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Risk Factors for Short Interpregnancy Interval -- Utah, June 1996-June 1997

The Utah Medicaid program provides pregnancy-related coverage for women whose household incomes are less than or equal to 133% of the federal poverty level. * For women who are not otherwise eligible, Medicaid coverage of family-planning and other services ends after the second calendar month following delivery. To assess whether increased access to family-planning services would benefit Medicaid recipients, the interpregnancy intervals (IPIs) of Utah residents whose most recent pregnancies were covered by Medicaid (Medicaid-recipient mothers) were compared with those of all other Utah resident mothers. This report summarizes the results of that study, which indicate that Medicaid-recipient mothers aged greater than or equal to 20 years were at increased risk for having short IPIs, and may therefore benefit from extended Medicaid coverage or other means of assuring access to family-planning services.

Data for live-born infants of Utah resident mothers from June 30, 1996, through June 29, 1997, were matched to the Medicaid eligibility database by using the mother's date of birth, last name, first name, and middle initial. IPI was defined as the time between delivery dates of consecutive live-born infants minus the gestational age (1) of the most recent child. A short IPI was defined as an IPI of less than 12 months; this cutoff was based on a parallel study that showed that IPIs of less than 12 months were associated with significantly elevated risks for adverse perinatal outcomes (Utah Department of Health, unpublished data, 1998). The risk for having a short IPI was examined in relation to maternal Medicaid status and age, marital status, education, and race/ethnicity. A mother's education was categorized as age-appropriate if she had completed high school or the usual number of grades for her age. The educational status of mothers aged greater than or equal to 20 years also was evaluated based on the number of years of school completed (0-11, 12, 13-15, 16, and greater than 16 years).

Of the 42,429 live-born infants of Utah resident mothers from June 30, 1996, through June 29, 1997, 15,810 (37.3%) were ineligible for the study because they were first-born infants. An additional 4773 (11.2%) infants were excluded because the date of the previous delivery of a live-born infant or information for estimating gestational age were missing on the birth certificates.

Of the 21,846 (51.5%) infants eligible for study, 3916 (17.9%) were born after a short IPI (Table_1). Medicaid-recipient mothers (22.9% of all mothers in the study population) were nearly twice as likely as other mothers to have short IPIs. Young maternal age correlated strongly with short IPIs. Short IPIs were more common among unmarried mothers (22.5%) than married mothers (17.4%), and among mothers with less than age-appropriate education (25.2%) than among those with age-appropriate education (17.1%). American Indian/Alaskan Native mothers and Asian/Pacific Islander mothers were more likely to have short IPIs than white mothers, and Hispanic mothers were more likely to have short IPIs than non-Hispanic mothers. Mothers who had either one or five or more previous live-born infants were more likely to have short IPIs than mothers who had three previous live-born children.

When stratified analyses were performed, the association between Medicaid status and short IPI differed by maternal age. Among mothers aged greater than or equal to 20 years at delivery (21,207 {97.1%} of all mothers studied), Medicaid recipients were more likely to have short IPIs than others (relative risk {RR}=1.7, 95% confidence interval {CI}=1.6-1.8). However, among mothers aged 15-19 years (631 {2.9%} of all mothers), no such association was found (RR=1.0, 95% CI=0.9-1.2).

To evaluate risk factors for having short IPIs while simultaneously controlling for other covariates, logistic regression analyses were performed. Among mothers aged greater than or equal to 20 years (Table_2), Medicaid-recipient mothers were more likely to have short IPIs than other mothers (odds ratio=1.6, 95% CI=1.5-1.8). Mothers from racial/ethnic minority groups had a higher risk for short IPI than white mothers. Married mothers were more likely to have short IPIs than unmarried mothers. The risk for short IPI was inversely correlated with maternal age.

Reported by: J Duncan, B Nangle, Bur of Vital Records; N Streeter, L Bloebaum, Div of Community and Family Health Svcs; DC Tingey, JA Olson, Div of Health Care Financing, Utah Dept of Health. State Br, Div of Applied Public Health Training; Div of Public Health Surveillance and Informatics, Epidemiology Program Office; and an EIS Officer, CDC.

Editorial Note

Editorial Note: Short IPIs have been associated with an increased risk for adverse birth outcomes (2-5). Pregnancies that are too closely spaced often are unintended, which can place substantial financial and psychologic burdens on the mother and her family (6). The findings in this study indicate that Medicaid-recipient mothers aged greater than or equal to 20 years were more likely to have short IPIs than other mothers of the same age. Young mothers and mothers of racial/ethnic minority groups had increased risk for short IPIs. Utah data were unavailable to evaluate whether Medicaid-recipient mothers tended to have a shorter duration of breastfeeding or greater desire to build their families quickly than other mothers -- two factors that may contribute to short IPIs. Utah women whose deliveries were covered by Medicaid, most of whom lose Medicaid coverage shortly after delivery, may have less access to family-planning services. Improving these women's access to family-planning services might help them prevent unintended pregnancies, and improve birth outcomes. Access could be improved by removing financial barriers through extending Medicaid coverage for all women after a Medicaid-covered delivery, or through increasing availability of family-planning services for low-income women. In Utah, 74 publicly funded clinics provide family-planning services to low-income women. However, only 26% of all such women are served; Utah ranks 49th among all states for providing access to family-planning services (7).

The higher risk for having short IPIs among mothers from racial/ethnic minority groups may have been due to cultural or socioeconomic differences or to unequal access to health care. An example of such a factor was observed in one study that found the length of breastfeeding differed by race (8).

Because the sociodemographic characteristics of the Utah population are different from those of other states, caution should be used in generalizing the results of this study. In addition, the findings of this study are subject to at least three methodologic limitations. First, Medicaid status was based on the most recent pregnancy. The number of women in the study population whose Medicaid status had changed since the previous pregnancy was unknown; an implicit assumption has been made that mothers on Medicaid for the most recent pregnancy were on Medicaid for the previous one. Second, this study included women whose Medicaid eligibility was independent of pregnancy and those who were eligible only because they were pregnant and met the program's income requirements. Finally, the retrospective approach of this study may have overestimated the risk for having short IPIs among young women, although such bias is unlikely to account for much of the elevated risk among those women.

In Utah, extending Medicaid coverage of family-planning services and improving use of family-planning programs should be considered to help low-income women prevent unintended pregnancies and improve birth outcomes. Public health programs for preventing short IPIs also should target young mothers regardless of their Medicaid-eligibility status. The analysis described in this report may be repeated to monitor the success of efforts to improve reproductive health of women in Utah.


  1. Ventura SJ, Martin JA, Curtin SC, Mathews TJ. Report of final natality statistics, 1995. Hyattsville, Maryland: US Department of Health and Human Services, CDC, National Center for Health Statistics, 1997. (Monthly vital statistics report, vol 45, no. 11, suppl).

  2. Brody DJ, Bracken MB. Short interpregnancy interval: a risk factor for low birthweight. Am J Perinatol 1987;4:50-4.

  3. Erickson JD, Bjerkedal T. Interpregnancy interval. Association with birthweight, still birth, and neonatal death. J Epidemiol Community Health 1978;32:124-30.

  4. Bakketeig LS, Hoffman HJ, Oakley ART. Perinatal mortality. In: Bracken MB, ed. Perinatal epidemiology. New York, New York: Oxford University Press, 1984.

  5. Institute of Medicine. Preventing low birthweight. Washington, DC: National Academy Press, 1985.

  6. Institute of Medicine. The best intentions: unintended pregnancy and the well-being of children and families. Washington, DC: National Academy Press, 1995.

  7. Alan Guttmacher Institute. Contraception counts: Utah information. New York, New York: The Alan Guttmacher Institute, 1998.

  8. Statistical Analysis Division, Center for Health Statistics. PRAMS surveillance report: Alabama, 1996. Montgomery, Alabama: Alabama Department of Public Health, 1998.

Poverty statistics are based on definitions developed by the Social Security Administration in 1964 (which subsequently were modified by federal interagency committees in 1969 and 1980) and prescribed by the Office of Management and Budget as the standard to be used by federal agencies for statistical purposes.

Note: To print large tables and graphs users may have to change their printer settings to landscape and use a small font size.

TABLE 1. Risk factors for short interpregnancy interval (IPI) (<12 months) -- Utah,
June 30, 1996-June 29, 1997
                                            Short IPI
Maternal risk factor           N*           No.    (%)      Relative risk    (95% CI+)
Delivery covered
 by Medicaid
 Yes                        4,995         1,348   (27.0)         1.8        (1.7- 1.9)
 No                        16,851         2,568   (15.2)      referent
Maternal age
 at delivery (yrs)
 15-17                         92            53   (57.6)         7.8        (5.5-11.2)
 18-19                        547           240   (43.9)         6.0        (4.3- 8.3)
 20-24                      5,024         1,392   (27.7)         3.8        (2.7- 5.2)
 25-29                      7,814         1,330   (17.0)         2.3        (1.7- 3.2)
 30-34                      5,536           639   (11.5)         1.6        (1.1- 2.2)
 35-39                      2,344           226   ( 9.6)         1.3        (0.9- 1.8)
 40-47                        489            36   ( 7.4)      referent
 Less than age-             2,248           567   (25.2)         1.5        (1.4- 1.6)
 Age-appropriate           19,441         3,319   (17.1)      referent
Marital status
 Unmarried                  2,262           510   (22.5)         1.3        (1.2- 1.4)
 Married                   19,583         3,406   (17.4)      referent
 American Indian/
  Alaskan Native              329           105   (31.9)         1.8        (1.6- 2.1)
 Asian/Pacific Islander       562           135   (24.0)         1.4        (1.2- 1.6)
 Black                        113            28   (24.8)         1.4        (1.0- 2.0)
 White                     20,671         3,615   (17.5)      referent
 Hispanic                   1,682           402   (23.9)         1.4        (1.3- 1.5)
 Non-Hispanic              20,105         3,505   (17.4)      referent
No. previous live-born
  1                         9,997         1,930   (19.3)         1.2        (1.1- 1.3)
  2                         5,903           968   (16.4)         1.0        (0.9- 1.1)
  3                         3,160           503   (15.9)      referent
  4                         1,461           240   (16.4)         1.0        (0.9- 1.2)
  >=5                       1,309           267   (20.4)         1.3        (1.1- 1.5)
Overall                    21,846         3,916   (17.9)
* Numbers may not equal overall number because of missing data.
+ Confidence interval.
& A mother's education was considered age-appropriate if she had completed high school or
  the usual number of grades for her age.

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TABLE 2. Logistic regression analysis of risk factors* for short interpregnancy interval
(<12 months) among mothers aged 20-47 years (n=20,982) -- Utah, June 30,
1996-June 29, 1997
                 Variable                         Odds ratio+    (95% CI&)
                 Delivery covered by Medicaid
                   Yes                                1.6        (1.5-1.8)
                   No                               referent
                 Maternal age at delivery (yrs)
                   20-24                              4.7        (3.3-6.8)
                   25-29                              2.6        (1.8-3.7)
                   30-34                              1.7        (1.2-2.4)
                   35-39                              1.4        (0.9-2.0)
                   40-47                            referent
                 Education (yrs)
                    0-11                            referent
                      12                              1.0        (0.8-1.1)
                   13-15                              1.1        (1.0-1.3)
                      16                              1.1        (0.9-1.3)
                     >16                              1.4        (1.2-1.7)
                 Marital status
                  Unmarried                           0.7        (0.6-0.8)
                  Married                           referent
                  American Indian/
                   Alaskan Native                     1.9        (1.5-2.5)
                  Asian/Pacific Islander              1.6        (1.3-2.0)
                  Black                               1.6        (1.0-2.5)
                  White                             referent
                  Hispanic                            1.3        (1.1-1.5)
                  Non-Hispanic                      referent
* The number of previous live-born children is not included in the model because of a multi-
  colinearity problem.
+ Controlling for all of the other variables in the table.
& Confidence interval.

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