When you're pregnant, travelling on public transport becomes a study in complex sociology.

The first few months are the worst.  There’s no outward sign you’re pregnant, but you feel vile and dread the Tube.  You feel a fraud asking for the ‘less able to stand’ seat without a bump – despite feeling sick, dizzy and tired.  Wearing a badge that says ‘Baby on Board’ is not possible for so many reasons, the obvious one (apart from misplaced vanity about wearing a stupid badge) being that colleagues don’t know yet. One morning I was fighting dizziness when a man, perhaps in his 60s, gazed over for a moment thoughtfully, before getting up and saying quietly ’You need this more than I do’.  I regretted not being able to express the depths of my gratitude.

As the bump starts to show, women are more likely to move for you: they are better at spotting the signs (and more willing to take the risk that you might be just fat). When nobody can mistake it, young men leap to their feet and insist you sit even if you are only going one stop. Particularly young Indian men – we must thank Indian mothers for their being so beautifully brought up.

During the middle stages, a subtle game of Top Trumps is played to select who takes a free priority seat. Elderly lady versus pregnant lady (how pregnant, how elderly?). Man in a neck brace versus pregnant lady. What about two pregnant ladies? What about a sweating man with a foot in plaster, carrying a large bag?

What’s really interesting is that somewhere beyond our conscious thought we have a code for this. Sociologists can probably give you a precise list.  Eye contact is exchanged for the briefest of seconds, and a deal is struck.

Towards the end, as you commute London carrying an extra 8 kilos, swollen ankles and a powerful internal travel radiator, you feel more entitled to your irritations about the failures of London transport.  You HATE the DLR for its lack of seats and design that ensures everyone can avoid your eye. The platforms don’t even have seats. When it’s 90 degrees and you give up on the stairs, you realise that the station lifts are perfectly designed to mimic the kind of dark hole where a sick cat would choose to die.

By contrast, you can forgive the Jubilee line anything for its good connections, the loos at Canary Wharf, and generous travellers who always do the right thing.  Even if you sometimes wonder whether it’s because everyone is watching. Before you get pregnant, you don’t even notice most of this stuff. Afterwards, the world has conspired to form a complicated obstacle course – without, in reality, a single detail changing.