It's easy to tell when my mother-in-law is expected. The house gleams (I finally get round to those blooming cobwebs), the fridge bulges with healthy home made food (pizzas shoved deep into the freezer) and my towel cupboard is folded to a Hadron collider level of precision.
All this, of course, is useless. If your mother-in-law is anything like mine, she will hone in unerringly on the spot you missed.
"Still haven't had time to sew up that cover?"
"What a shame that stain won't go."
"You need to fix that or the boys will tear it!" (Addressed, casually, at a strip of wallpaper that was peeling off the wall. Gah!)
She's not evil. I don't have one of those cartoon mother-in-laws who come round and push their talons along the mantelpiece to check for dust. My mother-in-law is one of the kindest, most supportive women I know, and I am deeply in her debt for the many times that she has taken my three boys off for a weekend, or come to babysit, or had us all to stay (the very first lie-ins I had after every baby were always thanks to her).
But she's also occasionally infuriated me so much that I've had to walk out of the room in order to calm down, with offhand suggestions that perhaps my children need professional help, or criticisms of their diet, or 'helpful suggestions' that feel very much like maulings of my parenting.
My husband has been known to mutter; "If she's so bloody keen on positive reinforcement, why doesn't she give us any!"
And I'm not the only one; one friend rings me whenever her mother-in-law is staying just to vent steam.
"She's so irritating! She'll only eat highly processed food so we have to get special bright pink sausages and white bread in for her, and she always refers to the salads I make as rabbit food," she squawks down the line! "She's been making that joke for eight years now. She means so well, but she drives me insane!"
Another friend can't believe the odd presents her mother-in-law gives her; "1001 uses for vinegar! Is this some sort of hint?"
And a friend who has recently had a baby nearly went round the bend when her mother-in-law came to stay and then spent her entire visit criticising feeding-on-demand.
"No wonder my husband's a bit messed up sometimes. Now I know the poor lamb had to lie crying on his own in a cot when he was just two weeks old!"
Part of the problem is, of course, that for your mother in law, your partner is still, in some ways, that little child.
As my own children grow older I understand better how, strangely, your children's babyhood stays with them in your eyes; it's part of the dislocation of being a mother to look at your lanky 10-year-old covered in mud from an energetic football match and have a sudden flash of him naked aged two, sitting on a bright green potty and demanding the "twinkle twinkle" song.
For my mother in law, I know that my husband – the youngest of three boys - is and will always her little boy with a pudgy tummy ("he always gets a bit frightened by lightening," she told me once during a storm).
It's obviously impossible for her to really believe that he is a man in his forties, fully capable of looking after three energetic young sons himself. During a hot spell recently, she even texted him; "Don't forget to give the boys lots of water!" It must be a constant source of amazement to her that we have managed to keep them alive this far.
And it is further complicated by the fact that if I'm honest, I am really a far from ideal daughter in law; quick-tempered, private, and extremely independent. I have rarely phoned her for advice (don't really need to) or for the help I know she would always be happy to give. In the very tired early days of motherhood, I would guard my territory like a bad-tempered dog; any sign of encroachment prompted a growl or even a snap.
The good news is that of all relationships this seems to be the one that eases most over time. It may be because my mother-in-law is changing; mellower, older, less quick to jump in with corrections and expertise.
But I think that I too am changing; less tired, more sure of my own mothering abilities, no longer so quick to perceive a slight where, perhaps, there was none.
Most powerfully of all however, as my children grow up and away from me, I see just why she loves being with us, what she pines for and what I will lose soon too. The years of motherhood with small children may be the hardest work of our lives, but they are also the most joyful and rewarding.
In fact, I'm already looking forward to being a mother-in-law myself...
More on Parentdish:
Why I love my mother-in-law
Our tongue-in-cheek guide to the mother-in-law lore of babies