Gaining weight during pregnancy is expected, healthy, and necessary. A study of more than a million Ontario pregnancies suggests that how much you gain matters more than many people realize, whatever you weighed when you started.
Researchers at McMaster University found that gaining more than the recommended amount was linked to a higher risk of severe, potentially life-threatening complications at every starting weight, according to the World Obesity Federation's release on the study. The findings, presented at the International Congress on Obesity this summer, make pregnancy weight gain a conversation worth having with every pregnant patient, whatever they weighed before pregnancy.
Key takeaways
The study followed 1,035,927 Ontario pregnancies from 2012 to 2022, and 52.8 percent of them gained more weight than recommended.
Gaining too much raised the risk of severe complications by 16 to 33 percent, depending on weight before pregnancy.
The biggest increase, 33 percent, appeared among people who were underweight before pregnancy.
A healthy-weight patient with chronic high blood pressure or diabetes who gained too much faced nearly three times the risk of a healthy-weight patient without those conditions who gained within the range.
Health Canada's recommended total gain runs from 5 to 18 kg, depending on your BMI before pregnancy.
What did the study of a million pregnancies find?
The research team, led by Associate Professor Laura Anderson and PhD candidate Sahar Khademioore of McMaster University, linked Ontario birth registry and hospital records from April 2012 to March 2022. Across those million pregnancies, about 1.5 per cent, or roughly 3 in every 200, involved severe maternal morbidity. That term covers serious complications during pregnancy, delivery, or the weeks after birth, such as major bleeding and sepsis, a dangerous reaction to infection.
Excessive gain was common, affecting 52.8 percent of the pregnancies, compared with 28.6 percent that stayed within the recommended range and 18.6 percent that gained too little. The study is part of the SERENE Project, a McMaster-led effort that works with people who have lived through severe complications to understand why they happen and how to prevent them.
Why does the risk show up at every starting weight?
The finding that stands out is how consistently the risk showed up across every starting weight. Excessive gain was linked to a 33 percent higher risk for people who were underweight before pregnancy, 24 percent for those at a healthy weight, and 16 percent for those who were overweight. Among people with a BMI of 30 or more, the increase ran from 21 to 28 percent, depending on the group.
The largest jump showed up in the group most people would assume had the most room to gain. Khademioore said in the Scimex summary of the study that healthy pregnancy weight gain matters for all pregnant women, whatever their weight. Gaining too little carried risks of its own, raising the risk of severe complications by 13 percent among healthy-weight patients and 19 percent among those who were overweight.
Gaining more than recommended raised the risk of severe complications in every weight group, most of all among people who started underweight.
Why do existing health conditions raise the stakes?
The risks also stacked on top of each other. Consider a patient who started pregnancy at a healthy weight but already had chronic high blood pressure or diabetes. If she gained more than recommended, her risk of severe complications was nearly three times that of a healthy-weight patient with neither condition who gained within the range.
That makes existing health conditions the place where early support pays off most. A separate McMaster-led study published in the CMAJ in March found that severe complications affect about 2.7 percent of Ontario pregnancies when tracked from conception to six weeks after birth. Our report on pregnancy complications among Black Canadians shows how unevenly those risks fall.
What is the recommended range for your body?
Recommended weight gain is not one number. Health Canada bases it on your body mass index, or BMI, before pregnancy, a measure that compares your weight with your height. Its weight gain calculator for pregnancy works out your range from your height and weight before pregnancy. For a single pregnancy, that means 12.5 to 18 kg for people who start underweight, 11.5 to 16 kg at a healthy weight, 7 to 11.5 kg if overweight, and 5 to 9 kg with a BMI of 30 or more.
Health Canada advises against trying to lose weight during pregnancy, because it could affect the baby's growth. The aim is a target you know, regular check-ins, and practical support early if the numbers drift. Everyday surroundings can matter more than people expect, as our report on heat exposure during pregnancy shows.
Health Canada's recommended total gain for a single pregnancy depends on your BMI before pregnancy.
What can you ask at your next prenatal visit?
Your weight is usually checked at routine prenatal visits, so the information exists, and the goal is to turn that number into a conversation. These steps, along with our tips on preparing for a doctor's appointment, can help you do that.
Ask for your number. Ask your doctor or midwife what the recommended weight gain range is for you, based on your weight before pregnancy.
Check in regularly. Ask how your gain compares with that range at each visit, especially in the second and third trimesters.
Ask for practical help. If you are gaining faster or slower than your range, ask for a referral to a registered dietitian who works with pregnant patients.
Plan ahead with a health condition. If you have high blood pressure, diabetes, or another chronic condition and are planning a pregnancy, book a preconception appointment.
Know the warning signs. A severe headache, vision changes, chest pain, trouble breathing, heavy bleeding, or sudden swelling during or after pregnancy needs urgent care, so call 911 or go to the nearest emergency department.
Frequently asked questions
How much weight should you gain during pregnancy?
The answer depends on your BMI before pregnancy. Health Canada recommends 12.5 to 18 kg for people who start underweight, 11.5 to 16 kg at a healthy weight, 7 to 11.5 kg if overweight, and 5 to 9 kg with a BMI of 30 or more. Twins and other multiples have different ranges.
Is it safe to diet during pregnancy?
Dieting to lose weight is not recommended during pregnancy, because your baby needs a steady supply of nutrients. If you are gaining faster than your range, the aim is to slow the gain through balanced meals and regular activity, with guidance from your care team. A registered dietitian can build a plan around foods you enjoy.
Does gaining too little weight carry risks too?
Gaining less than recommended can also raise risks, including preterm birth and a smaller baby. In the McMaster study, low pregnancy weight gain was also linked to more severe complications among patients who started at a healthy weight or were overweight. The goal is to land within your personal range, which your care provider can help you track.
Your next step
Whether you are planning, expecting, or somewhere in between, the right conversation is easier to have early. The Medimap Health Hub has more on women's health, and you can find a family practice near you or an obstetrician to talk through your personal range, or book a registered dietitian for practical help. No family doctor? Our guide to getting care without a family doctor walks through your options.
Source: World Obesity Federation news release, July 17, 2026; Scimex summary of the International Congress on Obesity abstract; Health Canada pregnancy weight gain calculator; Hospital News, April 2026, on the March 2026 CMAJ study; Health Canada prenatal nutrition guidelines on gestational weight gain.
Disclaimer: This article is for general information only and is not medical advice. It does not replace a conversation with a doctor or other qualified health professional who knows your history. If you think you are having a medical emergency, call 911 or go to the nearest emergency department.
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