But when Felix attended a routine check-up at 32 weeks, she was shocked to be told that she had severe pre-eclampsia, a pregnancy complication that causes dangerously high blood pressure levels and organ damage, and that she required immediate hospitalisation. The following day, doctors performed an emergency C-section, and her daughter Camryn was born two months early, subsequently spending the first month of her life in the neonatal intensive care unit.

Until then, there had been few signs that anything was amiss for Felix and her unborn baby, other than some swelling in her feet. “I wasn’t too alarmed by that, but I found out I was spilling protein and all these things about my blood pressure. It was terrifying. But our family got to go home,” she says.

While Camryn is now a healthy five year old, Felix is all too keenly aware of similar stories which have resulted in a far more tragic ending. In April 2023, her long-time team-mate Tori Bowie, a former world 100m champion and relay gold medallist at the Rio 2016 Olympics, died in childbirth from complications linked to pre-eclampsia. She was just 32.

“We were on numerous relay teams together, we competed against each other, with each other, and that was extremely shocking,” says Felix. “Someone that I’ve spent so much time with, it was really devastating.”

Unravelling a deadly enigma

Worldwide, pre-eclampsia is thought to be responsible for more than 70,000 maternal deaths and 500,000 foetal deaths every year, with many fatalities resulting from stroke or prolonged fitting as a result of the elevated blood pressure. It can occur without warning at any time during pregnancy, with some women developing early-onset pre-eclampsia before 34 weeks, and others experiencing late-onset forms of the condition. Women can even suffer from postpartum pre-eclampsia in the six weeks after giving birth.

Scientists have uncovered a few clues as to why this happens. Excessive inflammation, beginning in the uterus, disrupts the delicate communication patterns taking place between the mother’s body and the foetus. In particular, it impacts the reshaping of blood vessels within the uterus to form the placenta, the organ created to provide the foetus with the nutrients and oxygen it needs.

Because the flow of blood through the placenta is abnormal, it ultimately interferes with how the mother’s body controls blood pressure, gradually leading to hypertension and ultimately pre-eclampsia.

“When a woman becomes pregnant, her heart’s got to pump extra for the baby and the placenta,” says Ian Wilkinson, clinical pharmacologist and professor of therapeutics at the University of Cambridge, who is leading a UK-based population study of pre-eclampsia called Poppy. “The amount of blood she’s pumping each minute goes up one-and-a-half to two times [in normal pregnancy].”

Women with existing autoimmune disorders, those aged over 40 and women with a larger body mass index are known to be at greater risk, perhaps because they are unable to adapt as well to the physical toll that pregnancy places on a woman’s body.

But there are still many mysteries about why certain women develop pre-eclampsia, often without warning, and why others do not. In particular, rates are as much as 60% higher in black women, who are also more likely to experience severe forms of the condition.

Some researchers believe that the latter could be linked to poorer access to good nutrition and health insurance. “There’s structural racism, where certain patients and communities don’t have the same access to early interventions, detection screening, primarily because of where they get their healthcare,” says Garima Sharma, director of cardio-obstetrics and cardiovascular women’s health at the healthcare company Inova Health System in Fairfax, Virginia.

At the same time, Sharma says that this does not explain exactly why the condition begins in the first place. While doctors still rely heavily on clinical risk factors such as age, ethnicity and medical history to assess who might develop pre-eclampsia, the accuracy of predictions based on these factors is notoriously poor. “The sensitivity of clinical risk factors on their own is low,” says Sharma.

But with newer and improved diagnostics beginning to emerge, scientists may soon be able to shed more light on who is at risk and why.

Predicting pre-eclampsia

While specialists treating other diseases such as cancer or chronic infections can often take a biopsy of a patient’s internal tissues for further analysis, there is no easy way of studying the changes taking place in a pregnant woman’s uterus.

“We can’t just routinely go in and collect a sample of placenta [from a pregnant woman], because that can really increase the risk of miscarriage,” says Lana McClements, associate professor at the University of Technology Sydney. “And animals actually don’t develop pre-eclampsia, so rodent models, for example, are very difficult to create.”