New York: Women who are obese before they become pregnant face an increased risk of delivering a very premature baby, especially those that occur prior to 28 weeks of pregnancy, says a study.
However, the researchers found no link between maternal obesity and premature births that happen between 28 and 37 weeks of the normal 40-week gestation period.
The disparity suggests that premature birth may have different causes at different stages of pregnancy.
"Until now, people have been thinking about pre-term birth as one condition, simply by defining it as any birth that happens at least three weeks early," said Gary Shaw, professor of paediatrics at Stanford University School of Medicine in the US.
"But it is not as simple as that. Pre-term birth is not one construct; gestational age matters," he added.
The study examined nearly one million California births in the US between January 2007 and December 2009.
The findings appeared in the journal Paediatric and Perinatal Epidemiology.