About 56 percent of IVF babies are boys, even though sex selection is not permitted in the UK except in the case of some rare sex-linked genetic conditions.
Dr Helen O'Neill from University College London thinks she has found the answer, according to research presented at New Scientist Live.
It appears that male embryos develop slightly faster in the earliest stages after fertilisation, and are therefore more likely to be chosen for transfer by embryologists.
Read more in the Mail.