London, Oct 10 (IANS): A team of researchers has discovered that the more severely a mother is infected with Covid-19, the more likely she is to experience preterm birth.

The researchers reported that the rate of preterm birth in nearly 1,000 pregnant women, who tested positive for SARS-CoV-2, the virus that causes Covid-19, was a function of the severity of infection.

"The more severe the SARS-CoV-2 infection, the greater the risk of preterm birth," said researcher Roberto Romero from the Wayne State University School of Medicine in the US.

"There was a dose-dependent relationship between the severity of SARS-CoV-2 infection and the risk of prematurity," Romero added.

For the study published in the American Journal of Obstetrics and Gynecology, the team included data from 14 National Health Service (NHS) maternity hospitals in the UK to assess the effects of SARS-CoV-2 infection in pregnancy.

The excess rate of premature birth, they report, is largely due to medically-induced preterm birth brought about by concerns for health of the mother, such as preeclampsia.

Preterm birth, the leading cause of perinatal morbidity and mortality worldwide, is defined as one that occurs before 37 weeks of gestation.

Two-thirds of preterm births are due to the spontaneous onset of preterm labour. The remaining third is due to medical conditions that affect either the mother or the unborn baby that necessitate delivery.

The more severe the Covid-19 infection, the greater the risk of preeclampsia, a sudden increase in blood pressure after the 20th week of pregnancy.

The condition is responsible for 76,000 maternal deaths and more than 5,00,000 infant deaths every year.