Ian Sample, science correspondent
Thursday October 12, 2006

Keep clear of the cat if you want baby girls. It sounds like the lamest of old wives’ tales, but according to scientists women infected with a common cat parasite give birth to more sons than daughters.

The parasite, toxoplasma, infects around 15% of Britons, but up to 80% of the population in some countries. It is spread by contaminated cat faeces, but also lurks in uncooked pork and beef.

Researchers in the Czech Republic collected medical records from 1,803 newborn babies between 1996 and 2004 and checked them for information on the mothers and babies including gender, the number of previous pregnancies, and the mother’s levels of toxoplasma antibodies.

They discovered that women whose antibody count was high - suggesting a substantial infection - had a much higher chance of having baby boys. In most populations the birth rate is around 51% boys, but women infected with toxoplasma had up to a 72% chance of a boy. Toxoplasma causes congenital defects in newborns and can trigger miscarriages, but a link with the gender of newborns has never been identified before.

Jaroslav Flegr and his team at Charles University in Prague believe the parasite may interfere with the immune systems of pregnant women and make it more likely for male embryos to survive.