How much weight to gain in pregnancy depends on what you weighed before it. The Institute of Medicine — now the National Academy of Medicine — set four ranges in 2009, one for each pre-pregnancy BMI category, and they remain the standard used across obstetric care. This calculator applies them to your own measurements and spreads the total across the weeks so you can see where the recommendation puts you now.
- Guideline
- IOM/NASEM 2009, still current
- Underweight (BMI < 18.5)
- 28–40 lb (12.5–18 kg)
- Normal (18.5–24.9)
- 25–35 lb (11.5–16 kg)
- Overweight (25.0–29.9)
- 15–25 lb (7–11.5 kg)
- Obesity (30.0+)
- 11–20 lb (5–9 kg)
Formula
IOM/NASEM 2009 ranges, prorated across gestationPre-pregnancy BMI = weight (kg) ÷ height (m)² Expected gain by week w ≤ 13 = first-trimester allowance × (w ÷ 13) Expected gain by week w > 13 = first-trimester allowance + (w − 13) × (total − allowance) ÷ 27
- first-trimester allowance
- — 1.1–4.4 lb (0.5–2 kg) across the whole first trimester
- total
- — The recommended total gain for your BMI category
- 27
- — Weeks from 13 to 40, over which the remainder is spread
The IOM publishes a total range and a separate mean rate of gain for the second and third trimesters. Prorating the total, rather than compounding the weekly rate, guarantees the curve lands exactly on the published recommendation at 40 weeks. The two approaches agree to within about a tenth of a pound per week: for a normal-BMI singleton this works out at roughly 0.9–1.1 lb per week against the IOM's published 0.8–1 lb.
Pregnancy Weight Gain Calculator examples
| Input | Result | Category |
|---|---|---|
| 5'2" · 105 lb before · week 20 | 25–35 lb | Normal weight before pregnancy |
| 5'5" · 140 lb before · week 20 | 25–35 lb | Normal weight before pregnancy |
| 5'5" · 140 lb before · week 30 | 25–35 lb | Normal weight before pregnancy |
| 5'6" · 165 lb before · week 20 | 15–25 lb | Overweight before pregnancy |
| 5'4" · 190 lb before · week 20 | 11–20 lb | Obesity before pregnancy |
| 5'5" · 140 lb before · twins, week 24 | 37–54 lb | Normal weight before pregnancy |
How much weight should you gain in pregnancy?
It depends entirely on your pre-pregnancy BMI. The lower your starting BMI, the more you are advised to gain, because more of the gain in a leaner body goes into supporting the pregnancy rather than into maternal fat stores.
| Pre-pregnancy BMI | Category | Total gain | Rate, 2nd–3rd trimester |
|---|---|---|---|
| Below 18.5 | Underweight | 28–40 lb (12.5–18 kg) | 1–1.3 lb/week |
| 18.5–24.9 | Normal weight | 25–35 lb (11.5–16 kg) | 0.8–1 lb/week |
| 25.0–29.9 | Overweight | 15–25 lb (7–11.5 kg) | 0.5–0.7 lb/week |
| 30.0 and above | Obesity | 11–20 lb (5–9 kg) | 0.4–0.6 lb/week |
Note that the ranges are calculated from your weight before conception, not your weight today. If you do not know it, an estimate from a recent measurement is better than nothing — the categories are wide enough that a few pounds either way rarely changes which one you fall into.
How much should you gain with twins?
Substantially more. The IOM issued provisional twin guidelines in 2009, describing them as provisional because the evidence base was thinner than for singletons. They have not been superseded and remain what most obstetric services use.
| Pre-pregnancy BMI | Category | Total gain |
|---|---|---|
| Below 18.5 | Underweight | No guideline issued |
| 18.5–24.9 | Normal weight | 37–54 lb (16.8–24.5 kg) |
| 25.0–29.9 | Overweight | 31–50 lb (14.1–22.7 kg) |
| 30.0 and above | Obesity | 25–42 lb (11.3–19.1 kg) |
There is no published twin range for a pre-pregnancy BMI below 18.5: the 2009 committee found too few data to set one. If that is your category, the normal-weight range is the nearest available guide, and it is worth agreeing a target explicitly with your clinician rather than assuming one.
Where does the weight actually go?
Rather less of it is the baby than most people expect. At term, a singleton weighs around 7.5 lb — under a third of a typical 25–35 lb gain. The rest is the machinery that supported it, most of which is shed within weeks of the birth.
| Component | Approximate weight |
|---|---|
| Baby | 7–8 lb |
| Placenta | 1.5 lb |
| Amniotic fluid | 2 lb |
| Enlarged uterus | 2 lb |
| Breast tissue | 2 lb |
| Increased blood volume | 4 lb |
| Extra fluid in tissues | 4 lb |
| Maternal fat stores | 7–8 lb |
What if you gain more or less than recommended?
Both directions carry risk, which is why the guidance is a range rather than a ceiling. The associations are real but modest, and none of them is a reason for alarm about a single month that ran high or low.
- Gaining above the range is associated with a larger baby, a higher chance of caesarean delivery, and more weight retained after the birth.
- Gaining below it is associated with a smaller baby, preterm birth, and — for babies who are small for gestational age — a slightly increased need for neonatal care.
- Weight loss or no gain in the first trimester is common when nausea is severe, and is usually made up later without consequence.
- Deliberate weight loss is not recommended in pregnancy, even at a high starting BMI, without specialist supervision.
Does BMI still apply during pregnancy?
No — and this is the most common misunderstanding about these guidelines. BMI is not a valid measure once you are pregnant, because the weight gain is the pregnancy itself. The only BMI that matters here is the one from before you conceived, which is what selects your recommended range.
Recalculating BMI at week 30 and finding it has moved into a higher category is expected and means nothing. Your recommended gain does not change as the pregnancy progresses; it was fixed the day you conceived.
Frequently asked questions
How much weight should I have gained by 20 weeks?
For a normal pre-pregnancy BMI carrying one baby, roughly 7 to 12 pounds. Most of that accumulates after week 13 — the IOM allows only 1.1 to 4.4 pounds across the entire first trimester, and expects the steady rate of gain to start in the second.
Is it normal to lose weight in the first trimester?
Yes, if nausea is significant. A small loss in the first trimester is common and rarely harmful, and the gain usually catches up in the second. Persistent vomiting that stops you keeping fluids down is different and needs medical attention.
Should I calculate my BMI during pregnancy?
No. BMI is not valid in pregnancy because the added weight is the pregnancy itself. Use your pre-pregnancy weight to find the BMI category that sets your recommended gain, and ignore BMI thereafter — it will rise, and that rise means nothing.
Can I diet while pregnant if my BMI is high?
Deliberate weight loss is not recommended in pregnancy at any BMI without specialist supervision. The guidance for a BMI of 30 or above is a smaller gain of 11 to 20 pounds, not a loss. Focus on food quality and activity rather than restriction.
How much weight do you lose right after giving birth?
Usually 10 to 13 pounds within a day or two — the baby, placenta, and amniotic fluid. Most of the extra blood volume and tissue fluid clears over the following two weeks. What remains is largely the maternal fat laid down to support breastfeeding.
Are the IOM guidelines still current?
Yes. The 2009 report remains the reference standard and is endorsed by ACOG, which recommends discussing a target range at the first prenatal visit and tracking gain against it. No major body has replaced the four BMI-based ranges in the years since.
Sources
- Weight Gain During Pregnancy: Reexamining the Guidelines — Institute of Medicine and National Research Council, National Academies Press, 2009
- Weight Gain During Pregnancy (Committee Opinion 548) — American College of Obstetricians and Gynecologists, 2013
- Pregnancy weight gain: What's healthy? — Mayo Clinic
- Pregnancy — Eunice Kennedy Shriver National Institute of Child Health and Human Development