Are babies usually late?

Tim Harford writes about babies:

“Yes. Here – let me consult my copy of The Tiger That Isn’t, by my friends Michael Blastland and Andrew Dilnot.”

“Is it a book about a tiger?”

“Obviously not – since the tiger isn’t.”

“Is it a book about babies?”

“No. It is a book about numbers. Due dates in the UK are calculated by adding 280 days to the date of the mother-to-be’s last period.

“And Blastland and Dilnot say that 280 days is indeed the average. But in fact 50 per cent of mothers wait at least 282 days. And most commonly, the baby is actually born after 283 days. In other words, the typical baby is born two or three days late.”

“But if the typical baby is three days late, is it really late at all?”

“You tell me, my love. I’m just an economist. The doctors say that it is.”

There’s a good explanation: babies can be born quite prematurely, but if they would be born quite late they are usually forced out into the light in some way.  That means the median (50%) is higher than the arithmetic average (mean).