When her pregnancy scan revealed two tiny heartbeats, Melinda van Leeuwen felt mostly relief. ''My first thought was, 'I'm glad it's not triplets','' said the 39-year-old, who was already the mother of a toddler.
Five previous in-vitro fertilisation (IVF) cycles - two of them involving double embryo transfer - had resulted in one son, Benjamin, conceived on the third round of treatment for Dr van Leeuwen and her husband, Graham.
Though the couple's doctor at IVF Australia warned of the chance of multiple pregnancy and its possible attendant complications, their previous limited success meant this did not loom large in their minds.
But they hit the jackpot with Mariska and Madison, now six months old and born healthily at full term weighing more than three kilograms each.
''I feel very special and privileged to have twins,'' said Dr van Leeuwen, a palliative care specialist. ''Certainly it is more of a challenge but I don't look back and think I should only have put in one embryo. I was older and didn't have a lot of years to play around with.''
Stronger guidelines may be required to further limit the multiple birth rate of IVF and other assisted reproduction technologies, said Peter Illingworth, the president of the Fertility Society of Australia, after new figures showed a reduction over several years had plateaued.
Among new IVF mothers in 2009, the proportion who gave birth to twins or triplets was 8.2 per cent across Australia and New Zealand, down only fractionally on 8.4 per cent the previous year. And the report, from the Australian Institute of Health and Welfare, shows the number of twin births rose to 985, from 877 in 2008 - in line with a 14 per cent increase in IVF usage.
Double embryo transfer multiplies tenfold the chance of twins, who are at four times the usual risk of being stillborn, dying shortly after birth and suffering cerebral palsy.
''Until now Australia has led the way in the voluntary reduction of [multiple] embryo transfers,'' Associate Professor Illingworth said. ''It's possible additional guidelines may be needed if we don't make further progress.''
But fertility treatments inevitably involved more multiple births than the 1 per cent natural rate, he said. ''If we can get to less than 5 per cent [multiple births] nationally, [then] that would be a good thing,'' he said.
Mark Bowman, the medical director of fertility clinic Genea, said Medicare subsidy changes last year discouraged single embryo transfer because it was now more expensive to subsequently use a frozen embryo created and stored at the same time.
''If I put my foot down and say I'm not going to do it, I don't know that I'm fulfilling my doctor-patient relationship very well, and they might just find someone else who will,'' he said. But his clinic ''almost never'' agreed to double embryo transfer on a first IVF attempt.
Nick Evans, the head of the department of neonatal medicine at Royal Prince Alfred Hospital, said IVF twins were more likely to be premature, but they usually arrived between 30 and 36 weeks of pregnancy, mature enough to avoid serious complications. IVF twins were also less likely than others to share a placenta, which raises health risks.
But he cautioned that IVF twins might still be at risk of subtle damage, as evidence emerged that children born only four to eight weeks before their due date performed slightly worse at school than those carried to term.