A national snapshot of mothers and babies shows that one-fifth of mums give birth aged 35 or older.

The average age of first-time mums is 28 while about 21 per cent of mothers who gave birth in 2006 were aged 35 or older, up from 15 per cent in 1997.

Caesarean sections make up almost one-third of births in Australia.

About 84 per cent of mothers giving birth in 2006 who had previously had a caesarean section had another caesarean section.

The statistics are contained in the Australian Institute of Health and Welfare's 16th annual report on pregnancy and childbirth.

There were 9750 more births in 2006 than in the previous year, the report shows.

Of the babies born in 2006, eight per cent were pre-term (less than 37 weeks gestation), compared with seven per cent in 1997.

Six per cent of live births in 2006 were of low birth weight (less than 2500 grams) and 15 per cent were admitted to a special care nursery or neonatal unit.