Date of Award

January 2024

Document Type


Degree Name

Medical Doctor (MD)



First Advisor

Lajos Pusztai


Age-adjusted incidence rates of triple negative breast cancer (TNBC) are higher in Black women than in non-Hispanic White (White) women. The disproportionate incidence of TNBC in Black women partially drives the 40% higher mortality rate from breast cancer in Black women compared to White women. Several studies have identified lack of breastfeeding and younger age at first birth as risk factors for TNBC. There are large differences in breastfeeding uptake, breastfeeding duration, and the distribution of age at first birth across racial groups in the US: only 44% of Black women breastfeed for 6 months or longer, compared to 60% of White women, and 77% of Black women are younger than 25 at first birth compared to 51% of White women. To quantify the contribution of differences in breastfeeding pattern and younger age at first birth to disparities in incidence of TNBC, we calculate the population-attributable fraction (PAF), which is the proportion of the incidence of a disease that may be attributable to a particular exposure. A PubMed search was performed to identify relevant case-control studies. Pooled odds ratios for breastfeeding for < 6 months and age at first birth younger than 25 were calculated from data extracted from case-control studies using a common effect model. Population prevalence data were extracted from national surveys. PAF was calculated using the Levin formula (PRF × (RR-1))/(1+PRF ×(RR-1)), where PRF is the population prevalence of a risk factor. Pooled OR was used as an estimate of relative risk (RR). A combined PAF was calculated using the polychoric correlation coefficient between breastfeeding duration and age at first birth to account for contributions of multiple risk factors. Using race-specific ORs, the PAF of breastfeeding for less than 6 months was 12% (95% confidence interval (CI) 5-20%) among White women and 15% (95% CI 3-26%) among Black women. Among women aged 20-44, the PAF of breastfeeding for less than 6 months was 18% (95% CI 8-28%). The PAF of having a child before the age of 25 was not significantly different from zero for White women (2% (CI -6-11%)) but pronounced and statistically significantly different from zero for Black women (21% (CI 5-25%)). Combined PAF was 27% for Black women and 17% for all women in the US. Extrapolating to the US population, we estimated that 4,850 annual cases of TNBC (2,421 among White women and 1,533 among Black women) could be attributed to breastfeeding for < 6 months and age at first birth < 25 years. An estimated 195 annual cases of TNBC diagnosed in women under the age of 44 are attributable to breastfeeding for < 6 months. Given the protective role of breastfeeding against TNBC, policy changes aimed at supporting and more broadly enabling breastfeeding, addressing structural barriers, and promoting a culture shift could reduce TNBC incidence overall and lessen racial disparities in TNBC incidence in the US. For example, increasing awareness of the protective role of breastfeeding, improving workplace policies and access to pumps, lactation rooms, counseling, and support groups, and limiting the lobbying power of formula companies might increase breastfeeding rates across the nation.


This thesis is restricted to Yale network users only. It will be made publicly available on 04/30/2025