Women who underwent in vitro fertilisation (IVF), in which the resultant embryo was implanted in the womb five or six days after fertilisation, had boys on 56.1 per cent of occasions. By comparison the average number of boys per 100 births in Australia is 51.5 per cent.
The study, published today in BJOG, An International Journal of Obstetrics and Gynaecology, found that, overall, fertility treatment had no effect on the likelihood of having a boy or girl.
However, this masked significant variations between approaches used.
Babies born as a result of intracytoplasmic sperm injection (ICSI), in which a poorly swimming sperm is injected directly into an egg, were more likely to be female than those born as a result of IVF.
But the biggest influence on gender was the stage at which the embryo was implanted.
Embryos tend to be implanted at either ‘cleavage-stage’ – two or three days after fertilisation – or ‘blastocyst’ stage – five or six days after fertilisation.
For both IVF and ICSI, embryos implanted at the later stage were more likely to result in boys.
Those most likely to be female were babies born after ICSI, in which the embryo had been implanted at cleavage stage. In such cases, only 48.7 per cent were male.
Researchers are not sure whether fertility treatment has an active influence on gender, or if the results derive from natural causes.
However, by day five or six male embryos tend to have grown more than female ones. Researchers believe this might mean male embryos at this stage are more likely to be selected for implantation.
Jishan Dean, co-author of the study from the Reproductive and Perinatal Epidemiology Research Unit, School of Women’s and Children’s Health at the University of New South Wales, cautioned that the results should “not be used as a de facto tool for sex selection”.
She said the most appropriate type of fertility treatment was usually determined by the type of problem, such as male or female infertility.
Professor Philip Steer, editor-in-chief of BJOG, said it was important to understand how the gender balance could be unintentionally altered by fertility treatment as it becomes more prevalent.
About 13,000 babies are born following IVF and ICSI treatment in the UK every year, accounting for just over 1.5 per cent of all births.
Using fertility treatment for sexual selection is banned in the UK, Australia and many other countries.