Does the month in which you were born affect how fertile you are? Surprisingly the answer is yes. Women born during the summer tend to have fewer children than women born at other times of year.

Using birth statistics from Austria, dating back to 1967, Susanne Huber from the University of Veterinary Medicine in Vienna, and colleagues, show that women with a birthday in July had 13% fewer children than women with birthdays in December. Switching hemispheres to New Zealand, the researchers found mirror image results. The reason for these patterns is not entirely clear, but researchers think maternal nutrition along with weather conditions (and likelihood of catching infections) may affect foetal development at a critical stage.

Moving to Vietnam, Huber and her colleagues found women with birthdays during the July rainy season are more fertile than those born in the dry January period. They suggest that the lower birth rate of the January women is linked to the crucial third month of their foetal development. Poor nutrition or increased infection in mothers, associated with the more difficult weather, could have detrimental effects on the foetus at this stage, they suggest.

For men the story is slightly different, with autumn men producing the fewest children and springtime men producing the most. Again foetal development conditions are likely to be key, but perhaps the most vulnerable stage is different for boys and girls.