Pregnancy weight gain calculator
By Rick Campbell · Updated · Sourced to primary literature · Not medical advice
There is a published range for how much weight is typically gained across a pregnancy, and it depends on what your body mass index was before the pregnancy started. This page works out that range from the 2009 guidelines written by the Institute of Medicine and the National Research Council (the ones most midwives and obstetricians in Australia, the UK and the United States still work from), and shows the weekly rate that sits behind it.
It is worth being clear about what that range is before you read your number. It came out of modelling birth outcomes across large populations of women: the chance of a baby being born small, the chance of being born large, the chance of a caesarean, the chance of weight being retained afterwards. It was never designed as a target for an individual pregnancy, and the report itself says the evidence behind parts of it is thin. Plenty of healthy pregnancies sit outside it.
So this page gives you the range, the arithmetic that produces it, and the honest edges: where the guidance is provisional, where it publishes nothing at all, and why the person actually looking after you is watching a dozen things that matter more than the scales. What it will not do is tell you to eat less.
In brief
- Recommended total gain depends on pre-pregnancy BMI: roughly 12.5–18 kg under BMI 18.5, 11.5–16 kg for 18.5–24.9, 7–11.5 kg for 25–29.9 and 5–9 kg from BMI 30.
- The guidelines allow 0.5–2 kg in total across the whole first trimester, then apply a weekly rate from week 14, between about 0.17 and 0.58 kg per week depending on the band.
- These are population ranges from a 2009 report aimed at birth outcomes, not a target to hit; gain outside them is common and is a conversation for a midwife or obstetrician.
- Twin ranges in the same report are labelled provisional, and for the underweight band no twin range was published at all; this page says so rather than inventing one.
- Intentional weight loss during pregnancy is not recommended, and NICE advises against routine weighing unless there is a clinical reason for it.
Calculator
What you'll see here
The recommended total weight gain range for the whole pregnancy on the 2009 Institute of Medicine guidelines, the pre-pregnancy BMI band that sets it, the weekly rate for the second and third trimesters, and, if you add how many weeks pregnant you are, the range expected by this point.
Institute of Medicine weight-gain ranges by pre-pregnancy BMI
The 2009 guidelines used by obstetric care in most of the English-speaking world. The band is set by BMI before the pregnancy, and the weekly rate applies from week 14 onwards, after a first-trimester total of 0.5 – 2 kg.
| Pre-pregnancy BMI band | Total gain, one baby | Weeks 14–40, per week | Total gain, twins |
|---|---|---|---|
| Underweight (BMI below 18.5) | 12.5 kg – 18 kg | 0.4 – 0.6 kg | No range published |
| Healthy weight (BMI 18.5–24.9) | 11.5 kg – 16 kg | 0.4 – 0.5 kg | 16.8 kg – 24.5 kg |
| Overweight (BMI 25–29.9) | 7 kg – 11.5 kg | 0.2 – 0.3 kg | 14.1 kg – 22.7 kg |
| Obesity (BMI 30 or above) | 5 kg – 9 kg | 0.2 – 0.3 kg | 11.3 kg – 19.1 kg |
Twin ranges are marked provisional in the source report, and for the underweight band there is no published twin range at all: the data were too thin to set one, and nobody should be filling that gap with a guess.
Where these ranges come from, and what they were built to optimise
In 2009 a committee convened by the Institute of Medicine (now the National Academy of Medicine) and the National Research Council reopened guidance that had stood since 1990 and published Weight Gain During Pregnancy: Reexamining the Guidelines. The committee did something the 1990 version had not: it anchored the bands to the World Health Organization's BMI categories, it set an upper limit for women in the obesity category where the older guidance had none, and it built the ranges around outcomes rather than around any notion of an ideal shape.
The outcomes it modelled were obstetric ones. The committee weighed the chance of a baby being born small for gestational age against the chance of being born large for gestational age, the likelihood of preterm birth, the rate of unplanned caesarean section, and how much weight tended to be retained six months to a year afterwards. The ranges it published are the compromise that came out of that trade-off across a population: the span within which those risks were, on balance, lowest for the group as a whole.
That origin matters for how you read your own number. A range built to minimise population-level risk is not a prescription written for one pregnancy, and the report is candid that the evidence underpinning some of the bands, particularly the obesity band, was limited. It is guidance for a clinical conversation, which is exactly how the people using it treat it.
What the weight actually is, and when it arrives
Very little of the gain in a pregnancy is the baby. At term a singleton baby accounts for roughly three to four kilograms of it. The rest is the machinery and the reserves that got the baby there: the placenta, the amniotic fluid, a uterus several times its usual size, breast tissue, an expanded blood volume that can be forty to fifty per cent greater than before, extra extracellular fluid, and a store of fat laid down mostly in the second trimester that the body draws on late in pregnancy and while feeding afterwards.
The arrival of that weight is not even, which is why the guidelines split it. The first trimester contributes very little: the report's calculations assume a total of 0.5 to 2 kg across all thirteen weeks, and many women gain nothing at all in that stretch. Nausea and food aversion are common enough that a small loss in the first trimester is unremarkable on its own. From week 14 a steadier weekly rate takes over, and most of the total accumulates across the second and third trimesters.
Fluid is also why week-to-week readings jump about. Several litres of extra water are carried by the end of a pregnancy, and it shifts with heat, salt, standing, sleep and the time of day. A scale reading that moves a kilo between Tuesday and Thursday is describing fluid, not tissue.
- Baby: about 3–4 kg at term for a single pregnancy.
- Placenta and amniotic fluid: roughly 1.5 kg between them.
- Uterus and breast tissue: around 1.5–2 kg.
- Extra blood and other body fluid: commonly 3–4 kg.
- Fat and protein stores laid down for late pregnancy and feeding: the balance.
Why the range differs by pre-pregnancy BMI
The committee found that the amount of gain associated with the best balance of outcomes was not the same for everyone: it varied systematically with how much body fat a woman was carrying before she conceived. Women who started in the underweight band did better with more gain, because a low starting reserve plus low gain was associated with small-for-gestational-age birth. Women who started in the obesity band did better with less, because the stores to draw on were already there and high gain on top of a high starting BMI was associated with large-for-gestational-age birth, caesarean delivery and retained weight afterwards.
That is the whole logic of the four bands: 12.5–18 kg under a BMI of 18.5, 11.5–16 kg from 18.5 to 24.9, 7–11.5 kg from 25 to 29.9, and 5–9 kg from 30 upwards. The weekly rates follow the same ordering, from roughly 0.44–0.58 kg a week in the underweight band down to 0.17–0.27 kg in the obesity band.
Note what the BMI here is doing and what it is not. It is being used as a rough sorting variable to pick a band (a job it does adequately across a population), not as a statement about anybody's health. It carries every limitation adult BMI always has: it cannot tell muscle from fat, it reads differently across ancestries, and a single point on either side of a boundary moves you into a different row of the table without anything about you having changed.
What gain above or below the range is associated with
These are associations found in populations, not predictions about a pregnancy, and it is worth holding them at that distance. Gain below the recommended range has been associated with a higher rate of babies born small for gestational age and, in some studies, preterm birth. Gain above it has been associated with babies born large for gestational age, a higher rate of unplanned caesarean, and more weight retained in the year after birth. The size of these associations is modest, and none of them describes what will happen in any individual case.
There is also a well-documented problem of direction. Some of the conditions that matter in pregnancy change weight themselves (gestational diabetes, pre-eclampsia and the fluid retention that comes with it, hyperemesis), so weight that sits outside a range can be a signal of something rather than a cause of it. That is one of the reasons a reading outside the band is a prompt to talk to somebody, not a prompt to act.
The honest summary: an out-of-range figure raises a question worth asking at an appointment. It does not diagnose anything, it does not mean you have done something wrong, and it is a poor basis for changing what you eat without somebody looking at the rest of the picture.
Twins, and where the guidance runs out
The 2009 report published ranges for twin pregnancies too, but it flagged them as provisional, and the distinction is real rather than bureaucratic. The singleton ranges came from outcome modelling; the twin ranges are descriptive: they are the interquartile range, the middle half, of cumulative gain among women who delivered twins averaging at least 2,500 g between 37 and 42 weeks. They describe what happened in a group with good outcomes rather than a target that was tested.
Those provisional ranges are 16.8–24.5 kg for women starting in the healthy weight band, 14.1–22.7 kg for the overweight band and 11.3–19.1 kg for the obesity band. For women who start a twin pregnancy in the underweight band, the report publishes nothing: there was not enough data to set even a provisional figure.
This calculator reproduces that gap rather than papering over it. If you select twins and the pre-pregnancy BMI falls below 18.5, the page tells you there is no published range and shows the single-baby figure only for context, clearly labelled as such. Inventing a number to fill an empty cell in a table would be the single most misleading thing a page like this could do.
Weight is one line on a long list
Antenatal care watches a great many things, and weight is one of the less informative of them. Blood pressure and urine protein screen for pre-eclampsia. Fundal height and, where indicated, growth scans track how the baby is actually growing, which is the thing weight is being used as a crude proxy for. Blood glucose screening picks up gestational diabetes. Haemoglobin and ferritin check iron. Foetal movements, mood, sleep, nausea and how you are coping all carry more information than a number on a scale.
Guidance reflects that hierarchy. NICE, in its 2025 maternal and child nutrition guideline, recommends against routinely weighing people throughout pregnancy unless there is a clinical reason to do so, noting how much uncertainty remains about optimal weight change. The World Health Organization's 2016 antenatal care recommendations put the emphasis on nutrition counselling and staying active rather than on tracking a figure. ACOG asks clinicians to determine BMI at the first visit and counsel on appropriate gain, which is a conversation, not a weekly weigh-in.
Things genuinely worth raising with a midwife or obstetrician: sudden weight change over days rather than weeks, swelling in the face or hands, vomiting severe enough to stop you keeping food or fluid down, a sense that the baby is moving less, or worry about food and weight that is taking up real headspace. Each of those is a better reason to make contact than any figure this page produces.
Dieting during pregnancy is not recommended
This is the one flat statement on the page. Intentional weight loss during pregnancy is not recommended, because of the potential for harm to the baby. NICE puts it in those words, and it is the consistent position across the guidance. Restricting energy intake during pregnancy risks restricting the nutrients a developing baby needs, and there is no evidence that a number closer to the middle of a population range is worth that trade.
None of this is an argument against eating well or moving. Both are recommended in pregnancy, and both are worth doing for reasons that have nothing to do with the scales: blood glucose, blood pressure, sleep, mood, back pain, stamina in labour. The distinction that matters is between looking after yourself and chasing a number, and it is not a subtle one.
If weight or food is already a difficult subject for you, a calculator is a poor place to be working through it, and pregnancy is a period when that difficulty commonly intensifies. A GP or midwife is used to this conversation and it is a reasonable thing to open early.
How it's calculated
Pre-pregnancy body mass index
BMI = pre-pregnancy weight (kg) ÷ height (m)²
The weight before this pregnancy started, not today's weight; that BMI is what selects the band.
IOM/NRC 2009 total gain ranges, single pregnancy
BMI < 18.5 → 12.5–18 kg (28–40 lb) · 18.5–24.9 → 11.5–16 kg (25–35 lb) · 25–29.9 → 7–11.5 kg (15–25 lb) · ≥ 30 → 5–9 kg (11–20 lb)
Total gain across the whole pregnancy, for a single baby at term.
Rate of gain, second and third trimesters
BMI < 18.5 → 0.44–0.58 kg/week · 18.5–24.9 → 0.35–0.50 kg/week · 25–29.9 → 0.23–0.33 kg/week · ≥ 30 → 0.17–0.27 kg/week
Applies from week 14. The report's own means are 0.51, 0.42, 0.28 and 0.22 kg per week respectively.
Expected total gain by a given week
weeks 1–13: 0.5–2 kg · from week 14: expected = 0.5 + rate(min) × (week − 13) to 2 + rate(max) × (week − 13)
The first-trimester figure is a total for all thirteen weeks, not a weekly rate; the weekly rate is added on top of it.
Provisional twin ranges (IOM/NRC 2009)
18.5–24.9 → 16.8–24.5 kg · 25–29.9 → 14.1–22.7 kg · ≥ 30 → 11.3–19.1 kg · BMI < 18.5 → no range published
Descriptive interquartile ranges from twin pregnancies with good outcomes, not modelled targets.
Worked example: 165 cm, 62 kg before pregnancy, 24 weeks, 69 kg now
- Convert height to metres: 165 cm = 1.65 m.
- Pre-pregnancy BMI: 62 ÷ (1.65 × 1.65) = 62 ÷ 2.7225 = 22.8.
- A BMI of 22.8 falls in the 18.5–24.9 band, so the recommended total gain for a single pregnancy is 11.5–16 kg and the weekly rate from week 14 is 0.35–0.50 kg.
- Expected gain by week 24, lower end: 0.5 kg for the first trimester + (24 − 13) × 0.35 = 0.5 + 3.85 = 4.35 kg.
- Expected gain by week 24, upper end: 2 kg + (24 − 13) × 0.50 = 2 + 5.5 = 7.5 kg.
- Actual change: 69 − 62 = 7 kg, which sits inside the expected 4.35–7.5 kg range at this point, towards the upper end of it.
- Interpret it as such: one reading inside a population range at one moment in a pregnancy, worth roughly nothing on its own and worth a sentence at the next appointment if it is on your mind.
Where this number is used in the real world
- Antenatal appointments, where a pre-pregnancy BMI is recorded at booking and used to frame a conversation about gain, nutrition and activity.
- Obstetric and midwifery guidance in Australia, the UK and the United States, most of which still builds on the 2009 IOM bands.
- Screening and referral decisions: a pregnancy starting in the obesity band changes what is offered, from earlier glucose testing to anaesthetic review.
- Twin pregnancy management, where the provisional ranges are used cautiously alongside serial growth scans that carry far more weight.
- Research and public health surveillance, which measure the proportion of pregnancies gaining below, within and above the IOM ranges as a population indicator.
- Postnatal planning, since weight retained at six to twelve months is one of the outcomes the 2009 ranges were built around.
- Reassurance, in the ordinary case: checking whether a figure that has been worrying you is in fact unremarkable.
Frequently asked questions
How much weight should I gain in pregnancy?
For a single baby the 2009 Institute of Medicine ranges are 12.5 to 18 kg if your pre-pregnancy BMI was under 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 from 30 upwards. Those are population ranges built around birth outcomes rather than targets for one pregnancy, and a great many healthy pregnancies land outside them. Your midwife or obstetrician is the person to read yours with.
I have gained more than the guideline range. What does that mean?
On its own, very little. Gain above the range is associated at population level with larger babies, a higher rate of unplanned caesarean and more weight retained afterwards, but those are modest associations across thousands of pregnancies rather than predictions about yours. It can also be a signal of something else, such as fluid retention, rather than a cause of anything. The useful response is to mention it at your next appointment, not to eat less.
I have barely gained any weight. Should I be worried?
In the first trimester, gaining little or nothing is common and usually unremarkable; nausea and food aversion see to that, and some women lose a little before it settles. Later in pregnancy a flat or falling weight is more worth raising, particularly alongside anything else that has changed. Bring it to your midwife or obstetrician, who can check how the baby is growing directly rather than inferring it from your weight, which is the measurement that actually answers the question.
Can I diet or try to lose weight while pregnant?
No. Intentional weight loss during pregnancy is not recommended, and NICE states that plainly because of the potential for adverse effects on the baby. Restricting energy intake also restricts nutrients at the point they are most needed. Eating well and staying active are both recommended, and both are worth doing for blood pressure, blood glucose, sleep, mood and stamina in labour, but that is a different thing from trying to steer the number on the scales downwards.
How much of pregnancy weight gain is the baby?
Less than most people expect. A single baby at term is roughly three to four kilograms of a total gain that commonly runs to eleven or more. The rest is the placenta and amniotic fluid at around 1.5 kg, an enlarged uterus and breast tissue at around 1.5 to 2 kg, an expanded blood volume and extra body fluid at three to four kilograms, and fat and protein stores laid down for late pregnancy and for feeding afterwards.
Are the twin pregnancy ranges reliable?
They are explicitly provisional, and the source report says so. Unlike the singleton ranges, which came from modelling outcomes, the twin figures simply describe the middle half of cumulative gain among women who delivered twins weighing at least 2,500 g at 37 to 42 weeks. They describe what happened rather than what was tested. For pregnancies starting in the underweight band the report publishes no twin range at all, and this page reports that gap rather than filling it.
Should I be weighing myself at home during pregnancy?
There is no strong reason to, and NICE recommends against routine weighing throughout pregnancy unless there is a clinical reason such as gestational diabetes or hyperemesis. Fluid alone moves the reading by a kilogram or more across a single day, so frequent weighing mostly produces noise. If tracking your weight is making you anxious rather than informed, that is a good enough reason on its own to stop and to say so at your next appointment.
Does this calculator apply if my pregnancy is not full term yet?
The total range is for a pregnancy carried to term, so it will not be reached before then. That is exactly what the week field is for: enter how many weeks pregnant you are and the page shows the range the guidelines describe by that point instead, built from the first-trimester allowance plus the weekly rate for the weeks since week 13. Gain is uneven week to week, so treat even that as a broad band rather than a line.
Keep going
A single number rarely tells the whole story. Alongside the pregnancy gain result, the BMI calculator, the healthy weight range calculator, the TDEE calculator and the child BMI calculator each add a different angle on the same measurements. For the reasoning behind the numbers, read 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(1):210–12. doi.org/10.1097/01.AOG.0000425668.87506.4c
- 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
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- “Pregnancy weight gain calculator”, Body Stats, last updated 12 September 2026, https://bodystats.co/app/pregnancy-weight-gain-calculator
Every formula and threshold on this page is written out with its primary source on our methodology page. These results are informational and educational, not a diagnosis or a substitute for professional advice. See the medical disclaimer.
Last updated . Written by Rick Campbell; not medically reviewed. See review status.