The stress hormone and breastfeeding tug-of-war

New data has important implications for working mothers

As BIOMILQ conducts research to develop mammary biotechnology in North Carolina, we want to highlight and ‘cell’ebrate other mammary biology research in the greater field. Tara Fernandez, Ph.D. helped us learn from a peer-reviewed paper that was published in August 2021. Tara is a Science Communicator sharing stories on the latest discoveries, innovation, and technologies that are set to transform how we protect human health and wellbeing. You can read more of her work here.

Breastmilk is the original superfood. Complete and nutrient-dense, it provides everything that babies need to survive and thrive, from stress-reducing endorphins to pathogen-resisting antibodies. Lactation goes back over 200 million years in our evolutionary history — fossils of animals predating mammals show that they produced a rich, milk-like secretion to nourish their offspring (1).

And yet, for something so ancient and integral to our existence, it’s almost incomprehensible how little we know about it. Human lactation involves vast, interconnected networks of genetic, environmental, physiological, and psychological factors, of which only a fraction has been studied. Among the many unknowns are links between mental and physical wellbeing and milk production. For instance, how does maternal stress influence the intricate biological pathways that drive lactation?

In a first-of-its-kind study, an international team of researchers has gone in search of answers, examining the relationship between maternal work stress, hormone levels, and breastfeeding patterns (2). Bibi and colleagues recruited a cohort of 88 lactating women with children under 12 months old, half of whom had gone back to work. The women provided blood and saliva samples for hormone analyses and were given questionnaires to assess their moods and perceived stress levels.

The results showed that working mothers had significantly lower breastfeeding rates compared to the stay-at-home ones. This isn’t surprising. Several researchers have established clear associations between maternal employment with shorter breastfeeding durations (3,4). One study reported that in the month of returning to work, mothers are over twice as likely to quit breastfeeding as those who remained unemployed (5).

Certainly, these trends are affected by external factors such as logistical challenges and unsupportive workplaces (6), but do physiological mechanisms that suppress lactation under stressful conditions also come into play?

To investigate, Bibi’s team measured stress biomarkers among the study participants. They found that working women had elevated blood pressures and cortisol levels, and this group also reported more negative moods than their unemployed counterparts. These findings mirror results from prior studies involving professional mothers: over 63 percent said they experienced moderate to severe stress levels since resuming work (7).

Psychological stress triggers a cascade of changes in the body: it spikes inflammation, makes us more susceptible to infections, and topples hormonal balances. The brain responds to perceived threats (anything from charging bears to fast-approaching project deadlines) by signaling the kidney’s adrenal glands to churn out a cocktail of so-called stress hormones (8). This occurs via a feedback loop called the hypothalamic-pituitary-adrenal axis, or HPA. Adrenaline gets the heart pumping, while cortisol releases a surge of glucose into the bloodstream, setting off the fight-or-flight response.

Under normal circumstances, these hormonal responses are transient, subsiding once the threat is removed. However, this isn’t the case for the mothers juggling parenthood and professional pressures, whose uptick in cortisol levels pointed towards chronic stress.

Milk production takes a heavy metabolic toll on mothers. Some estimates suggest that the energetic cost of lactation is around 630 kcal a day, or around a third of the recommended caloric intake for women (9). Therefore, when subjected to chronic stress, mothers’ bodies tune down energy-draining lactation pathways to preserve resources. However, the mechanisms that influence the stress-lactation relationship are complicated — on the flip side, breastfeeding can protect mothers from both physical and psychological stressors (10–12).

The hormone profiles of working mothers in the study may offer some important clues on the hormonal interplay impacting lactation during stressful times. These women showed sharp dips in their levels of the thyroid hormone triiodothyronine (T3). The thyroid, a butterfly-shaped gland on the neck, is known to regulate lactation by boosting the production of prolactin, a hormone released from the brain that kickstarts milk production after childbirth (13). Unsurprisingly, employed women with low T3 levels also had less circulating prolactin than those who weren’t working. This difference in prolactin levels was found to be clearly linked to higher stress burdens (and not affected by overall breastfeeding durations).

Courtesy of BIOMILQ

The authors propose a potential mechanism for how all these pieces fit together: stress prompts cortisol production, which in turn inhibits T3. Eventually, a drop in T3 levels curbs prolactin, and breastmilk synthesis begins to taper off.

For now, these associations are correlations; future studies will elucidate whether circulating T3 has a direct, causative effect on diminishing prolactin levels. In addition, given the sheer complexity of lactation regulation, it is unlikely that T3 is the sole stress-induced mediator of prolactin production. Other factors such as progesterone and beta-endorphins are also likely to come into play (14,15).

Nevertheless, this pioneering study demonstrates the strong ties between maternal employment status, the mental and physical ramifications of stress, and lactation physiology. Clearly, workplace legislation to better support mothers as they navigate the stressors of work and home life is urgently needed. In addition, these new findings highlight the importance of tailored stress management strategies for breastfeeding mothers to promote and sustain lactation.


1. Capuco AV, Akers RM. The origin and evolution of lactation. J Biology. 2009;8(4):37–37. doi:10.1186/jbiol139 2. Bibi S, Shah M, Malik MO, Goosens KA. T3 is linked to stress‐associated reduction of prolactin in lactating women. J Neuroendocrinol. 2021;33(8):e13003. doi:10.1111/jne.13003 3. Calnen G. Paid Maternity Leave and Its Impact on Breastfeeding in The United States An Historic, Economic, Political, and Social Perspective. Breastfeed Medicine Official J Acad Breastfeed Medicine. 2007;2(1):34–44. doi:10.1089/bfm.2006.0023 4. Fein SB, Roe B. The effect of work status on initiation and duration of breast-feeding. Am J Public Health. 2011;88(7):1042–1046. doi:10.2105/ajph.88.7.1042 5. Kimbro RT. On-the-Job Moms: Work and Breastfeeding Initiation and Duration for a Sample of Low-Income Women. Matern Child Healt J. 2006;10(1):19–26. doi:10.1007/s10995–005–0058–7 6. Reat A, Matthews KJ, Carver AE, Perez CA, Stagg J, Byrd-Williams CE. Support for Breastfeeding Employees: Assessing Statewide Worksite Lactation Support Recognition Initiatives in the United States. J Hum Lact. 2019;36(2):328–336. doi:10.1177/0890334419865902 7. Kadale PG, Pandey AN, Raje SS. Challenges of working mothers: balancing motherhood and profession. Int J Community Medicine Public Heal. 2018;5(7):2905–2910. doi:10.18203/2394–6040.ijcmph20182620 8. Russell G, Lightman S. The human stress response. Nat Rev Endocrinol. 2019;15(9):525–534. doi:10.1038/s41574–019–0228–0 9. Raaij JM van, Schonk CM, Vermaat-Miedema SH, Peek ME, Hautvast JG. Energy cost of lactation, and energy balances of well-nourished Dutch lactating women: reappraisal of the extra energy requirements of lactation. Am J Clin Nutrition. 1991;53(3):612–619. doi:10.1093/ajcn/53.3.612 10. Altemus M, Deuster PA, Galliven E, Carter CS, Gold PW. Suppression of hypothalmic-pituitary-adrenal axis responses to stress in lactating women. J Clin Endocrinol Metabolism. 1995;80(10):2954–2959. doi:10.1210/jcem.80.10.7559880 11. Altemus M, Redwine LS, Leong YM, Frye CA, Porges SW, Carter CS. Responses to Laboratory Psychosocial Stress in Postpartum Women. Psychosom Med. 2001;63(5):814–821. doi:10.1097/00006842–200109000–00015 12. Heinrichs M, Neumann I, Ehlert U. Lactation and Stress: Protective Effects of Breast-feeding in Humans. Ann Ny Acad Sci. 2009;5(3):195–203. doi:10.1080/1025389021000010530 13. Motil KJ, Thotathuchery M, Montandon CM, et al. Insulin, cortisol and thyroid hormones modulate maternal protein status and milk production and composition in humans. J Nutrition. 1994;124(8):1248–1257. doi:10.1093/jn/124.8.1248 14. Camilletti MA, Ferraris J, Abeledo‐Machado A, et al. Participation of membrane progesterone receptor α in the inhibitory effect of progesterone on prolactin secretion. J Neuroendocrinol. 2018;30(9):e12614. doi:10.1111/jne.12614 15. Foley KM, Kourides IA, Inturrisi CE, et al. β-Endorphin: Analgesic and hormonal effects in humans. Proc National Acad Sci. 1979;76(10):5377–5381. doi:10.1073/pnas.76.10.5377

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