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BACKGROUND: Maternal age and interpregnancy interval (IPI) are associated with birth outcomes such as gestational age and birthweight. We examined the trends and relationships of maternal age and race-ethnicity, IPI, and preterm and low birthweight (LBW) births for an 11-year period, 1999-2009, in Chicago. The Chicago Department of Public Health (CDPH) has selected Adolescent Health and Healthy Mothers and Babies as two of 12 priorities in its public health agenda, Healthy Chicago. CDPH’s Women and Children’s Health Division has identified IPI as one modifiable risk factor of poor perinatal outcome that can be addressed through improved awareness and education of clients.
METHODS: We used records of Chicago births in the Illinois Department of Public Health annual birth certificate datasets for 1999-2009. Birth rates were calculated using annual population estimates based on a linear interpolation of counts from the 2000 and 2010 United States Census. Analyses of IPI excluded records for plural births, mothers with no prior pregnancy, and certificates with missing information on pregnancy interval.
RESULTS: Birth rates increased for all women 35-44 years of age during the 11-year study period, and decreased in all other age groups (except for Non-Hispanic [NH] white women aged 25-34). NH white women aged 35-44 years had the largest increase in birth rates from 1999 to 2009, 22.5 births per 1000 (69%), NH black women had the smallest increase, 2.2 births per 1000 (12%). In this same age group, there was an increase in the number of mothers with IPI of <18 months, except among Hispanics. NH white women aged 35-44 years had the largest increase in births with IPI of <18 months, 4 percentage points (24%). NH Asian and NH black women had similar increases, 2.5 and 2.2 percentage points (19% and 18%), respectively. Overall, IPI of <18 months in the 35-44 years age group did not significantly increase the risk of LBW or preterm birth during 2007-2009.
CONCLUSIONS: Chicago has experienced an increase in the birth rate for 35-44 year old women across the city’s four predominant racial-ethnic groups. In this age group, the prevalence of IPI of <18 months also increased. These concurrent trends could be related to increasing numbers of women in Chicago deferring childbirth to later years than in the past.