Of course the best laid plans of mice and men (and mums) can go awry, but fertility issues and family planning aside, I've always wondered how couples decide which camp they belong in. What determines how many children you choose to have? And how do you know when your family is 'complete'?
This question has been the topic of much debate among my friends in recent years. "Would you go again?" is a question I hear often. That's a lovely Irish euphemism for "Have you got your hands full with your two boys or do you think you might be mad enough to squeeze out one more baby while there's still time?"
My response? I've never really felt 'done' as a mother. The idea that my 'last' pregnancy has been and gone fills me with horror, and I've felt weirdly sorrowful at the notion that the childbearing chapter of my life is over. But maybe those feelings are just a trick that Mother Nature plays to keep the planet ticking over. Perhaps it's natural for women to feel reluctant at relinquishing their right to bear babies, whenever and however the time comes.
But deciding whether to have a third child seemed like the biggest decision of my mothering life. Starting our family was less dramatic - we simply took the plunge. And having a second baby seemed like a no-brainer. We wanted our son to have a sibling rather than be an only child so, once again, we went for it. Making the jump from two to three children, by comparison, is a minefield.
The very thought of returning to the realm of nappies, leaky boobs and sleepless nights seems like utter madness. At six and eight my sons are reasonable, independent young men who can tie their own shoelaces, get their own breakfast, and even bring me a cuppa in bed.
"I couldn't go back there," said a friend recently, visibly shuddering at the thought of trading her easy family life for the upheaval of that a new baby brings.
The fact that I'll be 37 this year seems to have brought the issue into much sharper focus. My friends and I know our fertile days are numbered, and none of us approach the possibility of pregnancy in our mid-30s with the naivety and nonchalance we had in our 20s.
The stakes are higher now - risks of complications loom large as we contemplate having another baby.
And with two healthy children and easy pregnancies and deliveries behind me, I can't help feeling that going for a third child at 37 is a risky move; like playing Russian roulette with my reproductive system.
Still, the news that I am expecting our third baby came as something of a shock to me, even though it was on the cards that we would 'go for one more' (another lovely Irish euphemism that makes the many indignities of pregnancy and childbirth sound so benign).
But 12 weeks on and just a few days away from my first scan, the news has now sunk in and I am overjoyed, as is my husband who has already named the baby. Our sons' reactions to their impending big brotherhood is worthy of a whole post of its own. But suffice to say they've helped make the whole thing feel wonderful, despite the nights I lay awake worrying about whether a big age gap of seven and nine years between siblings was going to ruin their lives.
And yet... despite the palpable excitement in our house, a slight sense of panic descends with the realisation that I'm not just over the hill in maternal years. I'm also officially on the cusp of being seriously outnumbered.