Evidence that males are more fragile than females and that fewer males are conceived in sub-optimal conditions is not new. What is new in research published (Thursday 27 March) in Human Reproduction is the extent to which nature tries to level the playing field to give boys a sporting chance.
In the first study designed specifically to focus on sex ratio (the proportion of boys to girls) at conception as opposed to at birth Dr Angelo Cagnacci and his team from the gynaecology unit at the Policlinico of Modena found that 535 males were conceived in the 'best' month for conception as against 465 females, while only 487 males were conceived for 513 females in the 'worst' month. (The normal sex ratio is 511 males to 489 females out of every 1,000 conceptions that produce a birth).
Dr Cagnacci explained that seasonal variations in reproduction have been described before, and a higher conception rate should indicate conditions more favourable for pregnancy and newborn survival for both boys and girls. This seasonal variation is different in different latitudes because it is linked to the length of daytime hours and environmental temperature. Optimum conditions are daytime of 12 hours and a mean temperature of 12C, so maximum conception rates occur at opposite times in regions equidistant from the equator but located in different hemispheres.
"In this study of over 14,000 births at our institute over a six year period, the lowest conception rates were from March to May and the highest from September to November. What is fascinating is the degree of disparity in sex ratio that the numbers of boys conceived compared with girls was so much higher in the favou
Contact: Margaret Willson
European Society for Human Reproduction and Embryology