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Follicle stimulating hormone (FSH) is directly impacted by prolactin in breastfeeding mothers. The release of prolactin prevents the release and production of FSH. This delays ovulation in many women who breastfeed exclusively during the early months of their infants' lives. A decline in FSH production has the opposite effect, and most women resume normal ovulation and menstruation once the breastfeeding relationship has ended.
Prolactin is a hormone produced by a woman's body soon after giving birth. It signals the breasts to make milk, and levels shift upward and downward throughout the day thanks to the brain's “supply and demand” system for breastfeeding. The more the baby nurses, the more milk a mother's body will produce in response, and more the prolactin her body makes in order to keep up with this demand. As prolactin levels increase, FSH decreases.
This correlation between prolactin and FSH acts as a natural birth control for women. Follicle stimulating hormone is the chemical which promotes maturation of follicles and eggs within the ovaries each month. By suppressing FSH production, prolactin prevents maturation of an egg and thereby, delays ovulation. Without ovulation a new pregnancy cannot occur.
Although the release of prolactin and FSH reduction does prevent ovulation for many, any dip in breastfeeding may throw this off balance. Additionally, prolactin levels may vary from woman to woman, and levels ebb and flow throughout the breastfeeding relationship. As an infant begins eating solid foods, typically between the fourth and sixth months of life, prolactin levels typically begin to decrease in production. This can lead to ovulation at any time thereafter. Because of this, couples should not rely on breastfeeding alone for birth control and should continue to use a backup method.
The exact impact of prolactin and FSH on a woman's body may vary with the individual. Some women are not able to produce enough prolactin on their own and medication may be required in order to achieve exclusive breastfeeding. There are also natural methods for increasing prolactin. Fluctuating levels of either hormone may lead a woman to have a period, followed by several months of irregular cycles as hormone levels stabilize.
Those who choose not to breastfeed will still produce prolactin for a limited amount of time as the milk “comes in" and then sometimes for several weeks or months after. These women typically have a normal period within a few months of giving birth, and if breastfeeding is not resumed, cycles should regulate within the first year. Women may have a cycle which is slightly different than what they experienced before giving birth. They may be longer or shorter, and periods may be heavier or lighter in nature.
Rarely, a woman may have an imbalance of prolactin and FSH. These women may continue producing elevated levels of prolactin even after breastfeeding has decreased or ceased altogether. Ovulation may be delayed in these women, and hormonal supplements may be needed to aid the body in regulating itself. This condition is relatively rare.