Birth control after pregnancy

It is normal for it to take a few months for a woman's period to resume after childbirth. It can take anywhere from four weeks to six months for a woman's periods to resume. It usually takes longer for breastfeeding women to have regular periods again. If your girlfriend is breastfeeding, she needs to switch to formula before she goes on birth control because the medicine can be transmitted to the baby through breast milk.
As for taking the birth control before she has a period, I would call and discuss this with the doctor. It IS possible for her to become pregnant even though she is not having periods so I understand your concerns.