It’s true that only 4% of babies are born on their due dates, and that plenty of first babies are born late, but plenty of others are born early. When computer scientist Allen Downey from the Olin College of Engineering in the US looked in detail at the data from a survey conducted in 2002 by the Centers for Disease Control and Prevention, he found that first babies are less likely to be born on the due date than subsequent children. His analysis showed they’re more likely to be late than subsequent babies, but they are also more likely than second or third babies to be born early.

He did find that first babies are on average born slightly later than other babies, but by just 16 hours, or a little more than that if you only include babies born at full-term. But since we’re looking at all babies, I think it’s fairer to include the premature babies too.

Sixteen hours isn’t much, so the assumption that first babies are usually several days or even a week late isn’t correct. It does happen sometimes, and everyone will have anecdotes about babies induced two weeks after they were due because they’ve still not arrived. But of course, data is not the plural of anecdote, so when you’re seeking trends, individual stories tell us little.

There’s also the question of whether we should even expect every pregnancy to last the same number of weeks. In one study, researchers measured hormone levels every day from the urine samples of women who were trying to conceive. This allowed them to determine exactly when ovulation and implantation of the fertilised egg had occurred, and to calculate an accurate length of pregnancy. They found it could vary by as