Bleeding that occurs in the middle of the menstrual cycle can have several causes. Hormone fluctuations in progesterone and oestrogen levels can cause bleeding before a period is due. Spotting may accompany the release of an egg from a mature follicle during ovulation, and the implantation of a fertilised egg may also cause unexpected bleeding. It is possible to have midcycle bleeding and still get pregnant.
The balance of hormones governs the reproductive system. Oestrogen dominates the follicular phase where follicles on the ovaries form mature eggs. If this oestrogen level falls suddenly, this can cause breakthrough bleeding which will look like a normal period. Women who have experienced this type of bleeding may be given oestrogen supplements to avoid midcycle bleeding, but they can still become pregnant during the cycle.
Progesterone governs the luteal phase of the menstrual cycle, following ovulation. This hormone is produced by the corpus luteum, a cyst which forms on the follicle where the mature egg was released. Production of progesterone continues until a fertilised egg implants and begins producing hormones related to pregnancy; if implantation occurs late or does not occur, the progesterone level will fall, signalling the start of menstruation. Menstruation that occurs too soon after ovulation and does not allow for the successful implantation of an egg is called luteal phase defect. Women who have this defect will often have difficulty conceiving, since once menstruation starts the process may not be halted. Progesterone supplements are prescribed to delay menstruation in these cases, but a woman who has experienced the start of her menstrual period due to a drop in progesterone will not become pregnant.
The act of ovulating itself may cause bleeding. Eggs are formed in follicles on the ovary and mature during the follicular phase of the menstrual cycle as the oestrogen levels remain high. At the peak of the egg's maturity, levels of a hormone called luteinizing hormone raise dramatically over the course of a few hours. This hormone surge signals the start of ovulation.
To ovulate, the follicle must rupture and release the egg. This rupture of the follicle can cause midcycle bleeding. The rupture may be accompanied by pain similar to menstrual cramping. Ovulation bleeding is light and may be accompanied by cervical mucus. If present, it should not last more than a few hours. Women who experience this type of midcycle bleeding can become pregnant; ovulation is the most fertile period of a woman's cycle.
Seven to ten days after ovulation, a fertilised egg begins to implant itself into the nutrient-rich uterine lining. This process is known as implantation and can cause bleeding to occur in the middle of a cycle. Bleeding due to implantation will be a very small amount of brown to reddish-brown blood and will not last longer than a few hours. Implantation bleeding is common in newly pregnant women and as long as the egg implants correctly and there are no defects, a woman who experiences midcycle bleeding due to implantation may become pregnant.