How many babies are actually born on their due date?

Question: How many babies are actually born on their due date?
- Keywords:
  babies,
  baby,
  born,
  due date,
  premature
Asked by yatestwh to Helen, Jenni, Mark, Martin, Stu on 23 Jun 2011.
- Mark Burnley answered on 21 Jun 2011:
  
  It’s a very small number (less than 10%) because the due date is calculated from the estimated date of the last period and therefore the estimated date of fertilisation 14ish days after that. The error is plus or minus 2 weeks.
- Helen O'Connor answered on 23 Jun 2011:
  
  The ‘due date’ of a baby is only approximately accurate: this is because it is calculated from the date of a women’s last period, and then they add 40 weeks to that. However, the woman might have conceived right after her last period, or up to several weeks later than that. So there is usually a couple of weeks either side of a due date when a baby might be born.
  
  When a baby is born really prematurely there are all sorts of possible reasons, although it’s not always possible to know what the reason is. These can include when it is a multimple birth (twins or triplets), an illness of the mother like diabetes or an infection, when there is something wrong with the baby, or the woman’s body just can’t hold the baby long enough

Question: How many babies are actually born on their due date?