Absolute risks of obstetric outcomes by maternal age at first birth: a population-based cohort

Abstract

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.

Conflict of interest statement

Conflicts of Interest: The authors have no conflicts of interest to declare

Figures

Figure 1

Predicted absolute risks (with 95% confidence intervals) of adverse pregnancy and birth outcomes at specified values of maternal age at first birth in British Columbia (Canada), 2004–2014 (n=203,414). Unadjusted and adjusted risks according to maternal age at first birth: i) gestational diabetes mellitus; ii) hypertensive disorders of pregnancy; iii) cesarean delivery; iv) preterm delivery; v) spontaneous preterm delivery; vi) indicated preterm delivery; vii) multiple gestations; viii) major congenital anomaly; ix) small-for-gestational age; x) NICU stay >=2 days; xi) stillbirth; xii) neonatal mortality; xiii) infant mortality; xiv) maternal mortality or near-miss morbidity; xv) severe adverse fetal-infant composite; xvi) health pregnancy and delivery composite. Models for gestational diabetes were restricted to those without pre-pregnancy type 1 or type 2 diabetes mellitus. Models for NICU stay ≥2 days restricted to births after April, 2008. Models for small-for-gestational age restricted to singleton pregnancies and gestational ages from 22 to 43 weeks. Adjusted models included pre-pregnancy diabetes, pre-pregnancy chronic hypertension, pre-pregnancy body mass index, smoking, indicator variables for calendar year, inadequate prenatal care, low income, rural residence, previous spontaneous or therapeutic abortion, diagnosed infertility, infertility treatment. Maternal age at first birth was modeled using restricted cubic splines with 4 knots (at the 5th, 35th, 65th, and 95th percentiles; ages 19.7, 27.1, 31.5, and 38.6) for hypertensive disorders of pregnancy, gestational diabetes, cesarean delivery, spontaneous preterm delivery, and healthy preterm delivery and modeled using restricted cubic splines with 3 knots (at the 10th, 50th, and 90th percentiles; ages 21.4, 29.3, and 36.5) for all other outcomes.