So what makes mothers - and fathers- add one extra? Do accidents really happen, or are they really moments of carelessness, coupled with a deep instinct for adding to your brood?
After replacing your own genes, and when the planet is struggling to cope with a growing population - why a third? Is it not downright selfish to replace your genes just one more time? And then there is the small matter of having to buy a bigger car, house and more school shoes than you ever imagined.
Is it a case of the more the merrier, or is it perhaps a case of mums (the ones who do most of the child care still) not having the confidence to pick up their own lives between their youngest starting school, and the oldest providing them with a grandchild - as was the case with my friend, Sue, who pushed out her fourth over forty.
Sue's first child was born when she was in her late twenties. Every few years she had one more until at forty and a bit, number four arrived. Sue's raison d'etre is to nurture. With brilliant timing, just as the youngest child was about to leave home, her eldest produced a grandchild. Sue is now a doting gran who loves nothing more than babysitting, day and night.
Sue's choices allowed her to postpone going back to work for almost thirty years and when she did, it was as a teaching assistant. More children to care for.
And then there are the accidents. Sometimes, passion takes over and a chance is taken, but sometimes the accidents are very carefully planned. Take Catherine for example: "We had two children, and I really wanted a third. My husband wasn't keen, for financial reasons, and persuasion hadn't worked. I stopped taking the Pill, but didn't tell him. When, four months later I became pregnant, it was easy to explain it as 'Pill failure'. He has no regrets now, and neither do I."
Kate was happy with her two boys: "Two were fine, and I was conscious about adding to the world's population problem. Then at my trip to the doctors, he removed a troublesome IUD. We hadn't decided what to use instead and after a bit too much to drink one Christmas,
'Oops' was confirmed on Valentine's Day. I was surprised, but at the same time knew I'd have been disappointed with a negative test. The biggest problem was the ten year age gap between him and my other children."
But did Kate stop there? "I didn't want my third child to feel like an only child, so it was in fact an easy decision to have a fourth, when I was thirty eight."
So what's the difference between having two and four? "Spacing is important. We struggled, emotionally with teens and terrible twos. I'd also like to think they have all left home before my husband retires. Would Kate recommend more than two? "You have to be really sure you can afford more than two."
But it's not all about money, surely?
Some parents like Judy take a very philosophical approach to family size: "We always wanted more than two. One good reason for going for more children is that, I believe, it allows them to be who they want to be rather than who their parents think they ought to be. I so often see parents who are living their own lives vicariously through their one son and one daughter."
Finally, from a purely selfish point of view - who is going to look after you in your old age?
Judy makes the point, " As one of several children, I also appreciated that not all my parents' ambitions for their offspring were weighted onto me but were spread across us all. Now that we are adult, we have been able to also spread the responsibility of ageing parents. My father is now widowed and we all take our fair share of making sure that he feels welcome in our homes and cared for."
So what do you think?
If money was no object, and planet earth could cope, would you still settle for two – or create that entire football team?
More on Parentdish: Meet the mum who thought her family was complete - then made her husband reverse his snip!