Weight Gain During Pregnancy: What the New 1-Million-Birth Study Means

Weight gain during pregnancy is one of the most common things parents worry about, and a new study has put it back in the news. Researchers at McMaster University in Canada looked at more than one million pregnancies in Ontario and found that gaining more weight than recommended was linked to a higher risk of severe maternal complications, whatever a person's weight was before pregnancy.

This guide explains what the study reported, what it cannot tell us, and what the CDC and NHS actually say about healthy weight gain in pregnancy. It is general information, not medical advice. Your midwife, obstetrician or GP knows your health history and is the right person to talk to about your own numbers.

TL;DR

Question	Short answer
What did the study find?	Gaining more than recommended was linked to higher risk of severe maternal complications in every pre-pregnancy BMI group.
Does it prove excess weight gain causes complications?	No. It is an observational study, so it shows an association.
How much should I gain?	The CDC gives ranges by starting BMI, for example 25 to 35 pounds for a typical starting weight and one baby.
What does the NHS say?	Most people gain 10 to 12.5 kg (22 to 28 lb), mostly after week 20, and there is no strict target.
Should I diet if I am above range?	No. Talk to your midwife or doctor first. The NHS does not recommend dieting in pregnancy.
When should I call?	For severe headache, vision changes, heavy bleeding, sudden swelling or reduced baby movements. Contact your maternity unit or emergency services.

What did the new study report?

The research was led by Sahar Khademioore and senior author Laura Anderson at McMaster University, using births recorded in the BORN Ontario birth registry. According to the Medical Xpress report on the work, the team looked at more than one million pregnancies and identified more than 15,000 cases of severe maternal morbidity. That is the term researchers use for life-threatening complications around birth, such as heavy bleeding, preeclampsia and admission to intensive care.

Reported findings include:

• About 53% of pregnancies gained more weight than recommended, and about 19% gained less.
• Gaining above the recommended amount was linked to higher risk of severe complications in every pre-pregnancy BMI category.
• Existing conditions such as diabetes, asthma and high blood pressure were linked to roughly double the risk of severe complications, and the risk rose further when weight gain fell outside the recommended range.
• Gaining less than recommended was also linked to higher risk, mainly in the typical-weight and overweight groups.

The paper is in JAMA Network Open (DOI 10.1001/jamanetworkopen.2026.40027). It is a retrospective cohort study of 1,035,927 single-baby pregnancies with a hospital birth at 20 weeks or later in Ontario between April 2012 and March 2022, with 15,119 cases of severe maternal morbidity (14.59 per 1,000 births). Weight gain was classed as inadequate, adequate or excessive using the 2009 Institute of Medicine guidelines, the same ranges the CDC lists. The paper reports that gaining less than recommended was linked to higher risk mainly in people with a typical or overweight starting BMI. The authors note that severe morbidity was not an outcome used to set those guidelines, and describe their findings as complementing existing guidance rather than replacing it. The study covers single-baby pregnancies, so it does not tell us about twins.

What can this kind of study show, and what can it not?

A registry study like this is powerful because of its size. With over a million births, it can pick up patterns in rare but serious events. Those are exactly the events that smaller studies miss.

But it has limits, and they matter for how you read the headline.

It shows an association, not a cause

People who gain a lot of weight in pregnancy may differ in many ways: diet, blood sugar, blood pressure, fluid retention, access to care and more. Some of the weight gain might even be a sign of a condition, such as fluid build-up with preeclampsia, rather than the cause of it. A study like this cannot untangle all of that.

Risk is relative, and the absolute risk is still small

Severe maternal morbidity is uncommon. In the study, 15,119 cases arose among 1,035,927 pregnancies, which is about 1.5% (14.59 per 1,000 births). A higher relative risk applies to a small starting number. Most people who gain above the range will not have a severe complication.

One study does not change guidelines

Official recommendations are built from many studies and reviewed by expert groups. The findings fit with what health bodies already say: gaining within the recommended range is generally linked to better outcomes. They are a reason to take the guidance seriously, not a reason to panic or to blame yourself.

What do CDC and NHS guidance say?

United States: CDC ranges by starting BMI

The CDC says that "how much weight you should gain during pregnancy is based on your body mass index (BMI) before pregnancy." For one baby, it gives these total gain ranges:

Weight before pregnancy	BMI	Recommended total gain (one baby)
Underweight	under 18.5	28 to 40 pounds
Typical weight	18.5 to 24.9	25 to 35 pounds
Overweight	25.0 to 29.9	15 to 25 pounds
Obesity	30.0 to 39.9	11 to 20 pounds

For twins the CDC lists higher ranges, and for triplets or more it advises talking to your health care provider about weight gain goals. The ranges come from the Institute of Medicine (now the National Academy of Medicine).

United Kingdom: a typical range, not a target

The NHS takes a softer approach. It says most pregnant people gain between 10 kg and 12.5 kg (22 lb to 28 lb), mostly after week 20, and that weight gain varies. It advises talking to your midwife or GP if your BMI is over 30 or under 18.5, or if you are worried about your weight. The NHS notes that gaining too much can raise the chance of high blood pressure, gestational diabetes and preeclampsia, while gaining too little can lead to premature birth or a low birth weight baby. It also says dieting is not recommended in pregnancy.

Why the two differ

The CDC gives numeric ranges by BMI, while the NHS describes an average and relies on individual advice from your midwife. Neither is wrong. If you are unsure which applies to you, follow the advice from your own care team.

What counts as healthy weight gain, week by week?

Gain is not even. Most people gain little in the first trimester, often a few pounds or none, particularly if they have nausea. Most of the gain comes later, as the baby, placenta, amniotic fluid, blood volume and breast tissue grow. This is why the NHS says most of the gain happens after week 20.

You can read how these stages fit together in our pregnancy week by week trimester guide. To see an example range based on your starting weight, you can try the free pregnancy weight gain calculator. It is an estimate to talk through with your care team, not a target to be judged against.

If you are above or below the range

First, a gentle word. Weight is a sensitive topic, and nobody should feel shamed about it. Weight gain is influenced by nausea, food access, medicines, stress, sleep, health conditions and genetics, and many of those are outside your control.

If you think you are above the range, the useful step is a conversation, not a crash diet:

• Ask your midwife or doctor how your gain looks for your stage, and how often they will check.
• Ask whether your blood pressure and blood sugar checks are up to date.
• Talk about food and gentle activity that suit you. Our pregnancy nutrition essentials guide covers balanced eating, and the pregnancy safe foods tool can help with what to avoid.

If you are below the range, perhaps because of nausea, vomiting or a poor appetite, tell your care team early. Severe sickness in pregnancy is treatable, and you do not need to wait.

When to call your midwife or doctor

Weight gain on its own is rarely an emergency. But sudden changes can be a sign of something else. Contact your midwife, maternity unit or doctor the same day, or call emergency services, if you have:

• a severe or persistent headache, especially with changes in vision such as flashing lights or blurring
• sudden swelling of your face, hands or feet
• pain just under your ribs, or feeling very unwell
• heavy vaginal bleeding
• your baby moving less than usual
• vomiting so often you cannot keep food or drink down

These are the kind of warning signs set out in our pregnancy warning signs guide. Trust your instincts. If something feels wrong, call.

What you can do now

1. Find your starting point. Note your pre-pregnancy weight and BMI, or ask your midwife to work it out. It sets which range applies.
2. Ask for your plan. At your next appointment, ask what weight gain range suits you and whether you will be weighed at each visit.
3. Eat regularly and well. Focus on a mix of vegetables, fruit, protein, whole grains and dairy or alternatives. You do not need to "eat for two".
4. Move in ways that feel good. Ask your care team what kind of activity suits you, then do what you enjoy.
5. Mention your health conditions. If you have diabetes, asthma or high blood pressure, make sure your care team knows, because the study linked these to higher risk.
6. Keep checks and scans. Blood pressure and urine checks are how teams spot preeclampsia early.
7. Be kind to yourself. The goal is a healthy pregnancy, not a perfect number.

Sources and related reading

Official and primary sources:

• CDC: Weight Gain During Pregnancy
• NHS: Your weight in pregnancy
• JAMA Network Open paper (DOI 10.1001/jamanetworkopen.2026.40027), McMaster University / BORN Ontario
• Medical Xpress news report on the study (news peg only)

Related Egspect guides and tools:

• Pregnancy nutrition essentials
• Pregnancy week by week: a practical trimester guide
• Pregnancy warning signs
• Pregnancy weight gain calculator
• Pregnancy safe foods tool

This guide is general information, not medical advice. Facts were accurate as of October 23, 2026; guidance changes, so check with your care team.