The human breast develops from a thickened ectodermal
ridge (milk line) situated longitudinally along the ante-
rior body wall from the groin to the axilla at about 6
weeks’ gestation. Regression of the ridge occurs except
for the pectoral region (2nd–6th rib), which forms the
mammary gland. Supernumerary glands may develop
anywhere along the ectodermal ridges, and in 2% to 6%
of women, these glands either mature into mammary
glands or remain as accessory nipples.
During the 7th and 8th weeks of gestation, the mam-
mary parenchyma invades the stroma, which appears as a
raised portion called the mammary disc. Between the
10th and 12th weeks, epithelial buds form; parenchymal
branching occurs during the 13th through 20th weeks.
Between the 12th and 16th weeks of gestation, the
smooth musculature of the areola and nipple are formed,
and at approximately 20 weeks’ gestation, between 15
solid cords form in the subcutaneous tissue.
Branching continues, and canalization of the cords oc-
curs, forming the primary milk ducts by 32 weeks’
At 32 weeks’ gestation the ducts open onto the
area, which develops into the nipple.
The adipose tissue of
the mammary gland develops from connective tissue that
has lost its capacity to form fibres, and it is considered
necessary to further growth of the parenchyma.
Shortly after birth, colostrum can be expressed from
the infant’s mammary glands. This is attributed to the
pro-lactation hormones present in the fetal circulation at
birth. Regression of the mammary gland usually occurs
by 4 weeks postpartum and coincides with a decrease in
the secretion of prolactin from the anterior pituitary
gland of the infant.