Breast milk takes longer to start flowing in new mothers using SSRI antidepressants than in moms not on the drugs, researchers report in the February Journal of Clinical Endocrinology & Metabolism.
Although the new study is too small to yield clear conclusions or change clinical practice, the authors say, the findings raise knotty questions about the use of selective serotonin reuptake inhibitors during late pregnancy.
“There’s no question” that this finding should lead to further study of SSRI use and of serotonin’s role in inhibiting breast milk production, says Robert Collier, a lactation biologist at the University of Arizona in Tucson who was not involved in the new work.
Researchers grew curious about the effects of SSRIs on lactation after animal tests reported in this study and in earlier work suggested that serotonin, a neurotransmitter, also plays a role in regulating lactation.
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SSRIs, which include the drugs marketed as Prozac, Zoloft and Paxil, work by keeping excess serotonin in circulation and thus enhancing the neurotransmitter’s effects in the brain and alleviating depression.
But tests on female mice showed that serotonin also works as a brake on breast milk production, says study coauthor Nelson Horseman, an endocrinologist at the University of Cincinnati College of Medicine. “There is an intramammary serotonin signaling system” that operates apart from the compound’s role in the brain, he says. “Serotonin interprets signals that a breast is full and shuts off milk synthesis and secretion,” the team’s tests in mice showed. The breast is a closed system and the stimulus to make milk is strong, so there needs to be a mechanism to inhibit milk production at some point. “That’s the role serotonin plays,” Horseman says.
The biology underlying these findings is supported by experiments in cattle, Collier says. “When we use a serotonin reuptake inhibitor, we see the kind of changes in [cows’] milk production and secretion rates that this research would have predicted,” he says.
Since SSRIs extend the time that serotonin is available in the bloodstream, Horseman wondered what effect the drugs might have on lactating women.
Using a database of women who had recently given birth for the first time, Horseman and his colleagues compared the time that elapsed between birth and each mother’s first copious milk secretion. Eight women were taking SSRIs and 423 were not. The median time before the first milk arrived was nearly 86 hours in the SSRI group, compared with 69 hours for the other women. In the SSRI group, one of the women began lactating at the 72-hour mark, six began on day four and one on day five.
The critical threshold for the arrival of breast milk — not colostrum, a thick, protein-rich substance secreted before a woman’s milk “comes in” — is pegged at 72 hours from birth, says study coauthor Laurie Nommsen-Rivers, an epidemiologist at the University of Cincinnati and Cincinnati Children’s Hospital. By then, a baby can become hungry and fussy, causing stress for the mother. Feeding the infant with formula is generally deemed necessary if a newborn loses more than 10 percent of its body weight. But formula takes longer to digest and makes the baby less eager to suckle, which is needed to stimulate breast milk production.
“It’s sort of a vicious cycle,” Nommsen-Rivers says. But counseling and support can keep a new mother from giving up and resorting to formula after passing the 72-hour threshold. Women can still start producing copious milk after that point, Nommsen-Rivers says. “There’s no reason to throw in the towel.”
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The World Health Organization recommends exclusive breast-feeding for the first six months of life for optimal newborn health.
A delayed start is associated with shorter duration of breast-feeding, she says. A 2009 study by Nommsen-Rivers and others found that 77 percent of women who started lactating within 72 hours were breast-feeding at six weeks, compared with only 53 percent who took longer than 72 hours to get started.
In the tests on mice, the study authors use an “impeccable, elegant methodological design” to establish that serotonin-based antidepressants can affect milk production, says Salvatore Gentile, a physician at the Local Health Authority in Salerno, Italy. But the group of eight women using SSRIs was too small to determine whether these experimental results apply more generally in people, he says. “For this reason, the finding that SSRIs may delay [lactation] activation must be considered merely speculative,” Gentile concludes.
Previous research has shown that SSRIs are reasonably safe for pregnant women and fetuses and that the drugs can benefit many people with depression, Horseman says. Although the new findings raise a potential drawback for SSRIs, he acknowledges that the data are preliminary. “This needs to be looked at in a larger population of women,” Horseman says.