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Absolute Risks of Obstetric Outcomes Risks by Maternal Age at First Birth: A Population-based Cohort

Schummers, Lauraa; Hutcheon, Jennifer, A.b; Hacker, Michele, R.a,c; VanderWeele, Tyler, J.a,d; Williams, Paige, L.a,d; McElrath, Thomas, F.e; Hernandez-Diaz, Soniaa

doi: 10.1097/EDE.0000000000000818
Reproductive and Perinatal

Background: First deliveries in women older than 35, 40, or 45 years are at increased risk for adverse pregnancy outcomes compared with those in younger women. However, specific relationships between each additional year of maternal age and pregnancy risks remain unclear, and absolute risks at each maternal age are not known.

Methods: Using a population-based cohort of nulliparous women in British Columbia, Canada, from 2004 to 2014 (n = 203,414), We examined relationships between maternal age (modeled flexibly to allow curvilinear shapes) and pregnancy outcomes using logistic regression. We plotted absolute predicted risks to display curves from age 20 to 50 estimated for two risk profiles: (1) population average values of all risk factors; (2) a low-risk profile without preexisting diabetes/hypertension, smoking, prior spontaneous/therapeutic abortion, diagnosed infertility, inadequate prenatal care, low income, rural residence, or obesity.

Results: Risks of hypertensive disorders increased gradually until age 35, then accelerated. Risk of multiple gestations, major congenital anomalies, and maternal mortality or severe morbidity increased slowly until age 30, then accelerated. Cesarean delivery and gestational diabetes risks increased linearly with age. While indicated preterm delivery increased rapidly with maternal age, spontaneous preterm delivery did not. Stillbirth, neonatal mortality, and infant mortality had j-shaped relationships with maternal age, with nadirs near 30. Despite age-related increases, risks of severe outcomes remained low for women 35 and 40: < 1–2% for severe maternal morbidity and 5–7% for fetal–infant composite.

Conclusions: This study provides risks for specific maternal ages to inform clinical counseling and public health messaging regarding the potential implications of delayed childbearing.

From the aDepartment of Epidemiology, Harvard T.H. Chan School of Public Health, Boston, MA; bDepartment of Obstetrics and Gynaecology, University of British Columbia, Vancouver, BC, Canada; cDepartment of Obstetrics and Gynecology, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA; dDepartment of Biostatistics, Harvard T.H. Chan School of Public Health, Boston, MA; and eMaternal-Fetal Medicine Division, Department of Obstetrics, Gynecology, and Reproductive Biology, Brigham and Women’s Hospital, Harvard Medical School, Boston, MA.

Submitted July 25, 2017; accepted March 4, 2018.

Description of process by which someone else could obtain data and computing code: The data used for this study are administered by Population Data BC. Population Data BC provides access to these data for research purposes, but does not allow the data to be shared publicly in order to maintain confidentiality and privacy of individual health information. Researchers who wish to access these data may submit a data request directly to Population Data BC.

Laura Schummers was supported by National Research Service Award 1F31HD086970-01A1 from the Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, a Training Grant in Pharmacoepidemiology from the Harvard T. H. Chan School of Public Health and received a CIHR-PHAC Family Planning Public Health Chair Seed Grant to support this project. Dr. Hutcheon holds New Investigator awards from the Canadian Institutes of Health Research and the Michael Smith Foundation for Health Research.

Disclosure: The authors report no conflicts of interest.

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Correspondence: Laura Schummers, Department of Epidemiology, Harvard T.H. Chan School of Public Health, 677 Huntington Avenue, Boston, MA 02115. E-mail:

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