on history taking

in obs and gynae, its standard to ask about their mentrual cycle.
so was taking history from a wmn who is couple of weeks pregnant.
for the whole term, i was surprised all the wmn i interviewed knew they menstrual cycle length! i dont even know mine and cant be bothered.

me: how long is your cycle?
patient: i don't know
me: but dont u need to know to fall pregnant?
patient: erm, we just did it all the time, like rabbits