Given the gap between the Queen's two batches of children, this was hardly surprising: Charles is less than two years older than than Anne, but is 11 and 15 years older than Princes Andrew and Edward respectively.
My father's theory
Had he still been alive, I'd have been able to interrogate my father on his theory about why the Queen and the Duke of Edinburgh decided to have a second batch of children after an eleven year gap.
His line was that, having decided against home tuition in favour of schools for their first two children's education, the Queen and Prince Philip had started to worry that letting them get a taste for the 'real world' might change their attitudes towards the desirability (or otherwise) of becoming monarch.
At worst, what would happen to the House of Windsor if both Charles and Anne decided it wasn't the job for them?
So the obvious answer (to him) was to have some more children to reduce the chances of our hereditary monarchy dying out through a shortage of willing recruits.
A grain of truth?
I've never heard anyone (other than my father) even speculate about what, if anything, the Queen and Prince Philip's family planning strategy might have been - let alone that there might have been a grain of truth to his theory.
Given that journalists and the media haven't been shy when it comes to speculating about so many other details about the private lives of the Royal Family over the past 60 years, I find this rather odd.
And, more than half a century since my father raised the question, it still intrigues me enough to hope that there might be a royal correspondent somewhere who can enlighten us on the matter...