What is Breastfeeding?

The World Health Organization (WHO) recommends that within one hour of birth, mothers initiate breastfeeding and continue to breastfeed infants for at least the first six months of life. Breast milk contains a wide range of nutrients that enable it to be the ideal dietary source for all infants.

Image Credit: Okasana Kuzmina / Shutterstock.com

Physiology of milk production

The production of breast milk is primarily mediated by the activity of various hormones that both indirectly and directly control this process. Both prolactin and oxytocin directly affect breastfeeding, whereas several other hormones, such as estrogen and progesterone, indirectly impact lactation.

When a baby suckles at the breast, sensory nerves within the nipple transmit this signal to the pituitary gland in the brain. Whereas the anterior lobe of this gland is responsible for secreting prolactin, its posterior lobe secretes oxytocin.


Throughout pregnancy, prolactin levels rise to support the growth and development of the mammary tissue to prepare for lactation. Notably, the high levels of progesterone and estrogen during pregnancy prevent milk from being secreted until after delivery, which is when the levels of these hormones drop significantly and rapidly.

In the first few weeks of life, prolactin levels will continue to rise as the baby continues to suckle and stimulate the nipple. As a result, lactation consultants will often recommend new mothers to breastfeed every two to three hours in order to increase their prolactin levels within the blood to support milk production.


Prolactin stimulates the production of milk by the alveoli of the breasts, where milk is subsequently stored until contraction of these sacs is initiated by increased oxytocin levels. Subsequently, milk can flow through the cells of the alveoli and fill ducts that will ultimately carry milk outside the breast.

In addition to stimulation by suckling, oxytocin levels also rise when the mother sees, smells, and touches the baby, hears the baby cry, or even thinks about her baby. Conversely, oxytocin levels can decline when the mother is in severe pain or emotionally upset, which can ultimately hinder her milk supply.  

Taken together, oxytocin is important for ensuring that the baby is able to receive milk quickly and easily in the current feed. This oxytocin pathway is often referred to as the “letdown reflex.”

Phases of breast milk

Within the first two to three days following delivery, colostrum is the first phase of breast milk to be produced. Typically, only about 40 to 50 milliliters (ml) of colostrum will be produced on the first day, as this is all the infant will need at this point in their life.

Colostrum is rich in immunologic components such as white blood cells and antibodies, particularly secretory immunoglobulin A (IgA), as well as various developmental factors. Furthermore, colostrum contains a significantly greater concentration of protein, minerals, and fat-soluble vitamins like vitamins A, E, and K as compared to the later stages of milk.

Transitional milk is then produced between seven to fourteen days after delivery. This type of breast milk contains a reduced concentration of certain bioactive substances like protein and, alternatively, has higher potassium and calcium levels.

Mature milk will be produced by two weeks after delivery. In general, mature breast milk is approximately 87% water, 7% lactose, 3.8% fat, and 1.0% protein. Lactose is the primary carbohydrate found within breast milk and provides about 40% of the total energy to the infant. Comparatively, the fat within breast milk, particularly that which is the form of triglycerides, is responsible for about 50% of the energy provided by human breast milk.

Benefits of breastfeeding

As one of the most effective strategies to support the growth of infants and maintain their health development throughout childhood, breastfeeding is associated with numerous benefits for the child. For example, breastfed infants have a reduced risk of asthma, diarrhea, food allergies, diabetes, and metabolic diseases in adulthood.

Breastfeeding has also been shown to reduce the risk of childhood infection-related morbidity and mortality rates as compared to infants who have never been breastfed. The anti-inflammatory properties of breastmilk are largely attributed to the transmission of maternal IgA and IgG from the mother to the infant. The presence of lactoferrin within breast milk, which is an iron-binding glycoprotein, also prevents bacterial growth to similarly provide passive immunity to the infant against gastrointestinal and respiratory tract infections.  

Breastfeeding is also associated with numerous maternal health benefits including a reduced risk of breast, ovarian, endometrial, and thyroid cancers. These benefits appear to be dose-dependent, with a longer duration of breastfeeding associated with an even more significantly reduced risk of developing breast and ovarian cancers.


Further Reading

Last Updated: Jul 14, 2022

Benedette Cuffari

Written by

Benedette Cuffari

After completing her Bachelor of Science in Toxicology with two minors in Spanish and Chemistry in 2016, Benedette continued her studies to complete her Master of Science in Toxicology in May of 2018. During graduate school, Benedette investigated the dermatotoxicity of mechlorethamine and bendamustine; two nitrogen mustard alkylating agents that are used in anticancer therapy.


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