THE risk of stillbirth is about twice as high for women with COVID compared to those without, and grew to about quadruple during the period when the Delta variant became dominant, a large US government study said Friday.

The analysis, carried out by the Centers for Disease Control and Prevention (CDC) was based on more than 1.2 million deliveries between March 2020 and September 2021 from a large US hospital database.

Overall, stillbirths were highly rare, accounting for 0.65 percent or 8,154 deliveries.

But, after using statistical methods to account for the effect of other variables that might bias the result, stillbirths were 1.47 times more common among COVID-positive mothers pre-Delta, 4.04 times higher after, and 1.90 times higher overall.

The authors wrote that previous research had suggested a potential biological cause for the increased risk could be the result of inflammation or decreased blood flow to the placenta.

Among deliveries with COVID-19, conditions like chronic high blood pressure, having more than one baby, heart injury, separation of the placenta from the uterus, sepsis, poor blood flow resulting in shock, life-threatening lung injury, and having to go on a ventilator or being admitted to ICU were linked to higher rates of stillbirth.

"Additional studies are warranted to investigate the role of maternal complications from COVID-19 on the risk for stillbirth," they said.

The study is among the strongest to date linking Covid and stillbirth, the authors said.

Some previous research on the subject failed to adjust for "confounders" such as pre-existing conditions, and a previous analysis using the same dataset did not detect a significantly increased risk.

"The current analysis includes an additional year of data, adding to the growing evidence that COVID-19 is associated with an increased risk for stillbirth," the authors wrote, explaining why the link was more clear now.