Lactation Physiology: Part 2

April 16, 2021

Remember that the IBCLC examination is an entry-level test of your knowledge. You are not expected to be an endocrinologist. Physiology of the breast can get pretty complicated but let’s not go there. Here are some basic facts to know.1 2 3

Facts about the placenta:

  • A healthy placenta is necessary for normal fetal growth.

  • The placenta produces the hormone, human placental lactogen (HPL).

  • Human placental lactogen is involved in the growth of ducts, areola and lobes in the breast.

During pregnancy:

  • Estrogen is important for ductal growth and ductal sprouting. (Remember E D – estrogen, ducts)

  • Human placental lactogen is involved in the growth of ducts, areola and lobes in the breast. Progesterone also stimulates lobuloalveolar growth.

  • Prolactin works to increase breast mass and helps with cell differentiation.

  • The nipple grows under the influence of prolactin, the areola under the influence of HPL.

Facts about progesterone:

  • Progesterone is required to maintain pregnancy.

  • Progesterone increases and remains high during pregnancy.

  • During pregnancy, progesterone stimulates lobuloalveolar growth. But, at the same time, the high levels of progesterone inhibit lactation as they interfere with the action of prolactin at the alveolar cell level.

  • The delivery of the placenta, and the resulting drop in progesterone, triggers the initiation of Lactogenesis II (which is also called secretory activation or the copious production of breast milk). KEY POINT

Facts about estrogen:

  • Estrogen levels are also high during pregnancy and decrease with childbirth.

  • However, the withdrawal of estrogen does not trigger the initiation of Lactogenesis II; the drop in the level of progesterone is the trigger for this.

  • During pregnancy, estrogen is important for ductal growth and ductal sprouting.

The hormone, prolactin

P and P: Prolactin Produces milk

Prolactin is secreted by the anterior pituitary gland

How to remember this: PAP smear

Prolactin Anterior Pituitary - PAP

Prolactin: normal levels

In nonpregnant, nonlactating women, the normal prolactin concentration is 10-20 ng/mL.

Prolactin levels increase the most in the immediate post-partum period, peaking at term to 200-400 ng/mL.

Afterward, these levels slowly decline, but rise and fall in proportion to the frequency, intensity, and duration of nipple stimulation. They will remain elevated, compared to the nonpregnant, nonlactating woman, for as long as the mother breastfeeds.  

The prolactin concentration in blood doubles in response to the infant sucking at the breast and peaks approximately 45 minutes after the beginning of a breastfeeding session.

The more feedings, the higher the level of serum prolactin.

Five prolactin fun facts:

  1. Prolactin levels follow a circadian rhythm. Levels during the night (sleep) are higher than during the day.

  2. If lidocaine is applied to the nipples to deaden sensation, the prolactin level does not increase.

  3. Prolactin levels drop with cigarette smoking.

  4. Prolactin levels rise with beer drinking.

  5. Depressed mothers have lower serum prolactin levels.

Prolactin-Inhibiting Factor (PIF)

  • A substance called Prolactin-Inhibiting Factor (PIF) is released by the hypothalamus (a part of the brain that sends signals to the pituitary gland).

  • As the name implies, PIF inhibits the response one would expect to see from prolactin.

  • PIF has something to do with dopamine. PIF is either dopamine itself or something that is mediated by dopamine.

  • Long ago, when breastfeeding rates were horribly low and breast milk coming in after birth was seen as a problem that needed to be dealt with because women weren’t breastfeeding, standing hospital orders included giving women the drug bromocriptine. Bromocriptine has a dopamine-like, prolactin-inhibiting effect. It is a drug that suppresses lactation.

  • On the other hand, if you block or suppress prolactin-inhibiting factor, prolactin levels will rise and the breast will produce more milk. Nipple stimulation and breast milk removal suppress PIF and dopamine, thereby causing prolactin levels to rise and the breast to produce more milk.

  • Two medications, domperidone (brand name: Motilium) and metoclopramide (brand name: Reglan) work by inhibiting PIF to effectively increase prolactin levels and increase breast milk production.

Facts about oxytocin:

Oxytocin is produced by the posterior pituitary gland.

Oxytocin causes the myoepithelial cells surrounding the alveoli to contract, initiating milk ejection.

Oxytocin causes increased uterine activity or uterine cramping, reducing the maternal risk of hemorrhage.

Oxytocin causes the skin temperature of the mother's breast to rise, providing warmth to the infant.

Also known as the love hormone, oxytocin increases calmness and social responsiveness and enhances bonding.


Share The Lactation College


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


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


Lawrence RA, Lawrence RM. Breastfeeding: A Guide for the Medical Profession, 7th edition. Mosby, an imprint of Elsevier Inc. 2011