Birth records for 2004 show 490 babies were delivered on June 30, one of the quietest days in the year for midwives and obstetricians. The babies' parents received no bonus. But on July 1, the first day of the $3000 bonus, the number of births doubled to 978, making it the busiest birth day in 30 years of Bureau of Statistics data. The next day, July 2, recorded 902 births. This was the seventh most popular birthday in the three decades. "We estimate more than 1000 births were 'moved' so as to ensure their parents were eligible for the baby bonus," said Joshua Gans, of the Melbourne Business School, and Andrew Leigh, of the Australian National University.

The two recently found many rich people managed to shift their deaths, possibly with the help of life support, from June to July 1979 to escape inheritance tax. Their study Born on the First of July estimates 1089 births were delayed from June to July 2004, representing 12 per cent of all births in those months. About 300 were estimated to have been postponed by more than a week, "potentially posing a significant risk to [mothers] and their children". The study was motivated in part by Professor Gans's own experience. His daughter was delivered on July 25, 2004, when staff struggled to cope with the huge number of births "pushed back" through the month because of the bonus. "We were very, very surprised by what we found," he said. "That hospitals would allow [mothers to delay their children for money] is extraordinary."

Most of the delayed births involved caesarean sections or induced deliveries. Natural deliveries were broadly unaffected, leading the authors to conclude that fraudulent reporting by parents and medical staff was not widespread. The then health minister, Kay Patterson, said at the time she did not believe mothers would put their children at risk. Professor Gans and Dr Leigh analysed 11,000 days of birth records. Babies were 41 per cent more likely to be born on weekdays than at weekends, mainly reflecting the work preferences of obstetricians.

Hospitals are preparing for a nightmare in 12 days, when they will be inundated with new arrivals after midnight on July 1 when the bonus rises to $4000. July 1 is a Saturday - traditionally golf day for obstetricians.