... but need to email in a reason why you a) will miss important meetings tomorrow, b) will try to wfh but may not be terribly productive and c) are not sure when you will next be in...
...what excuse do you use?
That sounds 'serious' enough that they realise you're not taking the piss, but without giving the game away.
I will tell him of necessary, and he will be sympathetic and fine, but I am in line for promotion and I don't ideally want it to be known that I'm TTC.