ONE for the prince and one for the duchess.
That appears to be the monarchy's version of the former treasurer Peter Costello's oft-quoted ''one for mum, one for dad and one for the country'', with Prince William this week telling a fan in Singapore he hopes to have two children.
The Duchess of Cambridge's choice of billowing dresses on their tour of Asia and the south Pacific and her decision to drink water over wine have prompted pregnancy rumours.
Australian families are more likely to go the royal way than take the advice of Mr Costello, who is a father of three.
The most up-to-date total fertility rate, an indicator of the number of children an average Australian woman will have in her lifetime, was 1.89 babies per woman in 2010, according to the Bureau of Statistics.
A 2012 bureau analysis of the fertility rate over time showed the average woman had four children at the beginning of the 20th century, before the rate declined during the next two decades.
The baby boomer era reached a new peak of 3.5 in 1961, with the rate dropping towards two babies per woman and hovering there during the 1980s and 1990s.
In 2001 the rate fell to a low of 1.73 during what researchers called the ''fertility crisis'', before Mr Costello's call to go forth and multiply in 2004.
Daryl Higgins, the deputy director of research at the Australian Institute of Family Studies, said research showed expectations about family size changed with age.
''When they're younger both men and women tended to aspire to a larger family size. When they were older they tended to revise down their expectations,'' Dr Higgins said.
''You realise the complexities of life when managing responsibilities of children at the same time as having work responsibilities and financial obligations and having already embedded yourself within social networks.
''The biggest explanation is in fact the consistently older age at which people are having their first child.''
The median age of first time mothers in 2010 was 28.9, compared to 27.5 in 1990, and women aged 30 to 34 have continued to have the highest birth rate of all age groups since 2000, bureau figures showed.
Terence Hull, from the Australian National University's Australian Demographic and Social Research Institute, said relationship breakdown or death of a spouse was a factor sometimes overlooked.
''The most important influence is chance, because you have to consider the situation that each individual is facing.
''There's a lot of people who might want more children than they have and they're just not able to biologically.''
