Birth control pills can be Estrogen-containing contraceptives or Progesteron-containing contraceptives. It might reduce milk supply during breastfeeding. However, I would strongly recommend you to seek your gynae advise before consuming any birth control pills. For me I would be cautious and choose not to consume any hormonal pills while breastfeeding as i worry the hormones pills will pass to my baby while he/she is nursing. There are natural alternative for birth control methds such as using condoms and avoid intercourse during fertile period.
My gynae wanted to prescribe to me the mini-pill which is safe for breastfeeding (she is pro breastfeeding). However I did not take it as I didn't want to be under any form of medication.
thanks