Purpose <p>While breastfeeding is known to impact unassisted conception, its effect on fertility treatment outcomes remains unclear. This study aimed to evaluate the effect of breastfeeding on in-vitro fertilization (IVF) frozen embryo transfer (FET) outcomes.</p> Methods <p>A retrospective cohort study at a single, urban, Canadian Fertility center, including women who underwent an FET(2015–2023). Breastfeeding women (study group) were compared to non-breastfeeding controls, matched 1:1 for age, treatment protocol, and year. We compared outcomes between the two groups. Primary outcomes were pregnancy rate (PR), clinical pregnancy rate (CPR),and live birth rate (LBR). Secondary outcomes included peak endometrial thickness prior to progesterone supplementation, biochemical PR and miscarriage rate. Multivariate logistic regression controlled for confounders. Two sub-analyses within the breastfeeding group examined treatment outcomes by breastfeeding duration (≤ 12 months; &gt; 12 months) and prolactin levels (normal;elevated).</p> Results <p>Overall, 118 women met inclusion criteria (59 breastfeeding, 59 controls). Breastfeeding women, compared to controls,were characterized by a younger median age at initial oocyte collection (34.8 years(31.9–37.11) vs. 35.85 years(33.2–39.2),p = 0.044); higher median parity (1(1–1) vs. 0(0–1), p &lt; 0.0001);and higher rates of irregular cycles (35.59% vs. 18.64%,p = 0.016);and elevated prolactin levels (16.95% vs. 1.69%,p = 0.003). Utilizing multivariate logistic regression adjusting for breastfeeding status,parity, age at initial oocyte collection, and cycle regularity, only maternal age at oocyte collection was associated with PR (adjusted odds ratio 0.84, 95% confidence interval 0.75–0.94,p = 0.003). Both sub-analyses revealed similar PRs, CPRs and LBRs between groups.</p> Conclusions <p>Breastfeeding during an FET cycle was not associated with treatment outcomes, including PR,CPR, or LBR, providing cautious reassurance to physicians and patients.</p>