One in three Australian women who have their first baby with the help of assisted reproductive technologies such as IVF fall pregnant again without treatment within two years.
A landmark study of 236 Melbourne and Sydney women who had a baby through IVF and other assisted reproductive treatments found that 33 per cent conceived a second child naturally within two years of the first birth.
Women whose infertility was initially diagnosed as ''unexplained'' were more than twice as likely as others with a specific infertility diagnosis to become pregnant naturally, suggesting some may not have needed treatment in the first place - rather, they kickstarted something that may have happened eventually regardless.
Researcher Dr Karen Wynter, of the Jean Hailes Research Unit at Monash University, said it was also possible that some of the women had undiagnosed endometriosis when trying to get pregnant, because it affects fertility but tends to improve after pregnancy.
''If these women hadn't been diagnosed with endometriosis - and the clinicians tell me this is quite common - that might be why they were struggling the first time,'' Dr Wynter said.
''The pregnancy itself helps to alleviate some of the symptoms and the second pregnancy comes around much easier.''
The women who became pregnant naturally were also more likely to have been in relationships of shorter duration.
''One possible explanation is that these women are having sex more frequently, but there might be other explanations, too,'' Dr Wynter said.
Age did not appear to be a statistically significant factor, with subsequent natural pregnancies occurring in 19 women aged 37 and older, 21 women aged 31 to 36, and six women under 30.
The study was published in The Australian and New Zealand Journal of Obstetrics and Gynaecology.