Lactation Physiology: Part 1

April 14, 2021

When studying the physiology of the breast, five important items to understand are:

  1. The stages of breast changes during pregnancy and lactation

  2. Endocrine versus autocrine control

  3. Human placental lactogen

  4. Prolactin

  5. Oxytocin

Let’s begin with a review of the five stages of breast changes during pregnancy and lactation.


  • Mammogenesis is, per the Core Curriculum, “The development of the mammary gland and related structures within the breast.”1

  • This occurs throughout fetal, adolescent and adult life.

  • Breast development begins early in gestation, by 4 to 5 weeks, when two parallel milk ridges develop from the axilla to the groin. These are the milk lines, along which breast tissue can develop.

Lactogenesis I

  • Lactogenesis I is also called Secretory Differentiation.

  • This is the beginning of secretory cellular activity (remember those lactocytes?).

  • This is the beginning of the production of milk.

  • This occurs around 16 weeks prenatally. Therefore, after 16 weeks of pregnancy, lactation occurs even if the pregnancy does not progress.

  • Lactogenesis I is under endocrine control.

Lactogenesis II

  • Lactogenesis II is also called Secretory Activation.

  • This is the onset of copious milk secretion.

  • This is triggered following the delivery of the placenta which causes a rapid drop in progesterone. Remember that progesterone, which the placenta is secreting, is a prolactin inhibitor. This is why women get into trouble with milk production if they have retained placental fragments after birth.

  • With the placenta delivered and progesterone out of the way, prolactin is now unopposed and causes the onset of copious milk production or full lactation

  • This stage starts between 30 and 72 hours postpartum. Milk volume increases rapidly postpartum and then levels off.

  • With lactogenesis II, citrate and alpha-lactalbumin levels goes up in the milk.

  • The Core Curriculum for Lactation Consultant Practice, 3rd edition, page 292 (Reference 1) says:

    “Lactogenesis II or Secretory Activation

    A. Lactogenesis II is the onset of copious milk secretion.

    1.     Between 30 and 72 hours following the delivery of the placenta.

    2.     Women do not typically begin feeling breast fullness until 50 to 72 hours (2-3 days) after birth.

    3.     Initially under endocrine control and now under autocrine, or local, control (De Coopman, 1993; Wilde, Addey et al., 1995; Wilde, Prentice et al., 1995).”


  • Galactopoiesis used to be called Lactogenesis III.

  • Galactopoiesis is the long-term maintenance of breastfeeding.

  • Some texts say that this occurs from day 9 after birth to the beginning of involution.

  • Galactopoiesis is under autocrine control.

What is autocrine control? Maybe it will help to remember the ICE MAKER analogy.

Autocrine control works like the ice maker in a refrigerator. If the ice bucket is not emptied, the ice builds up causing the silver lever to pop up - which tells the ice maker to stop making ice. Emptying the ice bucket frequently means that lever never pops up and the refrigerator keeps making ice. Empty the ice bucket, go back in a few hours, and it is filled up again. Empty ice, it fills with ice – empty ice, it fills with ice.

This is the breast when milk is emptied except, of course, there isn’t a silver lever. What happens in the breast is that if the milk accumulates, a substance called Feedback Inhibitor of Lactation (FIL) accumulates. FIL is what tells the mother’s system to stop making milk. This helps us understand why autocrine control is locally controlled.

From Breastfeeding and Human Lactation2

“An autocrine feedback mechanism, the feedback inhibitor of lactation (FIL), appears to locally control milk synthesis. The specific mechanism by which FIL works to inhibit breast milk synthesis is not clear, but it appears to involve a compound within the milk, not the distention of the breast that slows the build-up of milk.”


  • This occurs when the milk-producing system in the breast is no longer being used.

  • The secretory cells (lactocytes) stop functioning.

Maternal conditions that can cause delayed lactogenesis II or delayed secretory activation include:

  • Age of mother >30 years

  • Cesarean birth (a maternal illness that cause separation can interfere with milk production)

  • Labor: long stage II with vaginal birth

  • Labor: maternal fluid overload

  • Maternal health issues: diabetes; history of breast reduction surgery; hypoplasia of breast tissue, infertility, obesity, poly cystic ovary syndrome (PCOS), thyroid dysfunction

  • Obesity: elevated BMI

  • Parity: primips are at increased risk; lactogenesis II occurs earlier for multips

  • Retained placental fragments

  • Sheehan’s Syndrome – significant maternal hemorrhage can cause poor blood supply to the anterior pituitary gland, affecting prolactin release


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Core Curriculum for Lactation Consultant Practice, 3rd edition. Edited by Rebecca Mannel, Patricia J. Martens, and Marsha Walker. Jones & Bartlett Learning. 2013


Wambach K, Spence B. Breastfeeding and Human Lactation, 6th edition. Jones & Bartlett Learning. Page 62. 2021