The study was led by the Robinson Research Institute at the University of Adelaide showed preterm births in South Australia have increased by 40% over 28 years and early intervention by medical professionals has resulted in the majority of the increase. The study findings were published in the Obstetrics and Gynecology

Research author, Dr. Petra Verburg from the University, analysed statistics for 550,000 births in South Australia between 1986 and 2014. "The rate of preterm births (birth before 37 weeks' gestation) increased by 40% from 5.1% in 1986 to 7.1% in 2014.

"Natural" or spontaneous preterm births accounted for the majority of all preterm births during the 28 years of the study. They rose from 3.5% in 1986 to 3.8% in 2014 – only a modest increase," she said.

"However, 80% of the increase in the rate of preterm births has been due to medical professionals ending pregnancies prematurely. "The rate of clinician-initiated preterm delivery increased from 1.6% in 1986 to 3.2% in 2014.

Dr. Verburg says, "Clinicians may initiate preterm delivery due to pregnancy complications, by either inducing labour or performing a caesarean section. "Problems such as hypertension or impaired growth of the fetus are the reasons doctors initiate the majority of preterm births" she said.

While preterm birth rates have risen by 40 percent, the rate of stillbirths has fallen by 45 percent (4.2 percent – 2.3 percent) in the same time period.

Co-author and Professor Gus Dekker said, "Currently, more than a quarter of the South Australian pregnant population is obese or morbidly obese. Additionally, more than half are 30 years of age or older."

"These factors increase the likelihood of major pregnancy complications and hence, increase the likelihood that these pregnancies will end with a clinician indicated preterm birth.

"On the one hand, clinician-initiated preterm birth of growth restricted babies, or in cases of preeclampsia, may potentially prevent stillbirth, or maternal complications.

"On the other hand, babies born following totally uncomplicated pregnancies at term, have the best long-term health. Babies born preterm have a higher risk of type II diabetes, obesity and cardiac problems in later life.

"New developments in ultrasounds and blood tests which indicate how well the placenta is functioning, may hopefully help obstetricians decide what the best time is to initiate birth in complicated pregnancies," said Professor Dekker.