Elizabeth Decker had a stressful second pregnancy, plagued by daily vomiting and the worry that this baby, like her first, would drive her blood pressure dangerously high and need to be delivered preterm. Oddly, the most relaxing part was her final trimester, which overlapped with the world’s descent into Covid lockdown in spring 2020.
Ms. Decker, who is 36 and lives in North Reading, Mass., decided to leave her high-pressure job as a lawyer and stay home. Her husband, a teacher, started working from home at that time, teaching online. He cared for their toddler and handled meals while Ms. Decker rested and slept. “I was able to really not do anything for the last three months of my pregnancy,” she said. The blood pressure spike that her doctor had expected didn’t arrive until late June, a week past Ms. Decker’s due date, at which point she delivered a healthy baby.
An ambitious worldwide study of births suggests that Ms. Decker was not the only expectant parent who avoided the experience of a preterm delivery during the earliest months of lockdown. The study, published Monday in the journal Nature Human Behavior, showed that across a group of mostly high-income countries — such as the United States, Belgium, Canada, Chile, Denmark and Switzerland — in spring 2020, there were about 4 percent fewer preterm births than expected. At a global level, the study’s authors estimated, the change most likely added up to nearly 50,000 premature births averted in the first month of lockdown alone.
The finding could help researchers better understand the causes of preterm birth, which remain frustratingly elusive to medical science.
“This is a unique natural experiment, where the whole world experienced this pretty drastic lockdown at the same time,” said Meghan Azad, an associate professor of pediatrics and child health at the University of Manitoba and one of the study’s leaders. “So it was a neat opportunity to look at what that might mean for maternal-child health.”
Around the time Ms. Decker was caring for her full-term infant, doctors in a variety of countries were also seeing fewer premature babies than they had expected. Some of their observations, shared before peer review but since formally published, were striking: Across Denmark, for instance, numbers of the smallest preemies were down by 90 percent. At a hospital in Ireland, very early births dropped by three-quarters or more.
The Irish authors speculated that pregnant people who were locked down at home might be experiencing lower stress, less air pollution or fewer viral or bacterial infections, all of which might reduce their odds of delivering early.
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Responding to the Irish study on Twitter, Dr. Azad wondered aloud — morbidly, she admitted — if doctors were seeing drops in premature births that spring because some of those babies had been lost to stillbirth or miscarriage.
Within two days, Dr. Azad was teaming up with other scientists to study this very question. “It was this kind of crazy time,” she said. “A bunch of researchers had a lot of time on their hands, because their projects were slowed down or their conferences were canceled.” They were free to dive into a large-scale side project.
The collaboration ultimately grew to include over 100 scientists from across the world, and 52 million births. Using data from 2015 to 2020, the scientists modeled the expected numbers of preterm births and stillbirths in the first months of each country’s strict lockdown period.
They realized that data from smaller samples, such as a single hospital, might not tell a full story. For example, Dr. Azad said, what if that hospital had become a dedicated site for Covid treatment and had simply diverted its pregnant patients elsewhere?
For that reason, the researchers focused their main analysis on high-quality data sets that covered an entire country, or a large region of a country. That included 18 high-income and upper-middle-income nations, as defined by the World Bank. Although the results varied across sites, the researchers concluded that preterm births had dropped by an average of 4 percent during both the first and second months of lockdown.
In the third month, the statistical signal was weaker. By the fourth month of lockdown — as countries were most likely diverging in their guidelines and in how strictly people adhered to them, Dr. Azad said — the drop in preterm births was gone.
The authors did find a slight uptick in stillbirths in Brazil during the second and third months, and in Canada during the first month. “But that certainly didn’t seem to explain the overall decrease in preterm births” across the data set, Dr. Azad said.
The authors noted that Covid itself raises a person’s risk of both premature birth and stillbirth. But because infection with the virus was less widespread in spring 2020 than later periods, this probably didn’t affect the study’s results.
“The causes of preterm birth have been so elusive, despite considerable efforts,” said Dr. Denise Jamieson, an obstetrician at Emory University’s School of Medicine in Atlanta who was not involved in the new study. Even though the global study found a dip of only about 4 percent, “I think any reduction in preterm birth is noteworthy and important,” she said.
“The next step is to really look at the why,” Dr. Jamieson added.
Dr. Azad and Dr. Roy Philip, a co-author of the new paper and also the Irish neonatologist at University Maternity Hospital Limerick who in 2020 found a striking drop in very early births at his hospital, both said it was possible that lockdowns had quite different effects on different groups of people. A pregnant person like Ms. Becker who was able to stay home in a low-stress environment, with good support, might have benefited. A frontline worker without health insurance might have had a different experience.
In this way, the findings highlighted how much is still unknown about what causes preterm birth. “Even if there are 52 million births in the study, it is not going to immediately answer all the questions,” Dr. Philip said. “But at least this should trigger people to look more closely at what is ideal during pregnancy.”
The study also highlighted the uneven preterm birthrates across different countries. Across the five years of data, the United States had the highest preterm birthrate of any high-income nation included — just shy of 10 percent. Finland’s rate, by contrast, was below 6 percent.
The disparity isn’t surprising, Dr. Jamieson said. “Unfortunately, the United States is an outlier for a lot of important maternal and infant health outcomes when you compare it to other high-income countries.”
Future research could use this global data set to investigate such variations in maternal health. Dr. Azad said she had originally hoped to dig into the drivers of preterm birth during lockdown, not just its frequency: Were changes in air pollution correlated with changes in early births? What about hygiene, or income, or access to health care? But she lacked funding to investigate further, Dr. Azad said, and now those other projects that were deferred early in the pandemic have caught up with her and her colleagues.
Dr. Azad doubts one of her tweets today could launch a huge international research effort. People in spring 2020 had “this burning desire to do something, to either help the pandemic or make something of it,” she said. Some researchers even worked on the project without pay. “I’m a scientist; I don’t like using the word ‘magical,’” she said. “But it was kind of magical.”
Now the mysteries of preterm birth will have to wait for other investigators, Dr. Azad said, adding, “We don’t all have that extra time anymore.”