Importance: A diagnosis of breast cancer after childbirth is reported to increase risk for metastasis and death in patients ≤45 years of age. Data to discern if a postpartum diagnosis is an independent risk factor for poor outcomes remain limited.

Objective: Determine associations between time since last pregnancy and time to metastasis and breast cancer-specific death using cases available from the Utah Population Data Base (UPDB). Secondary objectives are time to metastasis and breast cancer specific-death delineated by tumor stage and ER status. Study Design: This is a population-based study of 2,977 women with stage I, II, or III breast cancer (BC) diagnosed at ≤ 45 years of age from 1996 to 2017. The study population was identified from the UPDB, a statewide dynamic database of linked data received from the Utah SEER Cancer Registry, birth certificates, death certificates, and medical records. Clinical prognostic factors include patient age at diagnosis, year of diagnosis, tumor size, stage, and estrogen receptor (ER) status. The primary exposures are no prior childbirth or time between the most recent childbirth and a breast cancer diagnosis, and primary outcomes are distant metastasis-free survival and breast cancer specific death.

Results: Among the 2,977 predominantly white women from the state of Utah, USA, the reproductive histories of the breast cancer patients were grouped as nulliparous (n = 867), and parous: diagnosed 0-5 years (n = 614), >5-10 years (n = 615) and >10 years (n = 881) since last childbirth. We find a breast cancer diagnosis within 5 years of childbirth is associated with elevated risk for metastasis after controlling for diagnosis year, diagnosis age, tumor stage, and ER status [HR (95% CI) =1.64 (1.16, 2.32)], particularly in Stage I/II disease [HR (95% CI) =1.89 (1.24, 2.87)]. The 0-5 year group also had elevated risk of breast cancer-specific death [HR (95% CI) =1.57 (1.07, 2.32)]. While ER-negative tumors were enriched in women diagnosed 0-5 years postpartum, this enrichment could be attributed largely to the younger age of this reproductive group. Further, postpartum breast cancer patients diagnosed with ER-positive disease within 5 years of a completed pregnancy had significantly worse distant metastasis-free survival than nulliparous patients with ER-negative disease.

Conclusions: In this population level study from the state of Utah, a diagnosis of breast cancer within 5 years postpartum is a risk factor for metastasis and breast cancer-specific death that is independent of known clinical parameters, including ER status. The increased risk for poor outcomes is highest in stage I/II diagnoses. Relevance: For women with breast cancers classically considered as having good prognostic tumor features, i.e. stage I or II and ER-positive, a postpartum diagnosis is a dominant demographic feature of increased risk for metastasis and death.

Citation Format: Pepper Schedin, Sonali Jindal, Solange Bassale, Zhenzhen Zhang, Alison Fraser, Emily Guinto, Virginia F. Borges, Motomi Mori, Ken R. Smith. A Utah Population Data Base Study of young women's breast cancer outcomes by parity status [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 790.