Tracking gain against the range that applies to you
1. Enter your pre-pregnancy weight and height
Pre-pregnancy, not current. The entire framework is indexed to the BMI you started at, and the four ranges differ by up to 9 kg between them — so using your current weight puts you in the wrong band and gives you the wrong target for the rest of the pregnancy. If you do not know it exactly, your booking appointment weight is the closest usable figure.
2. Say whether you are carrying one baby or twins
The twin ranges are substantially higher and are provisional in a way the singleton ones are not. The page is explicit about which parts of the twin guidance are published and which are the best available stand-in.
3. Enter how many weeks along you are
Gestational age from your last period or your dating scan — our pregnancy due date calculator works it out if you are not sure. The expected range at your week is derived from this, so a week out is a range out.
4. Enter your current weight
Weigh in comparable conditions each time — same scales, same time of day, similar clothing. Week-to-week fluctuation from fluid and food is normal and is not gain.
5. Read the range as published, without a tolerance added
The band shown is the IOM range for your BMI category at your week, exactly as published. The previous version applied an invented plus-or-minus 8 per cent on top, so at 40 weeks it called 9.2 to 16.7 kg “on track” where the guideline says 10 to 15.5. That is not a rounding decision a calculator gets to make.
6. Treat the result as a prompt for a conversation
Being outside the range is common and frequently unremarkable. It is worth mentioning at your next appointment rather than acting on alone — and pregnancy is emphatically not a time to be dieting on the strength of a web page.
Where these ranges come from
The figures are from Weight Gain During Pregnancy: Reexamining the Guidelines, published in 2009 by the Institute of Medicine and the National Research Council, now part of the National Academies of Sciences, Engineering, and Medicine. They replaced guidance dating from 1990 and remain the reference in the United States and in many other countries.
They are indexed to pre-pregnancy BMI. For a single baby:
- Underweight, BMI under 18.5 — 12.5 to 18 kg (28 to 40 lb)
- Normal weight, BMI 18.5 to 24.9 — 11.5 to 16 kg (25 to 35 lb)
- Overweight, BMI 25 to 29.9 — 7 to 11.5 kg (15 to 25 lb)
- Obesity, BMI 30 and above — 5 to 9 kg (11 to 20 lb)
Gain is not spread evenly. The first trimester accounts for only about 0.5 to 2 kg in total, and many women gain nothing at all in it or lose a little to nausea, which is not a problem. Most gain occurs in the second and third trimesters at a weekly rate that also depends on starting BMI: roughly 0.44 to 0.58 kg per week if you started underweight, 0.35 to 0.50 if normal weight, 0.23 to 0.33 if overweight, and 0.17 to 0.27 if you started in the obese category.
For twins, the 2009 report gives provisional ranges: 16.8 to 24.5 kg for normal-weight, 14.1 to 22.7 kg for overweight, and 11.3 to 19.1 kg for obese pre-pregnancy BMI.
The recommendation that does not exist, and used to be printed anyway
There is a fourth twin category, and the 2009 report deliberately does not fill it in. For a woman who was underweight before pregnancy and is carrying twins, it states that there is insufficient evidence to set a recommended range.
The previous version of this page silently substituted the normal-weight twin figures and presented them as guidance. That is the most serious kind of defect a health tool can have: it manufactures authority. A reader had no way to know that the range on her screen was not something the Institute of Medicine had said, and the group it affected — underweight women carrying twins — is precisely the group with the least evidence behind their care.
The tool still shows a figure, because an empty panel helps nobody and the normal-weight range is the nearest published thing there is. What has changed is that it is labelled for what it is: a stand-in for a recommendation that has not been made, and a clear signal that this is a conversation for your obstetric team rather than a target from a calculator.
Why the twin week-by-week figures needed rebuilding
The 2009 report gives total ranges for twins but no weekly rate. The previous build handled that absence by interpolating linearly from week zero — total gain multiplied by the current week, divided by forty.
That treats pregnancy as though gain were spread evenly from the first day, which it is not. The consequence was measurable against the tool’s own first-trimester constant: at ten weeks, the linear model expected 4.2 to 6.1 kg, while the first trimester in total is 0.5 to 2 kg. The low end of the expectation was 3.7 kg above the top of the first trimester.
In practice that meant a woman at ten weeks with twins, gaining entirely normally, was told she was below range — and the implication of being told you are below range in pregnancy is that you should eat more. The estimate is now trimester-aware, applying the shape of singleton gain to the twin total, and it is labelled as exactly that rather than as something the Institute of Medicine publishes.
What the weight is actually made of
A useful corrective to the idea that gain is fat. At term, an approximate breakdown for a singleton pregnancy runs: baby around 3 to 3.6 kg; placenta about 0.7 kg; amniotic fluid about 0.9 kg; increased blood volume about 1.4 kg; extra fluid in tissues about 1.4 kg; enlarged uterus about 0.9 kg; breast tissue about 0.5 to 1.4 kg; and maternal fat stores roughly 2.7 to 3.6 kg.
The fat stores are not incidental. They are laid down deliberately as an energy reserve for the third trimester and for breastfeeding, which is metabolically expensive. A pregnancy in which no fat is stored is not a better pregnancy.
The extra energy required is also smaller than most people expect. Common guidance is no additional calories in the first trimester, roughly 340 extra per day in the second, and roughly 450 in the third — a sandwich and a glass of milk, not eating for two.
These are American ranges, and not everyone uses them
This is worth stating plainly, because tools rarely do. The IOM ranges are US guidance. They have been adopted widely, but they are not universal.
NICE, which sets clinical guidance for England and Wales, does not issue recommended weight-gain ranges for pregnancy at all. Its position is that there is insufficient evidence to support setting them, and NG201 on antenatal care — published on 19 August 2021, twelve years after the IOM report — focuses instead on healthy eating, physical activity and clinical monitoring. So a woman in the UK comparing herself against a number on this page is comparing herself against a standard her own health service has declined to adopt.
There is also a live question about whether ranges derived largely from Western populations transfer directly to Asian populations, where pre-pregnancy BMI distributions differ and where lower BMI thresholds are used for other purposes. Several countries have developed their own guidance for this reason.
None of that makes the IOM ranges useless. It makes them one reference point rather than a verdict, and it is a reason to weigh your own clinician’s view above a calculator’s.
When being outside the range matters
Both directions carry documented associations, and both are common enough that being outside the band is not by itself a cause for alarm.
Gaining more than recommended is associated with a larger baby, a higher likelihood of caesarean delivery, gestational hypertension and greater weight retention afterwards. Gaining less is associated with a smaller baby and with preterm birth, though for women who began pregnancy in the obese category, gaining below the recommended range is not consistently associated with harm and is sometimes an acceptable outcome.
What warrants prompt medical attention rather than watchful waiting is sudden change: a gain of more than about 2 kg in a week, or rapid swelling of the face and hands, can be a sign of pre-eclampsia and should be reported the same day. Persistent vomiting preventing you from keeping food down, or ongoing weight loss after the first trimester, also needs assessment.
What this page will not tell you to do
It will not tell you to eat less. Deliberate weight loss during pregnancy is not recommended, and a calculator is not in a position to be prescribing intake to anyone who is pregnant. Nothing here is medical advice, a diagnosis or a nutrition plan.
Your midwife, obstetrician or a registered dietitian can see everything this page cannot: your blood pressure, your urine, your glucose tolerance, your baby’s growth on ultrasound, your history and how you are actually feeling. Take a result you are unsure about to them.
Nothing you enter is transmitted
Your weights, height, week and pregnancy type are used locally in your browser. Nothing is sent, stored or logged; there is no account and no upload, and the page keeps working with the connection off once loaded.
Questions people ask about pregnancy weight gain
How much weight should I gain in pregnancy?
It depends entirely on your pre-pregnancy BMI: 12.5 to 18 kg if you were underweight, 11.5 to 16 kg if normal weight, 7 to 11.5 kg if overweight, and 5 to 9 kg if you started in the obese category, for a single baby.
Why does it ask for my weight before pregnancy?
Because the ranges are indexed to pre-pregnancy BMI and differ by up to 9 kg between categories. Entering your current weight puts you in the wrong band and gives you a target that is wrong for the rest of the pregnancy.
I have gained nothing in the first trimester. Is that a problem?
Usually not. The first trimester accounts for only about 0.5 to 2 kg in total, and losing a little to nausea is common. Mention it at your next appointment if you are concerned, particularly if you are struggling to keep food down.
What if I am carrying twins and was underweight?
There is no published range for that combination — the 2009 report says the evidence is insufficient. The tool shows the normal-weight twin range as the nearest available figure and labels it as a stand-in. This is a case where your obstetric team’s guidance genuinely replaces the calculator.
Am I eating for two?
No. Roughly no extra calories in the first trimester, about 340 a day more in the second and about 450 in the third. The quality of what you eat matters considerably more than the quantity.
Can I diet during pregnancy?
Deliberate weight loss is not recommended in pregnancy. If your weight is a clinical concern, that is a conversation with your maternity team, who can manage it safely alongside your baby’s growth.
Does the UK use these ranges?
No. NICE does not set recommended weight-gain ranges, taking the view that the evidence does not support them. If you are in the UK, treat the numbers here as a reference point rather than as your target.
I gained 2 kg in a week. Should I worry?
Contact your midwife or doctor the same day, particularly if it comes with swelling of the face or hands, headache or visual changes. Rapid gain can be a sign of pre-eclampsia and is one of the few things on this page that is genuinely urgent.
Do you store my weights?
No. Nothing is saved or transmitted.
Sources behind these numbers
- Institute of Medicine and National Research Council — Weight Gain During Pregnancy: Reexamining the Guidelines, 2009 — every range on this page, including the explicit statement that no recommendation can be made for underweight women carrying twins.
- CDC — weight gain during pregnancy, for the US framing of the same ranges.
- NICE NG201 — Antenatal care, published 19 August 2021, which sets no weight-gain targets, and why.
- American College of Obstetricians and Gynecologists, for the clinical framing of gain outside the range.
- WHO — obesity, for the BMI categories the ranges are indexed to.