But when Donna Geddes, professor of biomedical sciences at the University of Western Australia (UWA), explains what the moving images are, it becomes clear just how remarkable this video footage is. Using ultrasound imaging techniques, Geddes and her colleagues at UWA have captured in real time the actual flow of milk through the ducts in a woman’s breast. Look closely, says Geddes, and you’ll notice “small white flecks (milk-fat globules) are seen moving within the duct towards the nipple. Also, sometimes surrounding ducts are seen to dilate...and some small ducts that are not obvious prior to milk ejection become visible.”
It is indeed an amazing, and rare, view of lactation in motion. The scientific data on milk ejection has been thin. As Geddes writes in a recent paper, “Although milk ejection is integral to lactation and thus the survival of the species, the lack of knowledge regarding milk ejection in women, in comparison to other lactating and dairy animals, is surprising.” Milk ejection, or the let-down reflex, is a complex neuroendocrine process. The baby sucks on the mother’s nipple, which stimulates nerve endings in the nipple and areola, which send signals along nerve pathways to the spinal cord and brain.
“It signals to the brain,” explains Sharon Donovan, “‘OK, I need the milk down here now.’”
The hypothalamus and pituitary gland release the hormone oxytocin, which then travels through the bloodstream back to the mammary gland and stimulates the contraction of specialized cells surrounding the alveoli. Milk is squeezed out, ducts widen, and breast milk begins to flow from the branching network to the nipples. And while the physical stimulation of the infant’s sucking most directly triggers this chain, many mothers find the let-down reflex also can be induced by sensory or emotional stimuli as well, hearing a baby cry, for example, even if it is someone else’s baby. On the other hand, let-down is inhibited by stress, fear, and high alcohol consumption.
Anthropologist Sarah Blaffer Hrdy notes in her book, Mother Nature, that “as I became more conditioned to nursing, just the thought of her, or her cry, was enough to trigger this ‘let-down reflex’ and cause a patch of wetness on my blouse.” It becomes automatic and, as Geddes points out, “while the first milk ejection is sensed by most mothers, subsequent milk ejections that occur during either breastfeeding or breast expression are rarely sensed.”
A suite of hormones are involved in lactation. Some of the hormones play roles early in pregnancy, some in late pregnancy, and others after birth. Though different hormones play different roles at different times, none of them work in isolation; some work in tandem with other hormones, and some actually inhibit the action of other hormones. The hormones oxytocin and prolactin are critical for lactation. Prolactin is the hormone that supports milk production and oxytocin is the hormone that promotes milk let-down. The two work hand-in-glove to regulate the daily rhythm of breastfeeding. It is a supply-and-demand system, or as Blackburn puts it: “Without stimulation by suckling and removal of milk, secretion of prolactin decreases and milk production ceases…. Milk production rate depends upon the milk removal rate.”
In addition to its role in lactation, oxytocin is also important in causing uterine contractions and for that reason it is sometimes given to women in order to induce labor. During nursing, the hormone continues to help contract the uterus, which in 9 months goes from the size of a fist to the size of a basketball. But in recent years, oxytocin has also gotten attention for its role in contentment, trust, and security when it comes to human bonding. Press stories dubbed oxytocin “the cuddle hormone.” According to researchers, oxytocin is a key player in the biological basis for human attachment and bonding and helps people maintain emotional closeness in interpersonal relationships.
Fetal Feat of Coordination
The sucking stimulation provided by the infant is also automatic, of course, but even here there is a biological learning curve. In fact, the fetus spends months rehearsing the skill of sucking and swallowing. And like virtually all other motor behaviors, it is “simple” only in the sense that virtually all infants come to perform it perfectly. In fact, sucking is a complex behavior dependent on a range of neuromuscular factors. As Susan Tucker Blackburn explains in Maternal, Fetal, & Neonatal Physiology, the swallowing reflex begins at 10-14 weeks, and by 16 weeks the fetus swallows a tiny amount of amniotic fluid each day (at term, the fetus is swallowing about 2 cups of amniotic fluid daily). The sucking reflex starts to kick in at 20 weeks gestation.
But that’s not the end of the training. The challenge for the fetus is to then coordinate sucking and swallowing. According to Blackburn, “Although all components of sucking and swallowing are present by 28 weeks, the fetus is often unable to coordinate these activities. Some suck-swallow synchrony is seen at 32-34 weeks; synchrony is complete by 36-38 weeks.”
And finally, once the baby arrives, she must coordinate sucking and swallowing with breathing. Practice makes perfect.
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