Babies need to be conceived by the first week of June to arrive before the magic February 28 cut-off date and associate professor of obstetrics at the Australian National University Stephen Robson said a hard cut-off rather than a slow decrease to the cash payment would result in more couples trying to conceive.

When the baby bonus first came into effect, mothers varied the timing of their births using caesareans and inductions.

According to Australian National University research 42 per cent of births in the final week of June 2004 were caesareans or inductions. The rate jumped to 52 per cent the following week.

Dr Robson expects a similar occurrence in the week before the bonus is indexed according to income. ''It's a real fact that employers are going to have to accept that the younger employees just need to duck out of work to try and achieve this,'' Dr Robson said.

There were three things couples could do to improve their chances of conception: have sex frequently, quickly and take a nap afterwards.

''There is a misconception that if men save things up for a few days the sperm is better quality but it's pretty clear that is a myth. The better advice is to actually have intercourse at least twice a day, the more the better.''

Giving up smoking is a must. And while there is a deadline for conception to get the baby bonus Dr Robson warned spontaneity was still important.

''It should be unexpected seduction to try and make out it's nothing to do with getting pregnant and that's an important psychological thing,'' he said.