A study of 13,000 Australian and New Zealand IVF mums found age had a profound effect on a baby's gender.
More than 10,000 babies are born through fertility treatment in Australia each year.
Most IVF mums are over 30, but researchers stopped short of saying the gender imbalance was causing a man shortage.
University of the NSW expert Prof Michael Chapman said in the normal population, 51 per cent of births were boys and 49 per cent girls.
The study of IVF mums showed women over 30 were 5 per cent more likely to have girls, while those under 30 were 5 per cent more likely to have boys.
And the probability of having a girl increased further with age. Women over 40 were up to 10 per cent more likely to have daughters than those under 30.
"So it's a substantial change ... and it will be having an impact on the gender ratio in Australia," Prof Chapman told a Fertility Society of Australia meeting in Perth yesterday.
He said it was the largest study of its kind, and provided clear evidence of an increased ratio of girls from older women after assisted conception.
Prof Chapman said the gender shift was not dramatic enough to create a serious shortage of men in the community, but it indicated IVF could change genetics.
"IVF certainly has some impact on the expression of the Y chromosome, and more importantly whether it has an impact in other ways on the genetics of a child," he said.
The reason for the link between maternal age and the sex ratio of babies was not yet clear, he said.