Progress her main story and after some time she will sit seductively on the couch, and you will have sex there, get caught and then you will have option "I want to have a baby" from which moment she will stop taking contraceiptives and can get pregnant.
In the future there could be added variant to steal the pills and she would be able to get pregnant earlier. (But as you continually realize she could be still with Mathew resulting in having a MC baby while being with Mathew, this is considered NTR and a lot of fans didn't like it so it is only an idea option for the future at the moment.)