Scientists in the U.S. say a significant number of premature births may be caused by infections which are undiagnosed and the heavier the infection, the more likely the women were to deliver younger, sicker babies.
The research team at Stanford University in California found bacteria or fungi in 15 percent of the amniotic fluid samples taken from women in pre-term labour.
Babies born prematurely are vulnerable to breathing problems, underdeveloped organs, infections and cerebral palsy.
By using new technology, a highly sensitive technique called polymerase chain reaction (PCR), the researchers were able to search for micro-organisms in amniotic fluid samples collected from women in preterm labour.
The 166 women were patients at Hutzel Women's Hospital in Detroit Medical Center between October 1998 and December 2002.
The new technology along with standard laboratory cultures, allowed the researchers to determine that 25 of the 166 samples were infected with either bacteria or fungi - 17 different bacterial and 1 fungal species were identified in the positive samples with the combined PCR and culture analysis - far more than the 11 found by conventional analysis.
They also discovered at least one unknown organism that could represent a new species.
Of the women, 113 went on to deliver premature babies, all 25 of the women with infected amniotic fluid delivered their babies pre-term, while 53 of the women with no infections were able to delay their labour.
Researcher Dr. Dan DiGiulio says if such infections were picked up sooner or prevented in the first place, some premature births might be avoided.
The researchers say babies are being born prematurely more often and in the U.S. 12% are born before the 37th week of gestation, and while their findings do not prove that underlying infections of the amniotic cavity is the cause of premature birth, more investigations are called for.
The study is published in PLoS ONE.