FROM the moment a woman gives birth, she takes responsibility for all those things new parenthood is likely to entail. From feeding to story-telling and nappy changing, she may have spent months, if not years, preparing for it.

But baby care manuals don’t mention another role suddenly thrust upon her, one that usually hasn’t crossed her mind.

For women must instantly transform themselves into pack horses. Every time she leaves the house, however briefly, a new mum must carry a giant bag containing enough food, drink, spare clothes, sun cream, changing mat, wipes, nappies, waterproofs and first aid kit to sustain her child in case they, somehow, inadvertently end up on an expedition to Kilimanjaro.

As the child gets older, other things – such as potty, favourite blanket, cuddly toy, teething ring, pop-up sun protection tent, play pen and books - will be added to the kit list.

And the scooter or trike they insisted on riding to the shops with you has to be carried all the way home again when they lose interest, along with the coats and hats they refuse to wear, even though it’s freezing.

That’s why so many new mums end up with one shoulder lower than the other, and biceps the size of cabbages.

But, as time goes on, we gradually manage to shed huge chunks of the clunky paraphernalia our children were once unable to leave the house without but now no longer need.

The one thing we haven’t been able to shake off, though, is our status as sturdy animals with the ability to carry loads on our backs. Our children have never known us before we became pack horses, so assume this is simply our job.

It’s not helped by the fact that husbands, too, take advantage of the incredible ability we have developed to carry our own weight in car keys, wallets and any other assorted bits and pieces which anyone cares to throw in our Mary Poppins-style magic handbags, which, as the years go on, have defied all laws of physics to house at least three times as much as their size would seem to allow.

So it was little surprise to me when we visited our eldest son, William, in Leamington Spa for the bank holiday weekend and my giant, groaning handbag ended up with an assortment of various wallets, keys, phones, sun cream, bottled water and sunglasses, as well as a spare jumper, scarf and the hoodie 15-year-old Albert decided he didn’t want to wear any more.

It did occur to me that, at 26 years old, William must by now have mastered the art of leaving his flat while carrying his wallet and keys somewhere about his person, all on his own, without too much difficulty.

But somehow, like his other brothers, he loses this ability when I am in the vicinity, regressing back to those childhood days, when Mum looked after everything.

It wasn’t until early on the Tuesday morning that I received a panic-stricken text: “Have you got my car and house keys?”

They were in the bottom of my bag. I didn’t point out that I have so many possessions belonging to others in my bag at any one time I can’t be expected to keep track of all of them. Nor that he should have been carrying his own keys, or at least have asked for them back before I left.

His girlfriend had already left for work and, without his keys, he wasn’t able to lock up the house or drive to the railway station to get to his London office. “Don’t you have spares?” I asked.

But he didn’t know where they were. I made a few suggestions, which didn’t help: “I can’t do much from 160 miles away,” I said. It turned out his girlfriend had them in her handbag.

They were coming to visit us last weekend anyway, so he picked his keys up then. This time, he didn’t once ask me to carry his phone, wallet or keys. ‘How grown up,’ I thought. “He’s learnt from this."

On the Sunday, we all decided to go out for lunch in a little market town, part-way into their journey back, so, with packed bags already loaded into their car, they could head directly home from there.

But, just as we were having coffee, he groaned: “Oh no. I left my wallet on the kitchen table at your house. We’re going to have to go back.”

What a shame I didn’t have it in my bag. But then, when we go out to eat, none of the boys ever bring their wallets, since they’re used to Mum and Dad paying for everything.

That’s another regressive childhood quirk they’ll hopefully all grow out of one day…