Certain types of assisted fertilization appear to result in more male than female babies being born, a large study in Australia and New Zealand has found.
The researchers have no answer why it is so, but they warn that their findings should not be exploited for sex selection.
All fertility clinics that took part in the study comply with Australia's national guidelines that ban gender selection, the researchers wrote in their findings published on Wednesday in BJOG: An International Journal of Obstetrics and Gynecology.
Led by co-author Jishan Dean from the School of Women's and Children's Health at the University of New South Wales, the researchers studied records of 13,165 samples who underwent in-vitro fertilization (IVF).
Among women who were given standard IVF, 53 percent had male babies, while 50 percent of women who were given intracytoplasmic sperm injection gave birth to boys.
Intracytoplasmic sperm injection is done when sperm is not motile and has to be injected directly into the egg.
In standard IVF, sperm and egg are incubated together in a culture medium for about 18 hours and the egg is usually fertilized within that time.
More boys seemed to result from embryos that were transferred to the womb 4 days after fertilization (54.1 percent) compared to 2-3 days after fertilization (49.9 percent), the study found.
Over the years there has been a big increase in the proportion of IVF births.
Philip Steer, editor-in-chief of the BJOG, said in a statement deliberate sex selection in countries like China and India has already led to significant social problems with men being unable to find a wife.
"It is important that we don't allow such imbalances to occur unintentionally, simply because we have neglected to study the factors that influence sex ratio in the increasing proportion of the population who use assisted reproductive technology," he said.