All babies are born with nipples. This anatomical fact highlights how the body forms before sex differences are established, setting the stage for later physical changes. The presence of nipples in both sexes at birth is a clear example of how fundamental biological blueprints are laid down early in gestation.
The Presence and Appearance of Infant Nipples
The foundation for nipples is established very early in pregnancy, before a fetus’s sex organs fully differentiate. This process begins around the seventh week of embryonic development with the formation of the mammary ridge, or milk line. This ridge is a thickened band of ectodermal tissue extending from the armpit to the groin.
The permanent nipples, located in the thoracic region, develop from this ridge. Their initial development precedes the activation of the gene that determines biological sex, which explains why males also possess nipples. At birth, a baby’s nipples appear small, flat, or slightly inverted, typically measuring just a few millimeters in diameter. They are composed of smooth muscle fibers and rudimentary milk ducts, and their appearance is virtually identical in both male and female newborns.
Temporary Changes Due to Maternal Hormones
In the first few days and weeks of life, a newborn’s chest area may exhibit temporary changes caused by maternal hormones. During pregnancy, high levels of maternal hormones, particularly estrogen, cross the placenta into the baby’s bloodstream. After birth, the sudden withdrawal of this hormonal exposure can lead to newborn breast swelling.
This swelling affects both male and female infants and typically appears by the third day after birth. It is a common source of parental concern, but usually resolves on its own as the hormones clear the baby’s system, often within the first two weeks.
In some cases, a small amount of milky fluid, historically called “witch’s milk” or neonatal galactorrhea, may leak from the nipples. This discharge is a result of the hormonal exposure and is generally harmless, usually stopping within two months. Parents should avoid squeezing or massaging the swollen area, as this can irritate the tissue or potentially lead to an infection. If the swelling is accompanied by redness, warmth, or fever, a healthcare provider should be consulted to rule out infection.
Future Development and Sexual Maturation
The similar appearance of nipples persists throughout childhood until the onset of puberty. The underlying mammary structure remains largely dormant for years, consisting of tiny ducts and connective tissue. The significant transformation of the nipple and surrounding tissue is driven by the surge of sex hormones during adolescence.
In females, the release of estrogen triggers breast development, including the enlargement of the nipple and areola. In males, elevated levels of testosterone inhibit the extensive development of glandular and fatty tissue. Although male nipples increase slightly in diameter during puberty, they remain smaller compared to those in females. The adult difference in function, such as the capacity for lactation in females, results from this puberty-driven hormonal differentiation.