Some public health researchers are seeing hints that the coronavirus pandemic might help solve a longstanding puzzle: What causes premature birth?

Studies in Ireland and Denmark this summer showed that preterm births decreased in the spring during lockdowns to stop the spread of the virus in those countries. Anecdotally, doctors around the world reported similar drops. They speculated that reduced stress on mothers, cleaner air or better hygiene might have contributed. A large study from the Netherlands, published on Tuesday in The Lancet Public Health, has yielded even stronger evidence of an association between the lockdowns and a smaller number of early births.

The authors used data from a national newborn screening program. As in the United States, nearly all babies in the Netherlands have a spot of blood collected a few days after birth and tested for certain diseases. Information including the babies’ gestational ages — at how many weeks of the mother’s term they were born — is collected at the same time.

The Dutch researchers examined newborn blood screening data from 2010 to 2020, which included more than 1.5 million infants. Over 56,000 of those babies were born after the Netherlands started locking down in response to the pandemic.

With their large data set, the researchers compared early births in windows one to four months before and after the lockdown. Looking at the same windows in earlier years let them account for any other trends, such as seasonal changes in premature birth. No matter which windows they used, the researchers saw that premature births had dropped after March 9, when the government in the Netherlands began warning the public to take more hygiene measures and to stay home if they had symptoms or possible exposures to the virus. Within the next week, schools and workplaces began to close down.

“We could see that this impact was real,” said Dr. Jasper Been, a neonatologist at Erasmus Medical Center in Rotterdam and the study’s lead author. The reduction was from 15 to 23 percent, he said.

Dr. Roy Philip, a neonatologist at University Maternity Hospital Limerick in Ireland and author of the earlier study finding fewer preterm births this spring in the country, said he was “truly delighted” to read the new paper. He said it suggested that the lockdown did, in fact, cause the drop in early births. Unlike the Irish and Danish studies, the new Dutch study showed a decrease across all ages of premature babies — not only the earliest ones.

But as is often the case after such an investigation, Dr. Been said, “You have more questions than you started out with.”

One of those unanswered questions is about stillbirths. In the best-case scenario, the preemies that were missing from hospitals this spring were born as healthy full-term infants. But it’s possible that some of those missing preemies died, instead.

Stillbirths “might actually be the dark side of this,” Dr. Been said. Researchers couldn’t measure stillbirths in the Dutch data set, because only live infants undergo newborn screening. If most of the missing preemies were actually stillborn, Dr. Been said, there would have to have been a huge increase in stillbirths — maybe three times the usual number, conservatively. No one has reported such a change so far.

At one hospital in London, a study showed an increase in stillbirths after the start of the pandemic (not the lockdown). This increase might have come from hidden coronavirus infections, the authors wrote, or from women’s reluctance to seek medical care during the pandemic. Studies in Nepal and India showed that mothers were less likely to give birth in hospitals in the spring, and that stillbirths increased in those countries. Further complicating the picture, Covid-19 itself may raise the odds of premature birth.

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To find out how pandemic lockdowns affected preterm birth across different countries, the Dutch researchers have joined an international consortium of nearly 40 nations sharing data. Lockdowns might have been good for the health of mothers and babies in some places and not others, Dr. Been said, adding, “In general, we see that this whole pandemic seems to increase inequalities a lot.” The Dutch study even hinted that the drop in preterm births was limited to wealthier neighborhoods, although the result wasn’t statistically significant.

“Boy, did this affect people really differently,” said Jennifer Culhane, a senior research scientist in obstetrics and gynecology at Yale School of Medicine.

Dr. Culhane said the new study confirmed what she had been hearing from colleagues in the United States about lower preterm birthrates this spring. “Anecdotally, this is what we’re all hearing and feeling,” she said. In the absence of strong evidence in the United States, she and her colleagues have partnered with an insurance company to begin a large-scale study of how lockdowns affected premature births in this country.

She speculates that the lockdown’s effects might have depended on economic circumstances. For mothers who had the resources to work from home, “This could have a been a stress-reducing moment,” she said. For essential workers or those experiencing economic hardships, it might have been a different story.

“We see gross inequities in this country,” Dr. Culhane said. “My guess is that women that had no reprieve from rat-racy-type things had, actually, an enhanced burden of stress.”

Scientists have long struggled to find the causes of premature birth, and good ways to prevent it. “We haven’t been able to reduce the preterm birthrate come hell or high water,” Dr. Culhane said. Future studies could finally reveal ways to prevent early births for all mothers — no pandemic needed.