Here are some stats from the UK on how often bubs are born at what week When am I going to pop? (numbercrunch) « A disgruntled motherhood blog

Gestation weeks ------- %

<=31 ----------------- 2.1%

32-36 ----------------- 4.7%

37 --------------------- 4.4%

38 --------------------- 10.8%

39 --------------------- 22.6%

40 --------------------- 34.1%

41 --------------------- 18.8%

42 & over -------------- 2.4%

n/a --------------------- 16.7%