I'm 30 years old but TTC has left me wondering if my biology needs a bit of updating 
As far as I understand it, sperm can only survive for about 5 days, so you need to DTD within 5 days of ovulating (or day after) for there to be any chance of the egg being fertilised.
But when trying to prevent pregnancy as a teenager, it was always drilled in that, although unlikely, you could get pregnant by having sex on any day of your cycle. And after a bit of a Google, it seems there is evidence of conception happening after a one time sex on any given day of a cycle. So how is that?!