She added, "The uterine lining is composed of a basal layer and a functional layer. The basal layer never sheds. It has stem cells which allow for regeneration in the next cycle. The functional layer — which responds to hormones — grows and sheds. When you bleed from oral contraceptive pills and ovulatory cycles, the functional layer, or the top layer, sheds. In contraceptive options that are progestin only (like the implant, the shot, or sometimes the hormonal IUD), you may not shed, but you can have breakthrough bleeding and spotting."