Same here - I’m assuming that Shiv has much stronger abs than me (to be fair, most people do!), but unless Tom is the least observant man on the planet he’d surely notice that her body has changed from all the many other times he’s seen her naked?
I estimate she’s somewhere between 13-16 weeks now, and it’s (as far as we know) her first pregnancy, but even so once undressed there’d be a bit of something there. Maybe Tom was just distracted by thinking of all the nice watches he’ll be able to buy if they reconcile