There were fewer premature births around the world during the first four months of COVID-19 lockdowns.

A study of 52 million births in 26 countries found a three to four per cent drop in preterm births - defined as birth before 37 weeks gestation - during that period.

However, the International Perinatal Outcomes in the Pandemic (iPOP) Study published in the journal Nature Human Behavior, found the decrease in premature births was limited to high-income countries, including Australia.

The research involved 167 collaborators across 42 countries with the University of NSW Sydney co-leading the project alongside The Children's Research Institute of Manitoba, University of Manitoba, Murdoch Children's Research Institute and the University of Edinburgh.

Study co-leader Scientia Associate Professor Helga Zoega from the UNSW said the project was one of the first large-scale analyses of birth outcomes during the early months of the global COVID-19 pandemic.

"The scope and breadth of the iPOP dataset allowed us to thoroughly examine global birth trends during the shifting pandemic landscape," she said on Tuesday.

Possible reasons for the drop in premature births could include fewer non-COVID infections due to improved hygiene practices and less air pollution because of reduced traffic during lockdowns.

Infections and air pollution can trigger inflammation, which contributes to preterm birth.

"In an average year, there are an estimated 14.8 million preterm births worldwide, meaning that even a modest reduction could have a large impact on global birth trends," Assoc Prof Zoega said.

The study estimated nearly 50,000 preterm births were averted in the first month of lockdowns.

"Understanding the underlying pathways linking lockdown with the reduction in preterm births could have implications for clinical practice and policy," she said.

The study recorded no difference in stillbirth rates in high-income countries.

Assoc Prof Zoega said the study aimed to better understand how the global health crisis affected mothers and babies.

The next step includes research to better understand the reasons behind the findings.

"Including the role of reductions in non-SARS-CoV-2 infections, reduced air pollution and changing stress levels among pregnant people during periods of lockdown."