Postnatal Development: Lactation

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Chapter: Anatomy and Physiology for Health Professionals: Pregnancy and Development

The mammary glands are continually stimulated during pregnancy.

Postnatal Development

The mammary glands are continually stimulated during pregnancy. Estrogens extend the ductile systems and deposit fat around them. Progesterone stimulates the alveolar gland development, assisted by placental lactogen. Breast size is normally doubled during pregnancy and the mammary glands become capable of secreting milk. However, milk production is temporarily inhibited by placental progesterone and lactogen.


After childbirth, placental hormones decline rap-idly in the mother’s blood. Prolactin stimulates the mammary glands to secrete milk, but before this, a watery fluid called colostrum is secreted. Colos-trum contains more protein but less carbohydrates and fat than milk. It contains antibodies that pro-tect the newborn from various infections. Milk becomes available to the infant through the milk let-down reflex, which continues to function until the infant is weaned, usually between one and two years after birth.

The ejection of milk requires specialized myoep-ithelial cells around the alveolar glands to contract. Stimulation of the nipple or areola elicits the reflex action that controls milk ejection. Oxytocin is released and the cells contract within 30 seconds of stimulation. As long as milk is ejected from the breasts, prolactin and oxytocin release continues and milk is continually produced. If milk is not removed from the breasts reg-ularly, the mammary glands are signaled to stop pro-ducing milk. Human milk is the best possible food for human babies, containing exactly the right balance of required nutrients.

The Neonatal Period, Infancy, and Childhood

The neonatal period begins at birth, extending to the end of the first four weeks. Newborns must quickly adapt to respiration, digestion, excretion, and regulation of body temperature. The first breath of a newborn must be forceful enough to begin to expand the lungs, which eases breathing. Their lungs are col-lapsed initially and the tiny airways are very resistant to air movement. Surface tension tends to hold the moist membranes of the lungs together. Secretion of surfactant occurs, reducing surface ­tension.

A newborn’s liver is unable to supply enough ini-tial glucose to support metabolism, so stored fat is used for energy. Their kidneys excrete a dilute fluid because they cannot yet process concentrated urine. Newborns often become dehydrated as a result and develop a water and electrolyte imbalance. Their ­temperature-regulating system may also not be normally functional.

After birth, the umbilical vessels constrict, with the umbilical arteries closing first. If not clamped or severed for a minute or more, blood continues to flow from the placenta to the newborn through the umbilical vein. The foramen ovale closes due to blood pressure changes as fetal vessels constrict. Blood pressure in the right atrium falls. Resistance to blood flow through the pulmonary circuit decreases and blood pressure in the left atrium increases (­FIGURE 26-16).

The valve on the left side of the atrial septum closes the foramen ovale. It gradually fuses with the tissues along the margin of the foramen. Adults have a depression called the fossa ovalis marking the site of the previous opening. The ductus arteriosus also con-stricts after birth, and blood can no longer bypass the lungs by moving from the pulmonary trunk into the aorta. Constriction of the ductus arteriosus may be complete within 15 minutes, but permanent closure of the foramen ovale may take up to one year. Within four months after birth, most of the circulating hemo-globin has become of the adult type.

Environmental factors may affect inherited traits and conditions. These factors include ­nutrition, ­exposure to toxins or pathogens, and physical ­activity. Environmental factors hold great importance for ­polygenic traits, which are those determined by more than one gene such as height, skin color, and intelligence. TABLE 26-4 summarizes major events during the neonatal period and later life, TABLE 26-5 explains changes related to aging, and TABLE 26-6 lists normal physical growth for females and males during childhood.

1. Explain the hormones involved in milk production and secretion.

2. How long does the neonatal period last?

3. Describe the major events that occur in the neonatal period.

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