London: Certain types of In Vitro Fertilisation (IVF) treatment may enhance odds of having a boy.
Some methods tip the gender balance to as many as 128 boys being born for every 100 girls. The ratio for natural births is 105 boys to 100 girls.
The discovery was made by Australian researchers, who believe that the IVF process may be affecting the sex ratio.
Although the reasons remain unclear, the researchers suggested it could be because male embryos may in some way be better equipped to survive the process, reports the Daily Mail.
They warned that more research is needed to ensure a serious imbalance of male to female children does not develop as IVF becomes increasingly common.
Study leader Michael Chapman from the University of New South Wales said there was no question of the ratio being manipulated by IVF clinicians.
Deliberate sex selection is banned in Australia, as it is in Britain except in very specific circumstances, such as to avoid gender-related hereditary disease.
Chapman said laboratory methods may be responsible for the imbalance, particularly the substance in which embryos develop in the test tube.
"It could be that fitter embryos are male and it may be the female embryos which fall away at the various hurdles," he added.
"If we can discover which techniques are responsible for the difference, and why, we may be able to ensure the sex ratio returns to normal."
Researchers studied all live births following fertility treatment in clinics in Australia and New Zealand between 2002 and 2006.
In total, 13,368 babies were born to 13,165 women who underwent the transfer of a single embryo, says the study.
When the normal process of conception becomes impossible then fertilisation is done out of the female body in a test tube, according to BJOG: An International Journal of Obstetrics and Gynaecology.