Interpregnancy Interval and Low Birthweight: The Long and Short of It

1. Interpregnancy Interval and Low Birthweight: The Long and Short of It K. Simeonsson UNC-Chapel Hill March 9, 2003

2. Purpose • Define the problem of low birthweight (LBW) • Discuss interpregnancy interval as a risk factor for LBW • Describe preliminary analyses of study • Discuss public health implications

3. Prevalence of Low Birthweight, 2000 National Vital Statistics Report, 2002

4. Low Birthweight • Risk factor for perinatal mortality • Multifactorial problem • Short and long interpregnancy intervals (IPIs) as a risk factor

5. Interpregnancy Interval • The interval between delivery and conception • Calculated as the time period between two consecutive live births minus the gestational age of the index birth

6. Interpregnancy Interval and Low Birthweight Outcomes • Theories • Maternal nutritional depletion • Loss of growth-supporting factors • Previous research • Optimal interval between pregnancies • Zhu et al. 1999 & 2001, Fuentes-Afflick & Hessol 2000 • Extreme intervals • Associated with LBW • Zhu et al. 1999 & 2001, Knoshnood et al. 1998, Rawlings et al. 1995 • Associated with other perinatal outcomes • Shults et al. 1999, Klerman et al. 1998, Rawlings et al. 1995

7. Research Question • Does the length of time between pregnancies have an adverse effect on birthweight? • Hypothesis • Extreme IPIs will increase the odds of low birthweight outcomes

8. Methods • Source of data • North Carolina birth certificate data1 from 2000 • Total sample 122,550 • Exclusion criteria • Births to primagravid women (N=41793) • Births of multiple gestation (N=3219) • Births with missing data for date of last birth and gestation (N=10155) • Interpregnancy intervals < 0 (N=39) • Sample size 67,344 births for analysis 1 State Center for Health Statistics and the Odum Institute

9. Methods • Dependent variables • LBW (<2500 grams) • VLBW (<1500 grams) • Independent variables • Interpregnancy interval • Six groups of intervals, reported in months • Race • Maternal age • Maternal education • Tobacco use • Statistical analysis (SPSS)

10. Birthweight Outcomes

11. Interpregnancy Interval Groups

12. Descriptive Statistics • Mean birthweight 3367 grams • Median IPI 29.4 months • Mean maternal age 28.0 years • Mean maternal education 12.65 years • Race • White 70% • Black 26% • Other 4%

13. Interpregnancy Intervals and Low Birthweight

14. Odds of Low Birthweight by Interval **p< 0.001

15. Odds of Very Low Birthweight by Interval *P< 0.05 **P< 0.001

16. Odds of Low Birthweight by Interval Interpregnancy Interval (months)

17. Adjusted1 Odds of Low Birthweight by Interval 1 Adjusted for race, maternal age, education and smoking * p<0.05 ** p<0.001

18. Adjusted Odds of Very Low Birthweight by Interval *p< 0.05 **p< 0.001

19. Adjusted Odds for Low Birthweight by Interval Interpregnancy Interval (months)

20. Conclusions • Extreme interpregnancy intervals are associated with low birthweight outcomes • Long interpregnancy intervals have the highest odds of low and very low birthweight outcomes • An interval of 18-23 months may be the optimal time between pregnancies

21. Limitations • Interpregnancy interval calculated from clinical estimate of gestation • Other potential confounders • Reproductive history of the mother • Adequacy of prenatal care • Births excluded for missing data

22. Implications • Interpregnancy intervals may be an independent risk factor for low birthweight • Further research • Determine independent effect • Analyze data for several years • Counseling of post-partum women • Optimal interval • Plans for contraception • Multidisciplinary approach