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BACKGROUND: Longer interpregnancy intervals (IPI) have clear benefits to the child as well as the mother, yet it is unclear if the average IPI in Wisconsin has improved over time. Further, teen childbearers are at greater risk than adult childbearers for rapid repeat births, but it is not known if the average IPI of teen childbearers has improved over time and how has it changed relative to that of adult childbearers.
METHODS: Using linked birth records that spanned 1990-2010 that identified all first-births and any subsequent births to the same mothers over time, the authors selected all second births during 1993 through 2010 to examine average IPIs for each year and by whether the mother had been a teenager (age<18) at the time of her first birth. A series of progressively more advanced analytics were used to examine trends over time and statistical differences between groups. These included graphing and tabulating means and medians, calculating a chi-square test for a linear trend, and OLS regression.
RESULTS: Overall, IPIs for all Wisconsin childbearers significantly increased from 15.6 months in 1993 to 32.3 months in 2010 (t = 5.84, p < 0.0001). IPIs for adult childbearers increased from 16.4 months in 1993 to 26.8 months in 2010 (t = 3.92, p < 0.001), while IPIs for teen childbearers increased further from 14.7 months in 1993 to 37.8 months in 2010 (t = 6.92, p < 0.0001). It was also found that teen childbearers had a significantly greater increase in IPIs than adult childbearers over the time period examined (t = 5.51, p < 0.0001).
CONCLUSIONS: The average IPI in Wisconsin has significantly increased from 1993 to 2010, and the increase in IPIs was greater for teen childbearers versus adult childbearers. IPIs shorter than 18 months are associated with increased risk for adverse perinatal outcomes (e.g., pre-term birth, low birthweight, and small for gestational age), so this increase above 18 months represents a positive change for Wisconsin childbearers, especially among teen childbearers. Further analyses are needed to control for possible confounding factors, as well as to better model the relationship between average IPI and time. Longer IPIs have clear benefits to the child as well as the mother, and additional efforts should aim to better understand the mechanisms behind the increase in IPIs. Policies and programs can then be implemented that promote healthy IPIs for Wisconsin childbearers.