Progesterone is a hormone that plays a broad role in the female reproductive system. It acts as a chemical messenger, directing various bodily functions, particularly in supporting menstruation and the early phases of pregnancy. This hormone also connects to the process of lactation.
Progesterone’s Role During Pregnancy
During pregnancy, progesterone is produced in high amounts, primarily by the placenta, to support the developing pregnancy. This hormone helps to thicken the uterine lining, creating an environment suitable for a fertilized egg to implant and grow. High progesterone levels also prevent new ovulation and suppress uterine contractions, which can prevent preterm labor.
Progesterone also plays a role in preparing the mammary glands for milk production. It promotes the growth of milk ducts and the development of glandular, milk-making tissue within the breasts. Despite this preparation, high levels of progesterone during pregnancy inhibit the full onset of copious milk production. Progesterone interferes with prolactin, the primary milk-making hormone, from fully binding to its receptors on the alveolar cells in the breast, thereby suppressing milk secretion until after birth.
The Hormonal Shift After Birth
The process of lactation involves two main stages, with the second stage, known as lactogenesis II or “milk coming in,” being particularly noticeable. This stage is marked by a significant increase in milk production. The primary trigger for this dramatic increase is a rapid decrease in progesterone levels.
This sudden drop in progesterone occurs immediately after the delivery of the placenta, which was the main source of the hormone during pregnancy. With progesterone’s inhibitory effect removed, prolactin, which has been steadily rising throughout pregnancy, can now fully bind to its receptors in the mammary glands. This allows prolactin to stimulate the alveolar cells to synthesize milk components like lactose, casein, and lipids at a much higher rate. Most individuals experience this surge in milk production, often accompanied by breast fullness, around 2 to 3 days postpartum, although it can begin as early as 30-40 hours after birth.
Influences on Lactation Onset
While the drop in progesterone after birth is the primary trigger for milk production, several factors can influence or delay this hormonal shift. One significant factor is retained placental fragments. If portions of the placenta remain in the uterus after delivery, they can continue to produce progesterone, keeping its levels elevated and inhibiting the full onset of lactogenesis II. Milk production may not increase until these fragments are removed.
Other factors can also interfere with the overall hormonal balance or the milk production process. Certain medical conditions, such as diabetes, can sometimes delay the onset of milk production. Additionally, some medications or complications during delivery, like stressful vaginal deliveries or a Cesarean birth, may also be associated with a delayed onset of milk production, though the exact mechanisms can vary.