The girls in that situation don't need to fast while they're pregnant, weak, or are going through menstruation, but they need to make up for the fasts they missed later because you must fast for the entirety of Ramadan, whether you are a man or women and it is obligatory to do so.
Thank you hun! The questions are so fun to answer!
Ummm... am I allowed to say this on TBTF? xD Basically, if there is any release of... "fluids", and it comes out of you in a "flowing motion," then your fast has broken. Same goes for vomiting and menstruation. It leaves you in a spiritually impure state, so that's why it breaks your fast.
And you start your fast at dawn and break it at dusk all while not eating or drinking anything in between. You can't restart your fast after intercourse; you're going to have to wait for the next day to start your fast xD