Saw this on the BBC website: www.bbc.co.uk/news/magazine-31046144
"For women ... whose pregnancies are straightforward and low risk, 60% of babies are born within a week either side of the estimated date of delivery.
More than 90% are born two weeks either side of the predicted date.
But, as noted above, only 4% (or 4.4%, ignoring pregnancies with complications etc) are born on the predicted date itself - in other words, the chance of this happening is less than one in 20."