“Please tell me,” I say to Jake. “Please tell me that’s not real.” I’m looking at his left calf, which has a large black pattern over it. A triangle with bits intersecting it. He looks down in surprise, as if I’ve just told him he has some mud stuck to his shoe, or a bit of fluff on his trousers. “Oh, that,” he says. “Yes, of course it’s real.”

“But,” I’m stammering. “Why on earth have you had some huge tribal pattern tattooed all over your leg? Your lovely leg.” I add hopelessly.

“It’s not a tribal pattern,” he says. “I designed it myself. Everything has significance. I can explain if you like.”

The desire for tattoos is like flu: it spreads rapidly from one person to the next. Within days of Jake getting his leg done, Megan wants Frida Kahlo’s face under her left shoulder. Lily yearns for a snake on her arm.

“You can’t,” I tell them. “I made you. I spent nine months getting fat and being sick to create your smooth, flawless skin.”

“Don’t be silly, Mum,” Megan says. “It’s our skin now.”

“Just think about how badly they’ll age, how they’ll droop when you’re old and wrinkly,” I persist.

They look vague.

Getting tattoos is compulsive, like getting botox. Once is never enough. Jake’s now got a band around his bicep, Megan’s having a portrait of the dog’s head on her inner arm; Lily’s wrists are inscribed with small, mystical designs. I want to weep. I don’t understand. And this makes me feel old.

They are piercing their skin too, not just inking it. The two decorations go together, like black eyeliner and red lipstick. Except those wipe off. Jake sits in the kitchen attending to his nipple piercing with hot, salted water. This is when I know that somehow I have circled back through my life to relive my student days, but like a weird, bad dream, my fellow students turn out to be my own children. The intimacies and mistakes and bonding rituals of student life are not supposed to be shared with parents. It’s all wrong.

We’re cleaning our teeth, taking it in turns to spit into the sink, when I notice the metal in my partner’s ear.

I stare at him. “When?”

“A day ago. It’s still a bit sore.”

“But why? I mean, that was a bit sudden, wasn’t it?”

“Well, I always wanted one,” he says, “and then I thought: ‘Why not?’”

As I leave the bathroom, I catch him admiring it in the mirror.

“You’ve got your ear pierced, cool,” Jake says as he passes by, cotton wool pressed to his nipple.

My mother told me to wait until my 16th birthday before I had my ears pierced. But that was a lifetime ago. As the months pass, I grow accustomed, or perhaps simply inured, to new patterns blooming on my adult children’s skin, to silver glimmering from different parts of my family’s anatomies. I look at my own ears in the mirror as if for the first time, and wonder if one of them would look good with a hoop in the outer cartilage. I don’t need permission this time, the rest of my tribe will be thrilled.

Buoyed up by this knowledge, I go straight to a shop called Angelic Hell. As I step into the pink and black room, a voice in my head whispers: “You are too old for this.” The pain is intense. I have to remind myself I’ve been through childbirth. I grip the arms of the chair. As I cycle home, my ear feels as if it is the size of a football and is skewered to my head with a rod of fire.

It continues to hurt. I can’t lie on my left side. It gets caught in my hair. I don’t have time for this, I think. I have enough things keeping me up at night already. I take it out when I notice that my ear is swollen and purple. “I don’t want blood poisoning next to my brain,” I tell Jake.

“Don’t be a wimp,” he laughs. “I get infections all the time – it’s part of having piercings.”

I am not convinced. I am not blase about infection. I am not my son.

I need antibiotics.

Names have been changed

• The Stranger by Saskia Sarginson (Piatkus, £7.99). To order a copy for £6.79, go to bookshop.theguardian.com or call the Guardian Bookshop on 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders minimum p&p of £1.99.