Breasts
Significant breast changes occur within the first half of pregnancy. These changes can cause rapid enlargement, visible veins, soreness, and even stretch marks.
Milk production also starts early (colostrum), then peaks in speed during the last few weeks of pregnancy. Some women may even leak colostrum prior to delivery.
There is no correlation between size of breasts, either before or during pregnancy, and the ability to breastfeed or produce enough milk.
Further, significant growth of breast tissue and ducts in a short amount of time can produce potential concerns of new lumps (most of which are completely normal) by women who feel and examine their breasts often.
Women should continue reading to understand breast changes during pregnancy and when to discuss any concerns they may have with their HCP.
Background
Breasts are made up of lobules (milk-producing glands) and ducts (tubes that carry milk to the nipple). These are surrounded by glandular, fibrous, and fatty tissue.
The darker area of skin around the nipple is called the areola. There are small raised bumps on the areola, called Montgomery’s Tubercles that produce fluid to moisturize the nipple.
Physical Changes
High levels of the hormones estrogen, progesterone, and prolactin result in structural changes of the breasts during pregnancy and lactation.
Under the influence of estrogen in the first trimester, ducts multiply and grow, and alveolar-lobular growth occurs as well.
Fatty tissue and blood supply also increase and is most pronounced during the first 20 weeks of pregnancy.
At the end of the first trimester, nipple and areola begin to darken, the nipple becomes more prominent, and more veins underneath the skin of the breasts can be seen.
Breast soreness is a common pregnancy complaint in all trimesters due to these changes, especially in the first half of pregnancy. The greatest amount of breast growth usually occurs up to 22 weeks, but this same growth can occur in the last trimester and postpartum period in some women.
As the uterus increases in size, the rib cage is forced out toward the sides, and the diameter of the chest may increase by 10 to 15 cm (and chest bands on bras may no longer fit, as well as original cup size).
In the last few weeks of pregnancy, the nipples become larger and the breasts continue to grow as the milk-producing cells get bigger.
In one study that measured breast changes, breast size increased an average of 96 ml, regardless of the initial breast volume.
Due to larger nutritional needs during lactation, male infants may stimulate greater changes in maternal breast size than female infants.
This increased weight distribution to the breasts and stomach can change the center of gravity, which can force a woman to adapt her posture to avoid neck and upper back pain.
During the final stages of pregnancy, breast fatty tissue nearly completely disappears and is replaced by hard, tight lobes; breast skin becomes much thinner.
This phase of new breast tissue slows and changes to focus on milk production. Breasts transition from not being able to produce milk to full capability shortly after delivery. This is known as lactogenesis.
Note: A few women may have occasional leakage of blood from the nipple, which is due to an increase in the number and size of blood vessels. If there is no associated mass, it is viewed as a benign symptom of pregnancy, but should always be evaluated by an HCP.
Milk Production
Milk production is almost completely controlled by hormones. Prolactin is produced from the pituitary gland and stimulates enlargement of the mammary glands in preparation for milk production.
Blood levels of prolactin rise during the 8th week of pregnancy and peak at ten times normal levels, with higher levels of prolactin associated with longer durations of lactation. Human placental lactogen is made by the placenta, gives nutrition to the fetus, and also stimulates milk glands in the breasts for breastfeeding.
The breasts can produce a yellowish-colored fluid (colostrum) around the 16th week of pregnancy, which can leak from the nipples.
Colostrum production occurs in alveolar cells and is made by the body using nutrients from the mother’s blood. It is secreted into the ductal system in the final days of pregnancy at the rate of only a few milliliters per day.
Colostrum is the most potent natural immune booster known to science; it is a high-density milk that contains more protein, minerals and fat-soluble vitamins than mature milk. It also possesses anti-inflammatory and anti-infective functions, promotes immune system formation, and supports organ development.
During pregnancy, high levels of estrogens and progesterone counteract prolactin, thus inhibiting milk production, except for colostrum. After delivery, estrogen and progesterone levels decrease significantly, allowing prolactin to signal the release of milk. Oxytocin is also released through the sucking action of the infant:
When the baby suckles, oxytocin and prolactin are released from the mother’s pituitary gland. Prolactin passes through the mother's blood to the breast, stimulating milk production. Then oxytocin stimulates milk delivery to the nipple.
Milk production does not correlate with the amount of glandular breast tissue, the number of ducts, or the mean duct diameter. There is also no correlation with breast size and milk storage capacity of the breast. Multiple factors influence the production of milk, not just breast size, and breast size alone does not determine the quality and quantity of milk.
After colostrum, breast milk is comprised of foremilk (“skim milk”) and hindmilk (“whole milk”), whose compositions are different from that of colostrum. Foremilk is ingested by the baby first, and then its composition changes to hindmilk on the same breast after about 10 minutes of feeding.
There is also evidence that human milk contains exactly what that specific newborn needs. For example, preterm milk has a different composition and is of more benefit to the preterm newborn than milk produced after the delivery of a full-term infant.
It takes about three months after discontinuation of breastfeeding in order for the breasts to return to somewhat of a pre-pregnancy state. This is done through atrophy (tissue breakdown) of the lobules.
Breast Lumps
Breast lumps commonly develop during pregnancy and breastfeeding and can cause significant anxiety and concern among women and HCPs. However, most “true lumps” found during pregnancy are benign (noncancerous). Read more detailed information on Breast Lumps during pregnancy.
Action
Women should be very familiar their breasts before pregnancy and as pregnancy progresses in order to identify anything unusually painful or different that may require examination by an HCP. Read more.
Women should wear soft and comfortable bras to ease any pain as the breasts grow. Support is important, especially for large breasts, to avoid strain on the neck and upper back.
Women should not be concerned about the size of their breasts and their ability to breastfeed – there is no correlation.
Women who are concerned about their ability to breastfeed during or after pregnancy should contact their HCP, who may be able to recommend a lactation consultant. Some practices may already have consultants on staff, who can be very valuable for women who wish to breastfeed.
Resources
Breast Anatomy (Mayo Clinic: Slideshow)