Boys are more vulnerable in the womb and just after birth but mother nature lends them a helping hand, suggest researchers.
The time of year of conception may affect a baby's sex
Scientists from Modena in Italy believe that men are more likely to be conceived so that they are born during warmer months, to give them a better chance of survival.
How the reproductive system might be programmed to do this is as yet undiscovered, and the change during the key months appears to be small at best.
However, the scientist involved has even suggested that couples who want to choose the sex of their child try to conceive in particular months of the year.
Nature, recognising that male foetuses and newborns are more vulnerable than females, treats conception as a handicap race and tries to give boys a head start
Dr Angelo Cagnacci, Policlinico Modena
It is already known that the human body may already be trying to compensate for the inherent weakness of the male foetus, simply by increasing the proportion of male foetuses conceived.
At present, there are 511 males to 489 females in 1,000 conceptions. This advantage, however, does not last long.
Boy foetuses are more likely to perish in the womb or suffer obstetric complications.
Dr Angelo Cagnacci from the Policlinico Modena looked at the records for more than 14,000 births over a six year period, and found that the lowest conception rates for boys were from March to May, and the highest from September to November.
The first period would mean babies born in the Italian winter, while a conception in the second would mean a summer birth.
In the 'best' month for conception, the researcher found that 535 males were conceived versus 465 females.
Boys of autumn
Dr Cagnacci said: "We would conclude from our data that if you want a boy in this region of the world, you would have a better chance if you tried to time conception between September and November.
"And, although the overall numbers of conceptions between March and May are lower than in the autumn, these are the months when you are more likely to conceive a girl.
"Whatever the explanation it's fair to say that nature, recognising that male foetuses and newborns are more vulnerable than females, treats conception as a handicap race and tries to give boys a head start."
He speculated that this sex selection, if it existed, happened early in pregnancy.
He said that cells in male embryos seemed to divide more rapidly and to have a higher metabolic rate so in certain hormonal conditions in the womb it could mean that a male embryo might implant and develop while a female would have a lesser chance.
He said that it was also possible that sperm concentrations might vary according to the season.
The research was carried in the journal Human Reproduction.