STOCKHOLM, Dec. 3 (Xinhua) -- Women who gain weight between pregnancies run a greater risk of stillbirths and of losing their babies within a year of giving birth, Swedish researchers said on Thursday.

Women who gain more than four body mass index (BMI) units between two pregnancies run a 50 percent higher risk of infant mortality than those whose weight remained flat, Stockholm's Karolinska Institutet said in a statement.

"As many women gain weight between pregnancies, our results are very important from a public health perspective," Sven Cnattingius, professor of reproductive epidemiology at Karolinska, said in a statement.

The study, published in the British medical journal The Lancet, was carried out on some 450,000 Swedish women over two decades, between 1992 and 2012.

One in five participants gained two BMI units in weight, enough to influence the risks of stillbirth and infant mortality, the researchers said.

In Sweden, about four births per thousand result in either a stillbirth or in the death of an infant, Karolinska said.

"These tragic events are still very rare among infants of mothers with high weight gain," said Cnattingius.