To call baby swimming lessons an ordeal would be to miss the point entirely, because the fun is absolutely worth the faff. That said, there are certainly moments, specifically during the preparation and the aftermath, that share some characteristics with an ordeal. The part that proves most challenging for me is laundering the paraphernalia immediately after the lesson, in the sense that I never feel like doing it. That’s how I find myself debating the appropriateness of wearing a bikini to a baby swimming lesson moments before I need to leave the house.

I realise too late that the perfectly nice one-piece I’ve been wearing every week since the term began is currently scrunched in the bottom of the laundry basket, dank from last week’s lesson. The baby has plenty of options, but I only own one one-piece.

The idea of sporting a bikini in this setting seems fraught. On-trend bikinis and cut-out one-pieces may be de riguer at other baby swim classes, but the vibe at mine has been more “safe space meets sporty mum” than “rooftop pool at Shoreditch House” so far. I rummage around in a state verging on panic and find a black Speedo that I bought while training for a charity swim. I have nothing against Speedo, but it’s a grey Tuesday and I’m sleep deprived and the idea of putting it on is vaguely depressing. A bouquet of questions opens up. Do I want to wear a bikini so soon after having a baby? Would it seem like some kind of feminist, or indeed anti-feminist, statement? Is it even practical? Given that I’m breastfeeding, the answer, in the case of most bikinis, is not really. Suddenly, a hazy memory surfaces of buying a silvery grey Hunza G bikini on Net-A-Porter while pregnant, for reasons that escape me now. I find it in a drawer with the labels still attached.

Hunza G’s USP is that all items are “one size only,” stretching or contracting as the body, stage or time of day requires. Perfect for breastfeeding, at least in theory. I put it on, glance in the mirror and unexpectedly feel a distinct lightening. The day suddenly feels less grey, or perhaps I do. A sense of fun and frivolity pervades the prospect of the swimming class. I have no time to evaluate whether I’m pulling it off. The other parents at the class may judge, either that the bikini is ill-advised and I look terrible, or that I’m smug and attention-seeking for thinking I don’t. I try to reassure myself with the Marianne Williamson passage about not hiding your light under a bushel, which urges us to understand that we serve the world best by shining. I’m not sure about shining, but I feel better in the bikini than the Speedo, and people at baby activities tend to be kind. I throw clothes on over the Hunza G, grab the baby and go before I can change my mind.

Nobody seems particularly bothered. There’s no evidence that anyone is appalled, or dazzled into insecurity, so I wear it again the following week. It becomes a fun ritual, a secret frisson I get from leaving my house apparently in leggings, a hoodie and a baseball cap but knowing that I’m actually wearing a bikini underneath.

A couple of weeks later, I show up at the pool and immediately realise that the game has changed. I sense a new energy in the changing room before I even enter. The glamour is palpable. Her honey-hued hair is wavy and her earrings are unmistakably expensive. She’s tanned and when she moves, her bandeau bikini top shifts to reveal a total absence of tan lines. That’s explained when she speaks to her baby and reveals that, of course, she is French. She seems slightly older than me, or maybe just aeons more self-possessed. The bikini is red and orange with scalloped edges. I’m pretty sure it’s Marysia. To say that it’s clear she didn’t debate for one moment whether or not it was appropriate is an understatement. I flush at the memory of my own vanity. How could I have thought that my bikini might intimidate or irritate others? Anyone rendered insecure by me would have far bigger problems now. All I can think is, thank God I’m not wearing the Speedo.

After the warm-up, we do Ring a Ring a Rosie, the finale of which involves submerging the babies and ourselves underwater. French Mum calmly informs the teacher that she doesn’t go under the water. Not that she can’t, not that she doesn’t want to. It’s simply not something she does. I am completely awed. My fascination grows as the weeks unfold. I sneak glances at her things in the changing room. She totes her swim gear and nappies around in a Louis Vuitton Neverfull. She slides her feet into Gucci mules as we leave. If wearing the bikini made swimming more exciting, she has made it something to dress up for. I consider buying another bikini and applying self-tan before the lesson. Maybe I’ll start wearing jewellery to the pool or waving my hair for the lesson. Of course, the latter would be a waste of time as long as I am still dunking my head.

One week, emboldened by the woman who has unwittingly become my spiritual guide, I tell the teacher that I won’t be going under the water. I’m going to a wedding dress shop with my sister straight afterwards and I need to look presentable. I’ve always felt I had a good rapport with the teacher, but on hearing this she returns my gaze evenly and gives a curt nod. I can see her reevaluating me, withdrawing her approval of my approach to swimming and motherhood. I over-explain and then walk back my announcement, saying I’m sure I can dry it off with a towel on the bus. The teacher defrosts. The other women avoid my gaze, processing the realisation that they too will never emulate French Mum’s unapologetic sureness. Her face remains open and impassive. I never suggest not dunking my head again. But I do keep wearing the bikini.