Most mothers’ breasts are normal, and normal includes many sizes and shapes. A girl’s breasts begin to develop early, well before she is born, but after birth the milk tissues in the breasts are inactive until puberty, when hormones stimulate their growth. During pregnancy, breasts grow bigger and feel heavier. Breasts that are making milk are made of fat cells, supporting structures, millions of little sacs (alveoli) in which the milk is made, and tubes (ducts) that bring the milk from inside the breast to the nipple during the let-down reflex. Breast milk components are unique to each mother and will change over time.
A) The structure of the breast
Breasts are rarely exposed in public, so many mothers believe their breasts are not “normal.” The breasts that are seen are often manipulated through clothing, the alteration of an image, or surgery.
Normal breasts can be smaller or larger and many mothers have breasts that are slightly different in size. Breast size is not related to the amount of milk that can be produced (Ramsay 2005).
Millions of little sacs in which the milk is made (alveoli)
Muscular (myoepithelial) cells that squeeze the milk out of the alveoli
Tubes that bring the milk from the alveoli to the nipple (milk ducts)
Blood vessels that bring blood cells and nutrients to the breast and remove waste products
The breast is divided into lobes. Each lobe is drained by a milk duct. There are between 5 and 25 milk ducts with an average of 9 ducts in each breast (Geddes 2007; Ramsay 2005).
The breast contains lymph, a watery, clear fluid that is similar to blood but does not contain red blood cells. Lymph is present between cells and circulates through the lymphatic system, a complex body-wide net of delicate tubes and nodes. It is a passive system that has one-way valves and relies on external pressure from muscles and joints to help the lymph move.
While milk leaves the breast through the nipple, lymph leaves the breast through the lymphatic system. It is then filtered through nodes and eventually enters the blood system.
When the breast is infected or engorged or develops cancer, the lymph can accumulate and cause the breast to swell.
E) The development of the breast
1) The breasts of a baby
Breast development starts when a fetus (an unborn baby) is only 20 days old. By 6 weeks after conception, the nipple and areola start to form and by 28 weeks the milk ducts open.
Newborn babies respond to their mother's hormones. The baby’s breasts can become quite swollen and even secrete milk. The milk tissues are then inactive until puberty.
2) Puberty and menstrual cycles
During puberty many hormones, including estrogen, progesterone, various steroids, and thyroid hormones, stimulate marked development of the alveoli and milk ducts. With every menstrual cycle, these tissues develop a little more.
3) During pregnancy
During pregnancy, many hormones support extensive development and remodeling of the breast. Among the changes:
Milk ducts become longer and develop branches.
Nipples and areolas grow.
Glandular tissues develop further.
The nipple root becomes more elastic as the pregnancy progresses.
Most of the increase in breast size is completed by Week 22 of pregnancy. By this time, the amount of blood going to the breast has doubled (Geddes 2007) and the breasts feel heavier.
Lighter-skinned mothers may notice more blue veins in the skin of their breasts. There can also be a pink flush on the breast and if mothers look closely, they may be able to see tiny pink lines in the skin. These are small blood vessels that become visible due to the increased blood flow.
4) After birth
Colostrum and milk are created in glandular tissue that consists of tiny sacs called alveoli. As the baby latches and starts sucking at the breast, the brain releases the hormone oxytocin into the blood. Oxytocin travels to the breast, where it makes the muscular cells around the alveoli squeeze. This sends milk into the ducts which transport it to the nipple. This process is called the let-down reflex.
During pregnancy, levels of the milk hormone prolactin rise but its action is blocked by progesterone, which is secreted from the afterbirth (placenta). With the passing of the placenta, progesterone levels decrease quickly. This allows the breast to start the switch to milk production from colostrum production. This change is described as the milk “coming in” and happens between Day 2 and 4 after birth. It is also known as lactogenesis II.
When the milk comes in, mothers tend to feel changes first in the upper outer area of the breast and toward the axillary tail.
The amount of milk made by the breasts increases quickly during the first weeks after birth as does the size of the breast (Cox 1999). Mothers make roughly the same amount of milk between one and six months after birth. This is supported by the hormones prolactin and oxytocin. Once the baby starts solid foods, the breast will slowly make less milk.
During weaning, the milk-producing cells die, resulting in the milk glands losing about 90% of their weight (Akhtar 2016). This space is taken over by fat cells (Macias 2012; Wang 2019).
F) Breastfeeding problems
Breasts can be affected by conditions related to breastfeeding, such as infections and plugged ducts, and conditions not related to breastfeeding, such as cancer. A small number of mothers cannot make enough breast milk.
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Macias H, Hinck L. Mammary gland development. Wiley Interdiscip Rev Dev Biol. 2012;1(4):533-57
Ramsay D, Kent J, Hartmann R, et al. Anatomy of the lactating human breast redefined with ultrasound imaging. Journal of Anatomy. 2005;206(6):525-534
Wang QA, Scherer PE. Remodeling of Murine Mammary Adipose Tissue during Pregnancy, Lactation, and Involution. J Mammary Gland Biol Neoplasia. 2019 Sep 12