I would ask to go to the doctor with her. Or at least ask to see her ultrasound. Maybe I'm a pessimist or have just seen what desperate people can do but I would want some kind of proof.
She got pregnant in the past while on birth control but told you she has a medical condition that makes it impossible to get pregnant? And then knowing she has become pregnant while on birth control talks you into having sex without condoms? Something doesn't add up.