11. doi.org/10.1007/s11892-020-1296-1
- NICE. Maternal and child nutrition: nutrition and weight management in pregnancy, and nutrition in children up to 5 years. NICE guideline NG247, 2025. www.nice.org.uk/guidance/ng247
- WHO. WHO recommendations on antenatal care for a positive pregnancy experience. Geneva: World Health Organization, 2016. www.who.int/publications/i/item/9789241549912
- CDC. Weight gain during pregnancy: the IOM ranges for single and twin pregnancies, with the note that the underweight twin figure is not from the Institute of Medicine. www.cdc.gov/maternal-infant-health/pregnancy-weight/index.html
Cite this page
Quoting a figure from here in an article, a report or a piece of coursework? Use whichever of these your style guide asks for.
- APA
- Campbell, R. (2026). Pregnancy weight gain, explained. Body Stats. https://bodystats.co/app/guides/pregnancy-weight-gain-explained
- Plain text
- “Pregnancy weight gain, explained”, Body Stats, last updated 13 September 2026, https://bodystats.co/app/guides/pregnancy-weight-gain-explained
This guide is informational and educational, not medical advice. Formula details live on the methodology page; see also the medical disclaimer.
Last updated . Written by Rick Campbell; not medically reviewed. See review status.