“T HE OH, OH , Ohhh-lympics!” ran one tabloid headline. “As record 150,000 condoms are handed out to a host of super-attractive athletes, could London 2012 be the raunchiest games ever?” Official statistics regarding the sex lives of toned athletes are hard to find. But away from the Olympic village, another reproductive record was indeed broken that year. According to the Office for National Statistics ( ONS ), mothers in England and Wales gave birth to 729,674 children, the most in over four decades. They have struggled to keep up since (see chart).

The boom followed a bust. Births began to slump during the 1990s, reaching a low of 594,634 in 2001. The subsequent growth reflected a number of factors (none actually related to the hosting of sports events). There was an echo of earlier baby booms, which increased the number of women of child-bearing age. This coincided with a rise in fertility rates, partly because of more births among older women, who had earlier postponed them. The average maternal age at birth is now 31, up from 28 two decades ago.

And then there was an increase in immigration. A paper by the ONS estimates that foreign-born mothers accounted for two-thirds of the rise in births between 2001 and 2007. In 2000, 16% of children born in England and Wales had mothers who had been born abroad; by 2012, 26% did. An influx of youngish eastern European migrants after the expansion of the European Union in 2004 contributed to the boom. In 2010 Polish mothers overtook Pakistanis to become the largest group of foreign mums. Polish and Romanian mothers now account for more than 5% of births in Britain.