Spontaneous preterm birth of liveborn infants in women at low risk in Australia over 10 years: a population-based study

Objectives  To describe a 10-year trend in preterm birth.
Design  Population-based study.
Setting  Australia.
Population  All women who gave birth during 1994–03.
Methods  The proportion of spontaneous preterm births (greater than or equal to 22 weeks of gestation and less than 37 completed weeks of gestation) was calculated by dividing the number of women who had a live spontaneous preterm birth (excluding elective caesarean section and induction of labour) by the total number of women who had a live birth after spontaneous onset of labour (excluding elective caesarean section and induction of labour). This method was repeated for the selected population of women at low risk.
Main outcome measure  Preterm birth rates among the overall population of women; preterm birth among all women with a spontaneous onset of labour; and preterm birth in a selected population of women who were either primiparous or multiparous, non-Indigenous; aged 20–40 years and who gave birth to a live singleton baby after the spontaneous onset of labour.
Results  Over the 10-year study period, the proportion of all women having a live preterm birth in Australia increased by 12.1% (from 5.9% in 1994 to 6.6% in 2003). Among women with a spontaneous onset of labour, there was an increase of 18.3% (from 5.7 to 6.7%). Among the selected population of low-risk women after the spontaneous onset of labour, the rate increased by 10.7% (from 5.6 to 6.2%) among first time mothers and by 19.2% (4.4–5.2%) among selected multiparous women.
Conclusions  Over the 10-year period of 1994–03, the rate of spontaneous preterm birth among low-risk women having a live singleton birth has risen in Australia.