Postpartum Hormones, Breastfeeding, and Cancer: Untangling a Complex Web

By The SABI

The postpartum body is a paradox. Within days of giving birth, oestrogen and progesterone, the hormones that soared during pregnancy, plummet to near-menopausal levels. Prolactin rises to stimulate milk production. Oxytocin spikes to contract the uterus and trigger let-down. Cortisol lingers, sustained by stress and sleep deprivation.

It’s a hormonal choreography that keeps a new mother alive, healing, and able to feed her baby. But this same web of hormones has long been linked to one of women’s greatest health threats: breast cancer.

For decades, epidemiologists have noticed an intriguing pattern. Women who breastfeed tend to have a lower lifetime risk of breast cancer. But the biology behind this protection isn’t straightforward. It’s not simply about “breastfeeding reduces risk”,  it’s about how postpartum hormones remodel breast tissue, alter cell division, and recalibrate long-term exposure to oestrogen and other drivers of tumour growth.

So what’s really happening in this complex interplay between postpartum hormones, breastfeeding, and cancer risk?

The Postpartum Reset: A Hormonal Earthquake

After birth, the body undergoes one of the steepest hormonal shifts in human biology.

• Oestrogen and progesterone: drop by over 90% within hours of delivery. This crash contributes to mood volatility, but also reduces proliferative signalling in breast tissue.
  

• Prolactin: remains elevated in breastfeeding mothers, stimulating milk synthesis. Beyond lactation, prolactin also has a role in immune modulation and may influence breast cell differentiation,
  

• Oxytocin: repeatedly spikes with feeding, strengthening maternal bonding but also exerting anti-inflammatory and possibly anti-tumour effects.
  

• Cortisol: persists at higher levels postpartum, particularly in sleep-deprived mothers, with complex implications for immune surveillance.
  

This reset changes not only how a mother feels day-to-day, but also how her cells behave in the years ahead.

Breastfeeding and Breast Tissue Remodelling

Pregnancy and lactation push breast tissue through cycles of expansion and regression. During breastfeeding, mammary epithelial cells are active, specialised, and producing milk. Once breastfeeding stops, those cells undergo involution, a process of cell death and tissue remodelling.

• Involution has been described as a “wound-healing–like environment”: inflammatory molecules surge, extracellular matrix is remodelled, and immune cells flood the breast.
  

• This temporary inflammation can, paradoxically, increase short-term breast cancer risk in the years immediately after childbirth. Epidemiological studies show a transient rise in risk in the first 5–10 years postpartum, particularly in women who delay childbirth until later in life.
  

• However, long-term, the story flips: the differentiation of breast tissue during pregnancy and breastfeeding appears to leave cells less vulnerable to malignant transformation. Meta-analyses show that women who breastfeed for 12 months or longer reduce lifetime breast cancer risk by around 4.3% for every 12 months of breastfeeding (Collaborative Group, Lancet, 2002).
  

In short: breastfeeding triggers changes that may transiently elevate risk in the near term, but substantially lower it over a lifetime.

The Oestrogen Connection

One of the clearest biological links is cumulative lifetime exposure to oestrogen. Oestrogen stimulates breast cell proliferation, increasing opportunities for mutations that can drive cancer.

Breastfeeding lowers lifetime exposure in several ways:

1. Delayed ovulation and menstruation: Lactational amenorrhea suppresses ovarian cycles, reducing the number of lifetime oestrogen peaks.
   

2. Lower circulating oestrogen: Breastfeeding mothers have reduced baseline oestrogen compared to non-breastfeeding mothers.
   

3. Tissue differentiation: Once cells have fully matured for milk production, they are less prone to malignant transformation than undifferentiated cells.
   

Together, these mechanisms help explain why longer breastfeeding duration confers greater protection.

Prolactin, Oxytocin, and Immune Modulation

The prolactin story is complex. While some studies suggest prolactin can fuel tumour growth under certain conditions, in the context of lactation it seems to have a protective role. Lactating breasts show greater differentiation, and prolactin interacts with immune pathways that may strengthen surveillance against abnormal cells.

Oxytocin, meanwhile, is increasingly studied for its potential anti-cancer properties. It reduces inflammation, lowers stress responses, and may directly inhibit tumour cell migration. The repeated oxytocin surges during feeding may therefore be part of breastfeeding’s long-term protective effect.

A Complex Timeline of Risk

What emerges is not a simple “breastfeeding prevents cancer” narrative. Instead:

• Immediately postpartum: inflammation during involution can temporarily elevate risk.
  

• With prolonged breastfeeding: reduced oestrogen exposure and tissue differentiation lower lifetime risk.
  

• The longer the breastfeeding duration: the greater the cumulative protective effect.
  

This is why public health studies consistently show lower risk of both hormone-receptor–positive and even some triple-negative breast cancers among women who breastfeed, especially with longer duration (Collaborative Group, 2002).

What This Means for Mothers

As a mother and product developer, I know this is more than abstract biology. It’s about understanding that the messy, exhausting act of breastfeeding — the night feeds, the clogged ducts, the mastitis battles, is also rewriting our long-term health story.

It’s also a reminder that supporting breastfeeding isn’t only about milk supply today. It’s about protecting mothers tomorrow. That’s why at The SABI, whether through The Breastfeeding Herbata to nourish supply and calm stress, or hormone-safe skincare to avoid unnecessary endocrine disruptors, our mission is to care for both sides of the equation: the baby and the mother’s long-term health.

HORMONAL & PROUD

Created as a brand to help women navigate the toughest moments in pregnancy, childbirth, postpartum — and practically every stage of life –– The SABI is changing the narrative around our hormones from one of taboo, embarrassment, and loneliness to awareness and even, pride. As more than a wellness brand, The SABI offers a carefully-crafted line of products to carry you through your hormonal journey, including rituals, supportive tools, and ancient herbal remedies that have been tested time and time again by women and now come backed by medicine. The SABI is a blend of science and nature conceived by women who have experienced the joys and deep struggles of bringing a child into the world, the pains of a heavy, difficult period, miscarriage, and difficulty conceiving.

We invite you to get to know your body and its cycles better –– to really understand what is going on inside. Learn to use your hormones to your advantage no matter your stage of life, and know that you can support and balance your hormone levels. We are here to help with the information, understanding and natural tools to support your body and the emotional process along with it.

DISCLAIMER

The SABI blog and articles are not meant to instruct or advise on medical or health conditions, but to inform. The information and opinions presented here do not substitute professional medical advice or consultations with healthcare professionals for your unique situation.

References

1. Stuebe AM. The risks and benefits of infant feeding practices for women and their children. J Perinatol. 2009.
   
   

2. Ben-Jonathan N, Hugo ER, Brandebourg TD, LaPensee CR. Prolactin in human breast cancer: regulation and clinical implications. Endocr Rev. 2008.
   
   

3. Saban KL, et al. Oxytocin and health: a review of biological mechanisms and potential clinical applications. Front Neuroendocrinol. 2020.
   
   

4. Schedin P. Pregnancy-associated breast cancer and postpartum involution. J Mammary Gland Biol Neoplasia. 2006.
   
   

5. Lambe M, et al. Transient increase in the risk of breast cancer after childbirth. N Engl J Med. 1994.
   
   

6. Collaborative Group on Hormonal Factors in Breast Cancer. Breast cancer and breastfeeding: collaborative reanalysis of individual data. Lancet. 2002.