ok, so it is a myth that you can't get pregnant if you only have sex during your period..

[Spink]

..but how exactly does it happen? As someone ttc and counting cycle days and trying to have sex in the (tiny) fertile window, can someone explain to me how can you get pregnant by having sex while AF is still in full flow?

Yes it is a myth.

Sperm can live in your body for up to five days, so if you had sex on say CD6, finished your period CD7, OV on CD 10 (which can and does happen) you can get pregnant from the sex on CD6.

sperm can live for days and days if conditions allow it

or they can live fast and die young

lots of threads on creating cervical mucus [queasy]

yes its a myth