Pregnancy and Breast Cancer: Pathways to Understand Risk and Prevention
Authors: Priscila F. Slepicka, Samantha L. Cyrill, Camila O. dos Santos
Journal: Trends in Molecular Medicine (2019)

The breast is a highly plastic organ which undergoes multiple, complex developmental changes throughout a woman's life – changes that are capable of permanently altering the mammary gland, either promoting or preventing oncogenesis.Breast oncogenesis mimics several mechanisms that are commonly activated during pregnancy, including augmented cell proliferation, decreased cell apoptosis, altered gene expression, and extracellular matrix modifications. By contrast, epidemiological studies have provided evidence of the preventive benefits of an early age of pregnancy.Understanding the molecular mechanisms underlying this dual effect will open new avenues for breast cancer prevention strategies. Several studies have made strong efforts to understand how age and parity modulate the risk of breast cancer. A holistic understanding of the dynamic regulation of the morphological, cellular, and molecular milieu of the mammary gland offers insights into the drivers of breast cancer development as well as into potential prophylactic interventions, the latter being a longstanding ambition of the research and clinical community aspiring to eradicate the disease. In this review we discuss mechanisms that react to pregnancy signals, and we delineate the nuances of pregnancy-associated dynamism that contribute towards either breast cancer development or prevention. Further definition of the molecular basis of parity and breast cancer risk may allow the elaboration of tools to predict and survey those who are at risk of breast cancer development. Several studies have made strong efforts to understand how age and parity modulate the risk of breast cancer. A holistic understanding of the dynamic regulation of the morphological, cellular, and molecular milieu of the mammary gland offers insights into the drivers of breast cancer development as well as into potential prophylactic interventions, the latter being a longstanding ambition of the research and clinical community aspiring to eradicate the disease. In this review we discuss mechanisms that react to pregnancy signals, and we delineate the nuances of pregnancy-associated dynamism that contribute towards either breast cancer development or prevention. Further definition of the molecular basis of parity and breast cancer risk may allow the elaboration of tools to predict and survey those who are at risk of breast cancer development. Breast cancer is the most frequently diagnosed malignance in women. It strikes >1.6 million women worldwide, and about one in eight 8 women in the USA will develop breast cancer in their lifetime (Box 1 and Table 1). Most breast cancers arise because of dysfunction of cells in mammary ducts (50–70% of tumors) or lobules (10–15% of breast cancers), which categorizes these tumors as carcinomas, specifically adenocarcinomas. Some breast tumors are sarcomas, originating in the stroma or muscle. Other types and subtypes of breast cancer are less frequent, and a single diagnosis of breast cancer may refer to a combination of different tumors (www.breastcancer.org) (Box 1).Box 1Classification of Breast TumorsHistological analysis of breast biopsies combined with blood tests and mammograms allows breast tumors to be classified according to their invasiveness, size, morphology, growth, and whether they have reached the lymph nodes (LNs). Invasive or non-invasive tumors depend on the ability/inability of cancer cells to spread into structures near the mammary ducts. Another system classifies breast cancer into stages 0 to IV, and this allows tumor prognosis and potential therapy to be addressed according to a variety of parameters, including tumor size and the presence of cancer cells in the LNs. TNM is another ranking system based on tumor size (T), LN involvement (N), and metastasis (M) (www.breastcancer.org). Ductal carcinoma in situ (DCIS) and lobular carcinoma in situ (LCIS) are non-invasive (stage 0) premalignant lesions, but are liable to become invasive if not treated [111Malhotra G.K. et al.Histological, molecular and functional subtypes of breast cancers.Cancer Biol. Ther. 2010; 10: 955-960Crossref PubMed Scopus (109) Google Scholar]. The most commonly diagnosed breast cancer is invasive ductal carcinoma (IDC), in which cancer cells leak out through the basement membrane of the mammary ducts and infiltrate the adipose tissue of the gland (stages IA and IB). When the cancer cells break through a lymphatic or blood vessel, they are termed metastatic because these cells commonly invade the axillary (stages I–III) and distal LNs, bones, lungs, liver, or brain (stage IV). Although histological analysis has been a valuable tool, it does not provide a precise stratification of patients and possible treatments. To meet this demand, a molecular classification system was generated using hierarchical clustering of microarray-based gene expression combined with immunohistochemical analysis of normal and tumoral breast tissue [112Sørlie T. et al.Gene expression patterns of breast carcinomas distinguish tumor subclasses with clinical implications.Proc. Natl. Acad. Sci. U. S. A. 2001; 98: 10869-10874Crossref PubMed Scopus (7109) Google Scholar, 113Perou C.M. et al.Molecular portraits of human breast tumours.Nature. 2000; 406: 747-752Crossref PubMed Scopus (9402) Google Scholar]. In combination with other molecules, the molecular subtypes of breast cancer have been defined as luminal A (ER+, PR+, HER2−, Ki67−), luminal B (ER+, PR+, HER2− or HER2+, Ki67+), normal-like (ER+, PR+, HER2−, Ki67−), HER2-enriched (ER−, PR−, HER2+), and basal-like (ER−, PR−, HER2−, K5/K6/K14+). This classification allows ER+ tumor subtypes to be further stratified into luminal A and luminal B subtypes which have distinct clinical outcomes and impact differently on patient survival. Although the immunohistochemistry of the normal-like subtype resembles that of luminal A tumors, it accounts for ∼8% of all breast cancer cases in the LN-negative group and shares a similar tissue profile with normal breast [113Perou C.M. et al.Molecular portraits of human breast tumours.Nature. 2000; 406: 747-752Crossref PubMed Scopus (9402) Google Scholar, 114Smid M. et al.Subtypes of breast cancer show preferential site of relapse.Cancer Res. 2008; 68: 3108-3114Crossref PubMed Scopus (430) Google Scholar]. Moreover, basal-like cancers are also called triple-negative (ER−, PR−, HER2−) and have an aggressive clinical outcome [115Ho-Yen C. et al.Characterization of basal-like breast cancer: an update.Diagnostic Histopathol. 2012; 18: 104-111Abstract Full Text Full Text PDF Scopus (0) Google Scholar, 116Rakha E.A. et al.Morphological and immunophenotypic analysis of breast carcinomas with basal and myoepithelial differentiation.J. Pathol. 2006; 208: 495-506Crossref PubMed Scopus (0) Google Scholar].Table 1Factors Associated with Risk of Breast Cancer DevelopmentRisk factoraRisk factors currently under study are not shown in this table.IncidenceRefsAnthropometric and lifestyle factors20–40% lower risk of developing overall breast cancer in obese premenopausal women, but increased risk of developing TNBC and ER− tumors.Higher risk of developing breast cancer in obese postmenopausal women.70% likelihood of developing ER+ breast cancer in obese postmenopausal women.>15% risk of developing breast cancer in women who gained 20 pounds or more after age 18 years.10–20% decrease in the risk of developing breast cancer in women who exercise regularly.20% higher risk for women who consumed 2–3 alcoholic drinks per day compared to non-drinkers.10% increased risk in women who drank 6–7 alcoholic drinks per week between her first period and the first pregnancy.91Rosner B. et al.Short-term weight gain and breast cancer risk by hormone receptor classification among pre- and postmenopausal women.Breast Cancer Res. Treat. 2015; 150: 643-653Crossref PubMed Scopus (15) Google Scholar, 92Reeves G.K. et al.Cancer incidence and mortality in relation to body mass index in the Million Women Study: cohort study.BMJ. 2007; 335: 1134Crossref PubMed Scopus (824) Google Scholar, 93Pizot C. et al.Physical activity, hormone replacement therapy and breast cancer risk: a meta-analysis of prospective studies.Eur. J. Cancer. 2016; 52: 138-154Abstract Full Text Full Text PDF PubMed Google Scholar, 94Hardefeldt P.J. et al.Physical activity and weight loss reduce the risk of breast cancer: a meta-analysis of 139 prospective and retrospective studies.Clin. Breast Cancer. 2018; 18: e601-e612Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar, 95Liu Y. et al.Links between alcohol consumption and breast cancer: a look at the evidence.Womens Health. 2015; 11: 65-77Crossref PubMed Scopus (0) Google ScholarAgebThe association between age and pregnancy in the risk of developing breast cancer is discussed in the text. and Race5% of breast tumors are seen in women <40 years of age.In women aged <40 years, non-Hispanic black women have a higher risk of developing breast cancer.80% of breast tumors are diagnosed in women aged >50 years.In women >50 years of age, non-Hispanic white women have a higher risk of developing breast cancer.Highest incidence in women >70 years of age.Black women are diagnosed younger than white women.13% and 11% lifetime risks of developing breast cancer in black and white women, respectively.8–10% lifetime risk of developing breast cancer in Hispanic and American Indian/Alaskan American.TNBC is more common in black/African Americans.96Stark A. et al.African ancestry and higher prevalence of triple-negative breast cancer: findings from an international study.Cancer. 2010; 116: 4926-4932Crossref PubMed Scopus (113) Google Scholar, 97Bandera E.V. et al.Racial and ethnic disparities in the impact of obesity on breast cancer risk and survival: a global perspective.Adv. Nutr. 2015; 6: 803-819Crossref PubMed Scopus (35) Google Scholar, 98Warner E.T. et al.Estrogen receptor positive tumors: do reproductive factors explain differences in incidence between black and white women?.Cancer Causes Control. 2013; 24: 731-739Crossref PubMed Scopus (10) Google ScholarRadiation exposureUp to sevenfold increased risk of breast cancer in women treated with radiation therapy to the chest area for Hodgkin lymphoma at a young age.Low increase of risk in women who underwent mammography.99Ibrahim E.M. et al.Risk of second breast cancer in female Hodgkin's lymphoma survivors: a meta-analysis.BMC Cancer. 2012; 12: 197Crossref PubMed Scopus (37) Google Scholar, 100Inskip P.D. et al.Radiation-related new primary solid cancers in the childhood cancer survivor study: comparative radiation dose response and modification of treatment effects.Int. J. Radiat. Oncol. Biol. Phys. 2016; 94: 800-807Abstract Full Text Full Text PDF PubMed Scopus (44) Google Scholar, 101Yaffe M.J. Mainprize J.G. Risk of radiation-induced breast cancer from mammographic screening.Radiology. 2011; 258: 98-105Crossref PubMed Scopus (148) Google ScholarHormone replacement therapyIncreased breast cancer risk in the first 5 years of hormone replacement therapy.Higher incidence of breast cancer in women who use the combination of estrogen and progestin compared to women who used estrogen-only therapy.102Collaborative Group on Hormonal Factors in Breast CancerBreast cancer and hormone replacement therapy: collaborative reanalysis of data from 51 epidemiological studies of 52,705 women with breast cancer and 108,411 women without breast cancer.Lancet. 1997; 350: 1047-1059Abstract Full Text Full Text PDF PubMed Scopus (2301) Google Scholar, 103Bakken K. et al.Menopausal hormone therapy and breast cancer risk: impact of different treatments. The European Prospective Investigation into Cancer and Nutrition.Int. J. Cancer. 2011; 128: 144-156Crossref PubMed Scopus (0) Google Scholar, 104Chlebowski R.T. et al.Influence of estrogen plus progestin on breast cancer and mammography in healthy postmenopausal women: the Women's Health Initiative Randomized Trial.JAMA. 2003; 289: 3243-3253Crossref PubMed Scopus (1553) Google ScholarBirth control pills20–30% increase in risk of developing breast cancer in women taking birth control pills.105Collaborative Group on Hormonal Factors in Breast CancerBreast cancer and hormonal contraceptives: collaborative reanalysis of individual data on 53 297 women with breast cancer and 100 239 women without breast cancer from 54 epidemiological studies.Lancet. 1996; 347: 1713-1727Crossref PubMed Google Scholar, 106Nachtigall L. et al.Contemporary hormonal contraception and the risk of breast cancer.N. Engl. J. Med. 2018; 378: 1265PubMed Google ScholarHereditaryTwofold increased risk of breast cancer in women with a first-degree female relative with a diagnosis of breast cancer.3–4-fold higher risk if she has more than one first-degree relative with a breast cancer diagnosis.If the first-degree relative was diagnosed before age 40 years, the risk of developing breast cancer increases by a factor of two.Increase risk of breast cancer in women with one or more first-degree relatives diagnosed with prostate cancer.107Collaborative Group on Hormonal Factors in Breast CancerFamilial breast cancer: collaborative reanalysis of individual data from 52 epidemiological studies including 58,209 women with breast cancer and 101,986 women without the disease.Lancet. 2001; 358: 1389-1399Abstract Full Text Full Text PDF PubMed Scopus (685) Google Scholar, 108Pharoah P.D. et al.Family history and the risk of breast cancer: a systematic review and meta-analysis.Int. J. Cancer. 1997; 71: 800-809Crossref PubMed Scopus (426) Google Scholar, 109Kharazmi E. et al.Effect of multiplicity, laterality, and age at onset of breast cancer on familial risk of breast cancer: a nationwide prospective cohort study.Breast Cancer Res. Treat. 2014; 144: 185-192Crossref PubMed Scopus (12) Google Scholar, 110Beebe-Dimmer J.L. et al.Familial clustering of breast and prostate cancer and risk of postmenopausal breast cancer in the Women's Health Initiative Study.Cancer. 2015; 121: 1265-1272Crossref PubMed Scopus (7) Google Scholara Risk factors currently under study are not shown in this table.b The association between age and pregnancy in the risk of developing breast cancer is discussed in the text. Open table in a new tab Histological analysis of breast biopsies combined with blood tests and mammograms allows breast tumors to be classified according to their invasiveness, size, morphology, growth, and whether they have reached the lymph nodes (LNs). Invasive or non-invasive tumors depend on the ability/inability of cancer cells to spread into structures near the mammary ducts. Another system classifies breast cancer into stages 0 to IV, and this allows tumor prognosis and potential therapy to be addressed according to a variety of parameters, including tumor size and the presence of cancer cells in the LNs. TNM is another ranking system based on tumor size (T), LN involvement (N), and metastasis (M) (www.breastcancer.org). Ductal carcinoma in situ (DCIS) and lobular carcinoma in situ (LCIS) are non-invasive (stage 0) premalignant lesions, but are liable to become invasive if not treated [111Malhotra G.K. et al.Histological, molecular and functional subtypes of breast cancers.Cancer Biol. Ther. 2010; 10: 955-960Crossref PubMed Scopus (109) Google Scholar]. The most commonly diagnosed breast cancer is invasive ductal carcinoma (IDC), in which cancer cells leak out through the basement membrane of the mammary ducts and infiltrate the adipose tissue of the gland (stages IA and IB). When the cancer cells break through a lymphatic or blood vessel, they are termed metastatic because these cells commonly invade the axillary (stages I–III) and distal LNs, bones, lungs, liver, or brain (stage IV). Although histological analysis has been a valuable tool, it does not provide a precise stratification of patients and possible treatments. To meet this demand, a molecular classification system was generated using hierarchical clustering of microarray-based gene expression combined with immunohistochemical analysis of normal and tumoral breast tissue [112Sørlie T. et al.Gene expression patterns of breast carcinomas distinguish tumor subclasses with clinical implications.Proc. Natl. Acad. Sci. U. S. A. 2001; 98: 10869-10874Crossref PubMed Scopus (7109) Google Scholar, 113Perou C.M. et al.Molecular portraits of human breast tumours.Nature. 2000; 406: 747-752Crossref PubMed Scopus (9402) Google Scholar]. In combination with other molecules, the molecular subtypes of breast cancer have been defined as luminal A (ER+, PR+, HER2−, Ki67−), luminal B (ER+, PR+, HER2− or HER2+, Ki67+), normal-like (ER+, PR+, HER2−, Ki67−), HER2-enriched (ER−, PR−, HER2+), and basal-like (ER−, PR−, HER2−, K5/K6/K14+). This classification allows ER+ tumor subtypes to be further stratified into luminal A and luminal B subtypes which have distinct clinical outcomes and impact differently on patient survival. Although the immunohistochemistry of the normal-like subtype resembles that of luminal A tumors, it accounts for ∼8% of all breast cancer cases in the LN-negative group and shares a similar tissue profile with normal breast [113Perou C.M. et al.Molecular portraits of human breast tumours.Nature. 2000; 406: 747-752Crossref PubMed Scopus (9402) Google Scholar, 114Smid M. et al.Subtypes of breast cancer show preferential site of relapse.Cancer Res. 2008; 68: 3108-3114Crossref PubMed Scopus (430) Google Scholar]. Moreover, basal-like cancers are also called triple-negative (ER−, PR−, HER2−) and have an aggressive clinical outcome [115Ho-Yen C. et al.Characterization of basal-like breast cancer: an update.Diagnostic Histopathol. 2012; 18: 104-111Abstract Full Text Full Text PDF Scopus (0) Google Scholar, 116Rakha E.A. et al.Morphological and immunophenotypic analysis of breast carcinomas with basal and myoepithelial differentiation.J. Pathol. 2006; 208: 495-506Crossref PubMed Scopus (0) Google Scholar]. Parity is known to have a dual effect on breast cancer risk. In many ways, breast tumorigenesis mimics several mechanisms that are commonly activated during pregnancy, including augmented cell proliferation, alterations in cell shedding, reduced cell apoptosis, altered gene expression, and extracellular matrix (ECM) modifications. On the other hand, epidemiological studies have provided evidence of the cancer-preventive benefits of pregnancy wherein an early age of pregnancy decreases the risk of breast cancer development. Although understanding the molecular mechanisms underlying these phenomena is still in its infancy, their elucidation will open new avenues to target breast cancer. The mammary gland is a complex and highly adaptive organ whose main function, in female mammals, is to produce milk during lactation for the sustenance of young offspring. The gland is composed of a variety of cell types, including fibroblasts, adipocytes, epithelial, endothelial, and immune cells. The epithelial cells can be further subdivided into multiple cell types that, together, form the branching structure of the gland and constitute the secretory alveoli during lactation. Two main epithelial cell compartments can be distinguished in the mammary gland: the luminal compartment [the inner cell layer, localized between the lumen (see Glossary),and the basal compartment] and the basal compartment (the outer layer surrounding the luminal cells and adjacent to the basal membrane) (Figure 1). The luminal compartment consists of alveolar, ductal, and progenitor cells. The alveolar cells line the lumen and are largely responsible for milk production. The ductal cells form the milk ducts that carry expelled milk to the nipples for the suckling offspring. The basal compartment also consists of three different cell types, including mammary stem cells (MaSCs), myoepithelial progenitor cells, and myoepithelial differentiated cells. In addition to myoepithelial cell contractions that expel milk from the luminal alveolar cells, these cells also act in the deposition of the basement membrane that separates the mammary epithelium from the stroma. Interestingly, myoepithelial cells have recently been reported to have a tumor-suppressor function, for instance via the expression of cathepsin inhibitors that have been shown to prevent metastatic invasion and angiogenesis [1Duivenvoorden H.M. et al.Myoepithelial cell-specific expression of stefin A as a suppressor of early breast cancer invasion.J. Pathol. 2017; 243: 496-509Crossref PubMed Scopus (12) Google Scholar, 2Sternlicht M.D. et al.The human myoepithelial cell is a natural tumor suppressor.Clin. Cancer Res. 1997; 3: 1949-1958PubMed Google Scholar] in cancer. The cellular complexity of the mammary gland supports the unique and highly plastic sequence of developmental stages that occur during the mammalian lifecycle. Between birth and puberty, the mammary epithelium remains quiescent and its growth is equivalent to body growth (allometric growth). During puberty, and in response to female sexual hormones, the rudimentary mammary glands expand and develop into potentially functional tissues. High-resolution single-cell transcriptomics revealed that the transcriptional profile of mammary epithelial cells (MECs) is vastly impacted at the onset of puberty. This major event modifies lineage-commitment gene signatures, creating diverse and heterogeneous cell lineages that accommodate the extensive tissue remodeling [3Pal B. et al.Construction of developmental lineage relationships in the mouse mammary gland by single-cell RNA profiling.Nat. Commun. 2017; 8: 1627Crossref PubMed Scopus (20) Google Scholar]. Structurally, highly proliferative terminal end buds (TEBs), composed of cap and body cells, surface to the tip of the ducts to promote ductal invagination into the fat pad, extension, and branching (Figure 1, upper right). MECs fulfill their main role – milk production – only in response to the signals that accompany conception. During the first days of pregnancy, adipocytes localized in the mammary fat pad undergo apoptosis to create space for branching morphogenesis (Figure 1, lower right). Pubescent MECs with multipotent progenitor features (a transcriptional profile similar to that of MaSCs, but with impaired self-renewal ability) are highly responsive to pregnancy and thus promote the differentiation and expansion of the mammary ductal system [4Kaanta A.S. et al.Evidence for a multipotent mammary progenitor with pregnancy-specific activity.Breast Cancer Res. 2013; 15: R65Crossref PubMed Scopus (12) Google Scholar]. In response to prolactin signaling, lobuloalveolar structures develop into branching secondary and tertiary ducts, and differentiate into milk production apparatuses, making the gland very dense. Copious amounts of milk are then produced in these luminal alveolar cells towards late pregnancy, and milk is then released from the mammary cells in response to increased levels of the peptide hormone, oxytocin (Oxt). Mechanistically, Oxt interacts with G protein-coupled receptors in uterine smooth muscle cells and mammary myoepithelial cells that regulate calcium metabolism and cause cell contraction [5Stewart T.A. et al.Mammary mechanobiology: PIEZO1 mechanically-activated ion channels in lactation and involution.bioRxiv. 2019; (Published online May 26, 2019. https://doi.org/10.1101/649038)Google Scholar]. The combination of the offspring suckling stimulus and Oxt release induces myoepithelial cells to contract. This constricts the luminal compartment, secreting milk into the alveolar lumen, from where it travels to the ducts and finally to the nipples. Lack of Oxt limits myoepithelial contraction, significantly impairing milk release [6Young III, W.S. et al.Deficiency in mouse oxytocin prevents milk ejection, but not fertility or parturition.J. Neuroendocrinol. 1996; 8: 847-853Crossref PubMed Scopus (0) Google Scholar]. At the end of lactation, the gland undergoes a remodeling process, called involution (Figure 1, lower left), which restores tissue architecture to one that resembles a pre-pregnancy state. The onset of involution occurs mainly due to milk stasis and a lack of suckling stimuli. In rodents, the first 2 days of involution are reversible, but the process is irreversible during the following 8 days [7Sharp J.A. et al.The fur seal – a model lactation phenotype to explore molecular factors involved in the initiation of apoptosis at involution.J. Mammary Gland Biol. Neoplasia. 2007; 12: 47-58Crossref PubMed Scopus (4) Google Scholar]; by contrast, involution in humans can last for 24 months or more [8Jindal S. et al.Postpartum breast involution reveals regression of secretory lobules mediated by tissue-remodeling.Breast Cancer Res. 2014; 16: R31Crossref PubMed Scopus (0) Google Scholar]. During involution, the mammary gland endures a series of orchestrated events that include (i) programmed cell death of the majority of alveolar cells, (ii) disruption of epithelial tight junctions, (iii) adipose, ECM, and vascular remodeling, and (iv) immune infiltration and response. Although immune cells monitor the mammary tissue at all developmental stages, the immune system, together with non-professional phagocytes (modified MECs), plays a pivotal role during involution to clear residual milk and accumulated cell debris that, if not removed, may lead to mastitis, inflammation, and breast cancer [9Atabai K. et al.Roles of the innate immune system in mammary gland remodeling during involution.J. Mammary Gland Biol. Neoplasia. 2007; 12: 37-45Crossref PubMed Scopus (51) Google Scholar]. Macrophages are particularly important for mammary gland involution, given that their absence impairs processes associated with MEC death and adipocyte repopulation [10O'Brien J. et al.Macrophages are crucial for epithelial cell death and adipocyte repopulation during mammary gland involution.Development. 2012; 139: 269-275Crossref PubMed Scopus (77) Google Scholar]. At the end of involution, the gland is once again ready to engage cellular and molecular programs to activate milk production in subsequent pregnancies. Several cellular mechanisms, including responsiveness to circulating hormones, stromal composition, and cell specification, contribute to the effects of age and parity on the risk of breast cancer, all of which are further explored in the following. The pregnancy cycle directly affects the metabolism, gene expression profiles, and proliferation dynamics of MECs in response to hormones. These alterations are so profound that they significantly impact on the risk that a woman develops breast cancer. In women, an early pregnancy (before the age of 20 years) reduces the likelihood of developing breast cancer by 50% compared to nulliparous women [11MacMahon B. et al.Age at first birth and breast cancer risk.Bull. World Health Organ. 1970; 43: 209-221PubMed Google Scholar, 12Albrektsen G. et al.Breast cancer risk by age at birth, time since birth and time intervals between births: exploring interaction effects.Br. J. Cancer. 2005; 92: 167-175Crossref PubMed Scopus (129) Google Scholar]. Subsequent pregnancies [13Husby A. et al.Pregnancy duration and breast cancer risk.Nat. Commun. 2018; 9: 4255Crossref PubMed Scopus (2) Google Scholar] extend the protection against breast cancer by ∼10% [14Merrill R.M. et al.Cancer risk associated with early and late maternal age at first birth.Gynecol. Oncol. 2005; 96: 583-593Abstract Full Text Full Text PDF PubMed Scopus (38) Google Scholar, 15Bernier M.O. et al.Breastfeeding and risk of breast cancer: a metaanalysis of published studies.Hum. Reprod. Update. 2000; 6: 374-386Crossref PubMed Google Scholar, 16Palmer J.R. et al.Parity, lactation, and breast cancer subtypes in African American women: results from the AMBER Consortium.J. Natl. Cancer Inst. 2014; 106dju237Crossref PubMed Scopus (0) Google Scholar]. The protective effect of pregnancy is not evident in women who have their first pregnancy between the ages of 30–34 years, whereas the risk of developing breast cancer is augmented for those whose first pregnancy occurs after age 35 [11MacMahon B. et al.Age at first birth and breast cancer risk.Bull. World Health Organ. 1970; 43: 209-221PubMed Google Scholar, 12Albrektsen G. et al.Breast cancer risk by age at birth, time since birth and time intervals between births: exploring interaction effects.Br. J. Cancer. 2005; 92: 167-175Crossref PubMed Scopus (129) Google Scholar]. Although pregnancy protection is associated with maternal age during her first pregnancy, the overall risk of developing breast cancer increases immediately follow parturition, and is independent of race, age, or the number of pregnancies [11MacMahon B. et al.Age at first birth and breast cancer risk.Bull. World Health Organ. 1970; 43: 209-221PubMed Google Scholar, 12Albrektsen G. et al.Breast cancer risk by age at birth, time since birth and time intervals between births: exploring interaction effects.Br. J. Cancer. 2005; 92: 167-175Crossref PubMed Scopus (129) Google Scholar, 17Sun X. et al.Association of parity and time since last birth with breast cancer prognosis by intrinsic subtype.Cancer Epidemiol. Biomark. Prev. 2016; 25: 60-67Crossref PubMed Scopus (7) Google Scholar]. In 2013, Callihan and colleagues showed 2.8-fold and 2.7-fold increases in the risks of metastasis and mortality, respectively, in breast cancer patients diagnosed within the first 5 years post-pregnancy compared to nulliparous cases [18Callihan E.B. et al.Postpartum diagnosis demonstrates a high risk for metastasis and merits an expanded definition of pregnancy-associated breast cancer.Breast Cancer Res. Treat. 2013; 138: 549-559Crossref PubMed Scopus (0) Google Scholar]. In another cohort of parous women diagnosed with breast cancer within the first 5 years postpartum (≤40 years of age), the identified tumor subt