Boys are more likely to experience complications in the womb than girls

A new study suggests that knowing the sex of a child may help monitor or even prevent problems during pregnancy.

Chris Arnold 4 minute read April 29, 2022
Ultrasound, 12 weeks old

Researchers aren’t certain why boys in the womb are more susceptible to problems than girls, but theorize that the speed at which boys grow could be a factor. GETTY

Pregnant women are more likely to have complications with baby boys than baby girls, new research suggests. 

Scientists at St. John’s College at the University of Cambridge suggest that boys grow faster in the womb and require more nutrients, as well as oxygen. Their findings were published in the medical journal Biology of Reproduction. 

The researchers say designing treatment plans for each individual, instead of generalized plans, and encouraging lifestyle changes based on the baby’s sex could result in lifelong benefits for the child. But to do this, the parents must know the sex of the baby before birth. 

10 per cent of women experience a pregnancy disorder

About 10 per cent of women experience some form of pregnancy disorder, which ranges from fetal growth restriction to pre-eclampsia, a type of high blood pressure that can be deadly for both the mother and child. 

“Often parents don’t want to know the baby’s sex because they want it to be a surprise,”  Amanda Sferruzzi-Perri, author of the study, said in a statement. But actually knowing the sex would help to identify whether a pregnancy may be at greater risk than another because we know that some conditions of pregnancy such as pre-eclampsia and fetal growth restriction can be more prevalent in women that carry male babies than females.”

Researchers aren’t certain why boys are more susceptible to problems than girls, but theorize that the speed at which boys grow could be a factor. 

To study why this was happening, the scientists looked into mice pregnancies. They found that just as in humans, the baby boy mice were causing different responses than baby girl mice. The mouse placenta also responded better to girls than boys. 

The demand for nutrients and oxygen from the placenta can be too much, which can result in stress that baby girls do not experience, as their need for nutrients and oxygen in the womb is lower. 

The baby’s growth depends on the placenta to function properly; that means turning the food eaten by the mother into energy for the child. 

“The placenta has an amazing skill in changing how it forms and how it functions,” Sferruzzi-Perri said. “This can be seen at multiple levels from the way in which the cells form in the placenta, its genes and proteins, even its mitochondria.”

A need for sex-specific therapies

The cells are able to adapt to what the mother eats, and can change what is delivered to the womb if there is more than one baby present at once. The body is able to change and regulate what the fetus gets. 

Researchers say the study highlights the need for sex-specific therapies for placental insufficiency and fetal growth abnormalities, plus changes to lifestyle, specifically mentioning obese pregnant women. 

“The data that we’ve been generating in the lab has really shown us that we have to consider the sex of the baby when monitoring a pregnancy,” Sferruzzi-Perri said, adding that this information may help health-care providers to tailor treatments. 

Researchers also say that maternal obesity increases the risk of complications further, including higher chances of miscarriage, diabetes, and still-birth, adding that there are few ways to treat any pregnancy complications other than rest, diet, or early delivery. 

In Canada, 24.7 per cent of women are considered obese, according to the federal government.

“We’re now building more and more evidence of what to measure in the mum in pregnancy like her starting body mass index, her growth, her gestational weight, but also considering fetal sex,” Sferruzzi-Perri said. “ Routinely, clinicians do consider sex when they’re looking at ultrasound images, because sex is an important determinant of how the fetus is growing. However, we’ve not really understood before how that might be determined; how that might be interacting with the environment of the mother or the way in which the pregnancy is occurring.”


Chris Arnold is a Toronto-based writer.

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