Boys and girls are equal in number at conception, but female embryos are less likely to survive, leading to a higher number of males being born, according to a new study that contradicts the previous belief.

Overturning a long-held belief that male embryos are more vulnerable in the first months of life, the study found that human sex ratio is equal at conception and net female mortality exceeds that of males during pregnancy.

Steven Orzack, a senior research scientist with the Fresh Pond Research Institute in Cambridge, Massachusetts and colleagues examined a broad data set to investigate the trajectory of the human sex ratio between conception and birth, a trajectory that remains poorly characterised.

Researchers analysed the sex ratios of foetuses at different gestational ages, including three-to-six-day-old embryos produced by assisted reproductive technologies, induced abortions, chorionic villus sampling, amniocentesis, and US census records of foetal deaths and live births.

Analysis of the data revealed that the human sex ratio is equal at conception.

Further, abnormal male embryos outnumber abnormal female embryos at conception, and male mortality is expected to exceed female mortality during the first week after conception.

Female mortality exceeds male mortality during the following 10 to 15 weeks, after which male mortality exceeds female mortality between weeks 28 and 35.

Previous reports suggest that the sex ratio at conception is biased toward males and that male mortality exceeds that of females during pregnancy.

However, rigorous analysis of the large data set suggests that the sex ratio is equal at conception and net female mortality exceeds that of males during pregnancy, according to the authors.

The study was published in the journal Proceedings of the National Academy of Sciences.