It ' s possible the white substance was from her , cervical mucus can change consistency throughout her cycle and at one point will look similar to semen . Arousal will increase it for natural lubrication . Also , cycles can vary a couple days or so even when mostly regular , one may be 25 , the next around 30 . It ' s definitely good you are using condoms , but as an added layer of security it might be a good idea to have your partner start tracking her cycles more in depth so you can avoid sex on her most fertile days . It ' s pretty much what women do to try for pregnancy , but also can work in prevention as well . There are tons of apps like Clue that she can use to freely track . Just an idea to help you guys be extra careful .