First-time mothers are getting older every year, with one in seven babies now born to women over 35, but the nation's caesarean rate is still climbing and thousands of women continue to smoke while pregnant, the latest report on Australia's mothers and babies reveals.

More than 282,160 babies were born in 2006, an increase of 3.6 per cent on the 2005 birth rate, and confirmation that the baby bonus, introduced by the then treasurer Peter Costello in 2004, has boosted the population by more than 30,000 in three years. However, more than one in three babies are now born by caesarean and more than half of Aboriginal mothers are smoking during pregnancy.

About 41 per cent of women in private hospitals delivered by caesarean section. 

The results of the review reflect the burgeoning strain on maternity units across Sydney, including Royal Prince Alfred Hospital, which is delivering more than 5500 babies a year in a unit built for 4000.

Overworked staff at the unit are regularly forced to divert women in labour to Canterbury Hospital, while midwives at Nepean Hospital have complained of working double shifts to cope with 3800 babies in the past year, up from 3300 the previous year.

The Australia's Mothers And Babies 2006 report, collated by the National Perinatal Statistics Unit at the University of NSW, found that the nation's caesarean rate, which has been steadily climbing in the past 10 years, has now hit 31.6 per cent, ranging from 28.8 per cent in NSW to more than 33 per cent in Queensland.

About 41 per cent of women in private hospitals delivered by caesarean section, compared with 27 per cent of public patients. About 84 per cent of mothers giving birth in 2006 who had previously had a caesarean section had another caesarean.

In NSW, about 75 per cent of babies were born in public hospitals and 25 per cent in private maternity units.

More than 17 per cent of women nationally admitted to smoking during pregnancy, including 52 per cent of indigenous mothers.

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