# Twin Pregnancy Weight Gain: The IOM Targets by BMI _Three ranges, one missing one, and a guideline that calls itself provisional — what the numbers are and how much weight to put on them._ - Source: https://proba.baby/en/blog/twin-pregnancy-weight-gain - Published: 2026-09-05 - Author: ProbaBaby Team - Category: Twins & Multiples - Tags: twins, multiples, nutrition, calculators - Language: English > Twin pregnancy weight gain has its own targets, and they are not the singleton numbers doubled. Here are the IOM 2009 provisional ranges by pre-pregnancy BMI, why there is no range at all for underweight women, when in the pregnancy the gain actually matters, and what the research since 2009 shows about gaining below or above them. > **Medical disclaimer.** This article is general information about twin pregnancy weight gain and is not medical advice. Twin pregnancies are monitored more closely than singleton ones, and your obstetrician or midwife may set a target that differs from any published range — theirs comes first. Contact your care team about sudden weight changes, swelling in your hands or face, or any symptom that worries you. Twin pregnancy weight gain has its own targets, and they are not the singleton numbers doubled. The Institute of Medicine's 2009 provisional twin ranges are **37–54 lb (16.8–24.5 kg)** at a normal pre-pregnancy BMI, **31–50 lb (14.1–22.7 kg)** if you started overweight, and **25–42 lb (11.3–19.1 kg)** at a BMI of 30 or above. If your pre-pregnancy BMI was under 18.5, there is no published range at all. That is not an omission in this article — it is the state of the guideline, and it is worth knowing before a calculator hands you a confident number. ## The three ranges, and the missing fourth | Pre-pregnancy BMI | Total gain (lb) | Total gain (kg) | |---|---|---| | Under 18.5 (underweight) | no range published | no range published | | 18.5 – 24.9 (normal) | 37 – 54 | 16.8 – 24.5 | | 25.0 – 29.9 (overweight) | 31 – 50 | 14.1 – 22.7 | | 30.0 and above (obesity) | 25 – 42 | 11.3 – 19.1 | One footnote on the metric column, because it explains why sources disagree by a few hundred grams. The IOM published the pounds and rounded the kilograms hard — 17–25, 14–23, 11–19 kg. The finer figures above are the direct conversions of 37–54, 31–50 and 25–42 lb, which is what ProbaBaby uses; the 2023 French analysis quotes the same conversions but puts the obesity floor at 11.4 kg rather than 11.3. Same guideline, different rounding; nothing hangs on the difference. Two things the table does not carry: a weekly rate for twins, and a number for the underweight row. The IOM publishes a total at term for twin pregnancies and stops there — the per-week rates in the 2009 report are singleton rates. So a "you should be up X kg this week" figure for twins is not a quoted guideline; it is someone spreading that total across the weeks. Reasonable to do, worth labelling as such. ## Why "provisional" is the most important word in it The singleton ranges came out of an analysis weighing maternal outcomes against infant outcomes. The twin ranges did not, and the committee said so: it was unable to conduct the same kind of analysis for women with twins as it did for women with singletons. What it did instead was descriptive. It took the interquartile range — the 25th to 75th percentiles — of cumulative weight gain among women who delivered their twins between 37 and 42 weeks, where the babies weighed at least 2,500 g on average. A 2023 French analysis describes the underlying data as a historical US cohort of term livebirths from four hospitals, spanning 1979 to 1999. So the range describes the middle half of women whose twin pregnancies went well, in one country, decades ago. That is genuinely useful information. It is not a trial, and it was never claimed to be one. ## The gap, and what gets filled into it Here is the sentence, from the 2009 report itself: *insufficient information was available with which to develop even a provisional guideline for underweight women with multiple fetuses.* Research has been working around it ever since. A 2020 US survey of twin pregnancies had to analyse its six underweight participants alongside the normal-weight group, explaining that the IOM does not specify guidelines for underweight women pregnant with twins. The 2023 French cohort went further and derived its own underweight range, 13–21 kg — published as a research finding for that population, not as a guideline anyone has adopted. Some consumer tools fill the hole rather than name it. Tick "I am carrying twins" on What to Expect's web weight-gain calculator and it lists "Pre-pregnancy BMI is less than 18.5: 50 to 62 pounds" — a range the IOM never issued (page checked 2026-09-05). ProbaBaby does the opposite: declare twins with an underweight pre-pregnancy BMI and the app returns no target and says there is no established guideline, rather than inventing one. If that is your situation, the number has to come from your care team, and it is a good thing to raise at your next appointment rather than at 1am with a calculator. ## When in the pregnancy the gain actually matters A total at term says nothing about *when* the gain has to happen — and that turns out to be the part that calms people down. A 2017 NICHD study followed 171 twin pregnancies with six ultrasound exams each. Maternal weight gain from 14 to 20 weeks and from 21 to 27 weeks was associated with fetal weight. Weight gain in the first 13 weeks was not. Neither was gain from 28 to 34 weeks associated with birth weight. The researchers framed the first-trimester result as reassurance, and specifically for twin parents: nausea is more likely in a twin pregnancy, and plenty of people gain nothing — or lose — before week 13. That early plateau is not the thing the evidence says to worry about. What it does suggest is that the middle of the pregnancy is where the curve is worth watching. That is also the stretch where a weekly weigh-in is easiest to keep up, before the third trimester gets uncomfortable. Our [singleton pregnancy weight gain guide](/blog/pregnancy-weight-gain-guide) covers the IOM's per-week rates, which exist only for one baby. ## What happens above and below the range Two studies do most of the work here, and they disagree about the range while agreeing about the asymmetry: the floor matters more than the ceiling. A 2017 retrospective cohort of 741 twin mothers and 1,482 infants — deliveries in Nova Scotia, Canada, between 2003 and 2014 — classified gain against the provisional guidelines. Small-for-gestational-age ran 30% in the below-guideline group, 21% within it, 20% above it. Gaining **below** — 27% of the cohort — carried adjusted odds of 1.44 (95% CI 1.01–2.06) for a small-for-gestational-age baby and 1.55 (1.07–2.23) for low birth weight, plus a mean 4.4 additional days of neonatal intensive care (95% CI 0.7–8.2). Gaining **above** — 30% — showed no reduction in small-for-gestational-age odds (0.92, 95% CI 0.62–1.36) and higher odds of labour induction (1.65, 95% CI 1.08–2.53). What the authors concluded from that was deliberately narrow: below-guideline gain was associated with some adverse neonatal outcomes, and alternate twin ranges need researching. They did not propose a new target. The French cohort adds the counterweight. Among 2,562 women who delivered term twins at healthy birthweights, mean gain was 16.1 kg and **46.5% were still classified as having gained too little** by the IOM twin ranges, against 10.0% who gained too much. The alternative ranges the authors derived came out lower than the IOM's in every category the IOM actually covers — 13–20 kg at a normal BMI against 16.8–24.5, 11–19 overweight against 14.1–22.7, 7–16 with obesity against 11.4–19.1. Read those two together and you get something more useful than a target: gaining meaningfully less than the range is worth a conversation, gaining more than it buys nothing, and being labelled "inadequate" by a range built on a different population is common enough that it should never be read as a failing grade. ## Twins get a shorter runway, so the denominator changes One structural point that trips up anyone using a singleton calculator with a twin pregnancy. The provisional totals describe twin pregnancies delivered at term **for twins** — which is not 40 weeks. For uncomplicated dichorionic twins, planned birth is usually offered around 37 weeks; ProbaBaby's timeline uses a 37+0 target with a 34+0 to 38+6 window, sliding to 36+0 for monochorionic diamniotic twins and 33+0 for monochorionic monoamniotic twins. One table drives all of those dates, grounded in NICE NG137 and ACOG Practice Bulletin No. 231 — the two guidelines the app's due date calculator names on screen. US practice as summarised in StatPearls puts uncomplicated di-di delivery at 38 0/7 to 38 6/7 weeks, mo-di at 34 0/7 to 37 6/7, and mo-mo at 32 0/7 to 34 0/7. The practical consequence: a larger total arrives across fewer weeks. A tool that divides a twin total by 40 weeks will quietly tell you that you are ahead of schedule when you are on it. And none of this replaces your medical due date — the 40-week estimate stays exactly where it is, on your notes and in the app; the twin target sits beside it. [Why a twin due date has two dates](/blog/twin-due-date-explained) works through that pairing properly, and the [twins hub](/twins) collects the rest. ## How ProbaBaby helps ProbaBaby's [pregnancy tracking](/pregnancy-tracking) module carries the IOM 2009 twin table in its weight-gain engine once you declare a twin pregnancy: 16.8–24.5 kg at a normal pre-pregnancy BMI, 14.1–22.7 kg overweight, 11.3–19.1 kg at a BMI of 30 or above, labelled on screen as provisional twin guidelines. Because the IOM sets no weekly rate for twins, the app ramps through the first trimester and then interpolates toward the term total at twin term — 37 weeks, not 40 — and holds it flat after, so the target you see is scaled to a twin timeline rather than a singleton one. With an underweight pre-pregnancy BMI and twins declared, it returns no target and tells you there is no established guideline, which is the honest answer. The standalone [weight gain calculator](/tools/pregnancy-weight-gain-calculator) uses the same term totals if you would rather not set up a profile. ## FAQ **How much weight should I gain with twins?** The Institute of Medicine's 2009 provisional twin ranges are 37-54 lb (16.8-24.5 kg) at a normal pre-pregnancy BMI, 31-50 lb (14.1-22.7 kg) if you were overweight, and 25-42 lb (11.3-19.1 kg) at a BMI of 30 or above. These are totals at term, not weekly targets, and your care team may set a different number for you. **Why is there no twin weight-gain range for underweight women?** Because the IOM had too little data to set one. Its 2009 report says outright that insufficient information was available to develop even a provisional guideline for underweight women with multiple fetuses. Any number you see for a BMI under 18.5 with twins was filled in by someone other than the guideline. **Are the IOM twin ranges based on trials?** No. They are the 25th to 75th percentiles of weight gain among women who delivered twins at 37-42 weeks whose babies averaged at least 2,500 g, in a historical US cohort. The committee said it could not run the outcome trade-off analysis it ran for singleton pregnancies, which is why the twin ranges are labelled provisional. **Is it normal not to gain weight in the first trimester with twins?** It is common, and a 2017 NICHD study of 171 twin pregnancies found that weight gain in the first 13 weeks was not associated with fetal weight, while gain between 14 and 27 weeks was. Nausea is more likely in a twin pregnancy, and the researchers framed the finding as reassurance. Tell your provider if you are losing weight or cannot keep fluids down. **What happens if I gain less than the twin range?** A 2017 Canadian cohort of 741 twin pregnancies found gaining below the provisional guidelines was associated with 44% higher adjusted odds of a small-for-gestational-age baby, 55% higher odds of low birth weight, and a mean 4.4 more days of neonatal intensive care. Gaining above the range showed no reduction in those odds, and higher odds of labour induction. The authors called for research into alternate twin ranges rather than declaring a new target. **Do twin weight-gain targets run to 40 weeks?** No. The provisional totals describe twin pregnancies delivered at term for twins, which is earlier than 40 weeks - planned birth is usually offered around 37 weeks for uncomplicated dichorionic twins, and earlier for shared-placenta twins. The same total therefore arrives over fewer weeks than in a singleton pregnancy. **Should I weigh myself every week in a twin pregnancy?** Weekly, on the same scale at the same time of day, gives you a trend you can read; daily weighing mostly measures hydration and meals. Bring the trend line to your appointments rather than any single number, because twin pregnancies are monitored more closely anyway. ## Sources - Institute of Medicine & National Research Council - Weight Gain During Pregnancy: Reexamining the Guidelines — 2009; Rasmussen KM, Yaktine AL, eds. The provisional twin ranges, their derivation from the 25th-75th percentiles, and the statement that insufficient information existed for underweight women with multiple fetuses: https://www.nationalacademies.org/read/12584/chapter/2 - Amyx M, Korb D, Zeitlin J, Schmitz T, Le Ray C - Gestational weight gain adequacy among twin pregnancies in France (Maternal & Child Nutrition) — 2023; JUMODA cohort, 2,562 term twin pregnancies with optimal birthweights - mean gain 16.1 kg, 46.5% inadequate / 10.0% excessive by the IOM twin ranges; derived ranges 13-21 kg underweight, 13-20 normal, 11-19 overweight, 7-16 obesity; describes the IOM data as a historical US four-hospital cohort, 1979-1999: https://pmc.ncbi.nlm.nih.gov/articles/PMC9749591/ - Lutsiv O, Hulman A, Woolcott C, et al. - Examining the provisional guidelines for weight gain in twin pregnancies: a retrospective cohort study (BMC Pregnancy and Childbirth) — 2017; Nova Scotia, Canada, deliveries 2003-2014; 741 mothers and 1,482 infants - below-guideline gain aOR 1.44 (1.01-2.06) for small-for-gestational-age and 1.55 (1.07-2.23) for low birth weight, mean +4.4 NICU days (0.7-8.2); above-guideline gain aOR 0.92 (0.62-1.36) for SGA and 1.65 (1.08-2.53) for labour induction: https://pmc.ncbi.nlm.nih.gov/articles/PMC5622523/ - NICHD - Second trimester weight gain associated with fetal growth in twin pregnancies — 2017; NICHD Fetal Growth Studies, 171 twin pregnancies with six ultrasounds each, published in the American Journal of Clinical Nutrition: https://www.nichd.nih.gov/news/releases/102317-twin-pregnancies - Whitaker KM, Baruth M, Schlaff RA, et al. - Association of provider advice and gestational weight gain in twin pregnancies (BMC Pregnancy and Childbirth) — 2020; underweight participants were analysed with normal-weight women because the IOM does not specify twin guidelines for them: https://pmc.ncbi.nlm.nih.gov/articles/PMC7376962/ - What to Expect - Pregnancy Weight Gain Calculator — Checked 2026-09-05; the "I am carrying twins" option lists "Pre-pregnancy BMI is less than 18.5: 50 to 62 pounds" - a twin range the IOM 2009 report never issued: https://www.whattoexpect.com/pregnancy-weight-gain-calculator - StatPearls - Multiple Birth Delivery (NCBI Bookshelf) — Delivery timing for uncomplicated twins: di-di 38 0/7-38 6/7, mo-di 34 0/7-37 6/7, mo-mo 32 0/7-34 0/7 weeks: https://www.ncbi.nlm.nih.gov/books/NBK599524/ --- ProbaBaby (listed on the App Store as “Pregnancy & Baby Tracker - PB”) is a free iPhone, iPad and Apple Watch app for pregnancy, newborn and postpartum tracking, built for twins, in 32 in-app languages, with an optional AI baby-face prediction that computes Mendelian genetics (Punnett squares) from the traits you enter before any image is generated. Tracking, tools, PDF reports and family sharing are free; only AI predictions use credits — 5 starter credits cover your first Scientific prediction — and no subscription is required. Requires iOS 26 or later. Not available on Android.