News Release

Italian research reveals a new twist in the battle of the sexes

How nature tries to compensate for the vulnerability of male babies

Peer-Reviewed Publication

European Society of Human Reproduction and Embryology

New research from Italy reveals that mother nature tries very hard to compensate for the fact that male foetuses and newborns are more fragile than females by allowing significantly more boys to be conceived at a time of year when conditions for pregnancy and birth are optimal.

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[1] 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[2] – 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[3]. (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 12ºC, 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 favourable months when overall conception rates were high and so much lower in the unfavourable months when overall conception rates were low.

"Our findings, showing a prevalence of females conceived in seasons with reduced fertility and, by contrast, an increased prevalence of males in seasons with increased fertility support the hypothesis that there is a greater attrition on males in all situations in which reproductive conditions are sub-optimal," said Dr Cagnacci.

"We could 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."

Dr Cagnacci said that humans were not seasonal breeders and there were outside influences that affected the sex ratio: for example, whether parents smoked or whether there was environmental pollution. However, human conception did follow a seasonal rhythm and it was still not clear how nature acted to favour one sex or another according to conditions.

There were a number of possibilities. "It is tempting to speculate that sex selection occurs in the early stage of pregnancy, probably already at the time of conception and/or embryo implantation," he said.

"Cells in male embryos seem to divide more rapidly and to have a higher metabolic rate so it is possible there are hormonal situations in which one embryo survives more easily than another. The lining of the uterus may favour the implantation of one of the two types of embryos. It's possible that there is a higher rate of very early miscarriage among boys (before a woman even knows she is pregnant). Sperm concentration may vary according to the seasons and we can't exclude the possibility that the composition of Y sperm versus X is influenced by the seasons. It's also possible that sex may be influenced by the formation of an oocyte more receptive to Y rather than X sperm, or by the presence of more or less 'strong' Y sperm."

He concluded: "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 by favouring them in optimal reproductive conditions."


[1] The male disadvantage and the seasonal rhythm of sex ratio at the time of conception. Human Reproduction. Vol. 18. No. 4. pp 885-887.

[2] This was a retrospective study of 14,310 births at the Policlinico of Modena between 1995 and 2001. For each single pregnancy, time of conception was obtained on the basis of the last menstrual period and gynaecological examination, and in 98.5% of cases the calculated time of conception was confirmed or redefined by an early pregnancy ultrasound examination. In order to check whether the institutional sex ratio reflected that of the area generally, the researchers also estimated the sex ratio of all 199,454 pregnancies on the Modena county database between 1936 and 1998.

[3] Of every 1,000 conceptions that produce a birth, 511 are boys and 489 are girls. But the proportion varies throughout the year. The maximum proportion of males conceived in the study was in October – the expected 511 + 24=535 males, compared with 489-24=465 females and the minimum was in April with 511-24=487 males and 489+24=513 females. The peaks and troughs in proportion of males align with the peaks and troughs in overall conception. So for both sexes over a 12 month period with, for example, a mean of 1,000 conceptions a month there will have been 1,070 babies conceived in October and 930 in the following April.

1 PDF version of this press release and full embargoed text of the paper with complete results can be found from 09.00hrs GMT Tuesday 25 March on: or is available immediately from Margaret Willson.
2 Human Reproduction is a monthly journal of the European Society of Human Reproduction and Embryology (ESHRE). Please acknowledge Human Reproduction as a source. Dr Helen Beard, Managing Editor. Tel: +44 (0) 1954 212404 Email:
3 ESHRE's website is:
4 Abstracts of other papers in ESHRE's three journals: Human Reproduction, Molecular Human Reproduction & Human Reproduction Update can be accessed post embargo from: Full text of papers available on request from Margaret Willson.

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