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Determine association of home visiting with subsequent pregnancy outcomes
Retrospective study of Ohio mothers delivering their first infant from 2007–2009. First, we compared mothers enrolled in home visiting to a matched eligible group. Second, we compared outcomes within home visiting based on program participation (low < 25% of recommended home visits, moderate 25–75%, high 75–100% and very high >100%). Time to subsequent pregnancy within 18 months was evaluated using Cox proportional hazards regression; logistic regression tested the likelihood of subsequent preterm birth.
Of 1,516 participants, 1,460 were matched 1:1 to a comparison mother (n=2,920). After multivariable adjustment, enrollment was associated with no difference in pregnancy spacing or subsequent preterm birth. Among those enrolled, moderate vs. low participants had reduced risk of repeat pregnancy over 18 months (hazard ratio 0.68, p=0.003).
Increased pregnancy spacing is observed among women with at least moderate home visiting participation.
Preterm birth (< 37 weeks gestation) affects roughly 500,000 infants annually in the U.S. and is a leading causes of infant mortality1. As one strategy to address disparities, home visiting programs have received federal investments to provide education, social support, and interventions in a home-based setting2. Based on social determinants of health theory3, these programs seek to mitigate 1) barriers to healthcare, (2) behavioral risk factors, and (3) psychosocial stressors4. Despite several promising studies5–9, a recent meta-analysis suggests that on average, home visiting is not effective in changing birth outcomes10. A key limitation is that most pregnant women who enroll in home visiting do so relatively late in pregnancy, minimizing the opportunity to address modifiable risk factors11–14. For home visiting programs, the greatest opportunity is likely in addressing risk factors for future pregnancy.
A growing body of literature has demonstrated the importance of adequate pregnancy spacing to reduce risk of preterm birth, particularly among high-risk women15–17. Pregnancy spacing may also have important consequences for maternal educational attainment and economic self-sufficiency18, underscoring the long-range impact of this outcome for the maternal life course. Home visitors have an opportunity to impact pregnancy spacing through emphasis on self-efficacy and connection with family planning services. Prior literature describing home visiting effects on pregnancy spacing has been inconsistent19–22. With recent and ongoing expansion of home visiting programs across the U.S., evaluation of subsequent pregnancies is critical to assessing long-term programmatic benefits.
The goal of this study was to evaluate the association of pregnancy spacing with home visiting enrollment among high-risk first-time mothers. We hypothesized that participants would be more likely to delay subsequent pregnancy compared with eligible, non-enrolled women. We also sought to determine the effects of varying participation in this voluntary program by enrolled women. For home visiting programs, which often struggle to engage families in consistent visit schedules and to retain families in services over time, the issue of attrition and loss to follow up has important implications for interpretation and evaluation of program outcomes. We hypothesized that among enrolled participants, greater program participation after birth of the first infant would be associated with improved pregnancy spacing.
This retrospective cohort study evaluated a population of high-risk, first time mothers enrolled from 2007–2009 in a well-established, regional home visiting program serving 7 counties throughout Greater Cincinnati. This program contracts with 10 home visiting agencies in Ohio which adhere to program, training and evaluation standards established by Cincinnati Children’s Hospital Medical Center (CCHMC), employing a core curriculum based on one of two national model of Healthy Families America23 (HFA, 9 sites) and Nurse-Family Partnership24 (NFP, 1 site). Overall program goals are to: 1) provide pregnancy education and care coordination, 2) promote a nurturing home environment, 3) optimize child development, 4) link families to health services, and 5) promote economic self-sufficiency. Eligible participants must be first-time mothers with at least 1 of 4 characteristics: unmarried, low income (up to 300% of poverty level, receipt of Medicaid, or reported financial concerns), < 18 years of age, or suboptimal prenatal care (broadly defined with no specific gestational age cutoff). Of these 4 criteria, suboptimal prenatal care is the least commonly used with <5% of participants enrolled based on this alone. Participants are enrolled during pregnancy or before their child reaches 3 months of age. Home visitors hold bachelor’s degrees with training in child development and related disciplines (HFA), or are licensed nurses (NFP). To address pregnancy spacing, the program uses both formal approaches, such as handouts and tools to promote maternal health, life course planning, and connection to postpartum care, as well as informal approaches, e.g. motivation and confidence building for mothers to achieve their goals. The schedule of visits starts with weekly home visits, tapering to fewer visits as the child ages, with completion at 3 years of age.
For each participant, home visiting data were abstracted from a web-based data entry system used to document service provision data and for billing25. To derive information for the index pregnancy, program records were linked to Ohio vital statistics and birth-related hospital discharge of both mother and infant. Record linkage was accomplished using LINKS (University of Manitoba), a SAS-based probabilistic and deterministic matching program26. Similarly, linkage to subsequent live births for these mothers was conducted using Ohio vital statistics through 2013, such that information on subsequent pregnancies was examined within a three-year time frame from birth of the index infant (i.e. fixed type 1 censoring). The Ohio Department of Health and CCHMC Institutional Review Boards approved this study, including a waiver of consent based on minimal risk to subjects and feasibility..
Interpregnancy interval (IPI) was defined as length of time between index birth and date of conception for the second pregnancy, derived by subtracting the second infant’s gestational age from her or his birth date and then calculating the interval between the index birth and conception of the second infant. Using this method, only second conceptions resulting in an Ohio live birth were counted as events. IPI was assessed as a continuous variable as well as a binary variable (< or ≥18 months)15. As in previous analyses, we also evaluated a secondary outcome of very short IPI (≤ 6 months) to reflect rapid repeat births22. Among women with a subsequent pregnancy identified within the study time frame, preterm birth was assessed using the obstetrical estimate of gestational age documented in vital statistics.
This was a two-phased analysis of home visiting participation and pregnancy spacing. First, we examined outcome differences between those enrolled in home visiting and those not enrolled. Our prior research has shown that among women eligible for home visiting in the region, approximately one third are referred and among those referred approximately half enroll26. Therefore, using Ohio vital statistics from the same years of 2007–2009, a matched comparison group of first-time mothers was selected based on program eligibility criteria. To minimize the potential for self-selection bias in the enrolled group27, we excluded from the comparison group any women who were referred to home visiting but declined enrollment. We also excluded women with an index neonatal death, because home visiting requires participants to either be pregnant or parenting and therefore they would no longer be eligible for services. Similarly, women with a previable gestational age at delivery (< 23 weeks) for the index pregnancy were also excluded, as such infants have a low likelihood of survival or eventual discharge by 3 months of age thereby precluding home visiting participation. We then performed a 1:1 matching of potential comparison mothers to home visiting participants using the following matching characteristics: race, maternal age, index year of birth, and residence within a ZIP code with either higher or lower penetration of the home visiting program (< or ≥ 25% of eligible women in the area enrolled in home visiting). Exact matching constraints were permitted to vary modestly for maternal age (1 year; approximately 20% standard deviation in age) and birth year (2 years) to prevent unnecessary exclusion of matched comparisons. We also matched by propensity score28, derived using logistic regression for the individual conditional probability of participating in home visiting based on the following baseline characteristics: maternal age, education level, tobacco use, marital status, race, ethnicity, ZIP code of residence, hypertension before or during the index pregnancy, and diabetes before or during the index pregnancy. These characteristics were selected on the basis of prior research demonstrating their importance in predicting referral and enrollment into home visiting26. A 1:1 matching ratio was selected to minimize bias and ensure balance of covariates across enrolled and comparison mothers. Matches were constrained by a caliper of 20% of the propensity score standard deviation29. A greedy algorithm with weighted Euclidean distance was used to perform sample matching with SAS30. Balance of covariates across controls and the enrolled participants was assessed using absolute standardized differences in means.
For the second phase of the analysis, we evaluated whether level of program participation was associated with outcome differences within the group of women enrolled in home visiting. As above, women with a gestational age < 23 weeks at delivery and those with a neonatal death were excluded. Based on empirical evidence from multiple programs that attrition from home visiting is common in the first year31, we measured participation as percentage of expected home visits completed over the first year of life for the mother’s first infant per the program’s curriculum (complete participation would approximate 32 total home visits). We categorized these percentages a priori into levels of participation deemed low (0–25% of expected visits), moderate (25–75% of expected visits), high (75–100% of expected visits), and very high (>100%). Our characterization of 75–100% of expected visits as high participation aligns closely with previous research on home visiting engagement31,32. Characteristics of participants by participation level were analyzed by the Pearson chi-squared test or Student’s t test.
For both phases of analysis, Cox proportional hazards regression models tested differences in timing to subsequent pregnancy. Models were tested for the assumption of proportional hazards using Schoenfeld residuals and adjusted for clinical and demographic covariates. For the analysis of program participation, modelling was also stratified by agency to account for site-level differences in the baseline hazard. Cumulative hazard function curves were used to display results graphically. Finally, for both phases of analysis we used multivariable logistic regression to evaluate the likelihood of preterm birth among those women experiencing a subsequent pregnancy. Analyses were conducted using STATA 11.1.
To derive the sample for the first phase of the analysis, we identified 1,516 women with an Ohio birth certificate between 2007 and 2009 and who were enrolled in home visiting either prenatally or after birth of the infant. After applying matching algorithm constraints, 1,460 participants were matched 1:1 to comparison women who were neither referred to nor enrolled (total sample 2,920 women). Based on this sample size we had >80% power (at alpha=0.05) to detect a 25% reduction in repeat pregnancy by 18 months among enrolled participants using a two-sided test, which is similar to effect sizes shown in prior research10,19. As shown in Table 1, characteristics were balanced across the two matched groups. Across this total sample, 485 (16.6%) had an IPI < 18 months (17.4% among enrolled vs. 15.8% among comparisons), and 4.6% had an IPI < 6 months (4.8% among enrolled vs. 4.3% among comparisons).
As shown in Table 2 and Figure 1 (Panel A), after multivariable adjustment enrollment was not associated with a significant difference in timing to repeat pregnancy over an 18 month timeframe (HR 1.13, 95% CI 0.94, 1.37). Tests for interaction with key covariates (race, maternal age, and education level) were not statistically significant. Covariates that were associated with significant risk differences were obesity (HR 1.26, 95% CI 1.01, 1.57), college education (HR 0.68, 95% CI 0.50, 0.92), Hispanic ethnicity (HR 0.46, 95% CI 0.24, 0.89) and index Cesarean delivery (HR 0.59, 95% CI 0.47, 0.75).
Among this sample of enrolled and non-enrolled women, 854 (29.3%) had a repeat pregnancy over a 3 year timeframe; the preterm birth rate for these second pregnancies was 12.3% (12.1% among enrolled vs. 12.5% among comparisons). In multivariable logistic regression shown in Table 2, enrollment was not associated with a significant reduction in odds of preterm birth for the subsequent pregnancy (adjusted odds ratio (AOR) 0.78, 95% CI 0.49, 1.24). However, prior preterm birth and IPI < 6 months were both significant predictors of subsequent preterm birth.
The second phase of our analysis included only those women enrolled in home visiting. Approximately 37% were classified as low participation, 50% as medium participation, and 10% as high participants. Finally, a small number (3%) received > 100% of the expected intervention. Because we observed significant differences in risk characteristics of participants with this very high level of home visiting service, these were categorized separately (see Table 3). Among all enrolled participants, 19% had a repeat pregnancy within 18 months of first birth, and 5% had a repeat pregnancy within 6 months.
Compared to low program participation, moderate participation was associated with reduced risk of repeat pregnancy over 18 months (HR 0.67, 95% CI 0.52, 0.87). High participation was also associated with a reduced risk (HR 0.55, 95% CI 0.33, 0.91). However, those with the highest number of visits did not demonstrate a significant difference in repeat pregnancies compared with low participants (HR 1.39, 95% CI 0.78, 2.50) (see Figure 1, Panel B). Tests for interaction effects with key covariates (race, maternal age, and education level) were not statistically significant. We did not observe significant differences in timing to repeat based on home visiting model (approximately 5% served by an NFP agency) or on prenatal vs. postnatal enrollment (after birth of the index child). However, covariates that were associated with significant risk differences were maternal smoking (HR 1.19, 95% CI 0.91, 1.57), obesity (HR 1.44, 95% CI 1.08, 1.91), college education (HR 0.63, 95% CI 0.44, 0.91), and index Cesarean delivery (HR 0.64, 95% CI 0.48, 0.86). As a sensitivity analysis, we omitted 87 participants who based on program records were discharged from home visiting due to a move outside of the service area; results for all predictors were substantially unchanged.
For the 498 enrolled in home visiting with a repeat pregnancy over 3 years, the preterm birth rate for the second pregnancy was 12.3%. We evaluated for possible mediation by modeling the analysis of preterm birth with and without IPI as a covariate. In multivariable logistic regression, high home visiting participation demonstrated a trend towards reduced odds of subsequent preterm birth, but the effect was not statistically significant (AOR 0.45, 95% CI 0.10, 2.06). When IPI < 6 months was added to the model of preterm birth (Table 4), the effect of high home visiting participation was not substantially different, while IPI itself was a significant predictor of preterm birth (AOR 4.30, 95% CI 2.21, 8.38).
Several aspects of real world programmatic implementation likely contribute to the inconsistency of published findings on home visiting and birth outcomes, including referral bias towards women with more complicated pregnancies27, as well as an inability of home visiting programs to reach most participants until they are late in pregnancy13. Given these limitations, the question of whether home visiting influences subsequent birth outcomes has significant public health implications. In this study of high-risk first-time mothers, we observed no difference in pregnancy spacing or subsequent preterm birth between those enrolled and not enrolled in home visiting, which suggests limited overall impact of this intervention. These findings may be related to lack of sustained engagement among participants in this intensive, voluntary program, of whom roughly one quarter had fewer than 5 home visits after delivery of their first infant. Importantly, we observed that among enrolled participants, those with at least moderate program participation did have increased time to subsequent pregnancy compared with those who had very few home visits. While we observed no significant effect of participation level on subsequent preterm birth, our ability to detect differences in preterm birth was likely affected by the limited number of subsequent pregnancies, particularly among those women with moderate and high program participation. Shorter IPI was a significant risk factor for preterm birth within this cohort, indicating that pregnancy spacing is important for home visiting programs to achieve long term benefits for participants.
Prior studies on home visiting and pregnancy spacing have yielded mixed results21. A recent cohort study comparing women enrolled and not enrolled in NFP home visiting in Pennsylvania demonstrated positive effects on pregnancy spacing, particularly for young women and those living in rural locations20. Conversely, evaluation of HFA home visiting in Hawaii has demonstrated no improvement in pregnancy spacing among enrolled participants compared with those not enrolled19. We do not suspect our null findings are due to insufficient power to detect small but meaningful differences, as the trend for enrolled women actually appeared slightly worse than comparisons. Lack of effect has previously been attributed to several factors, including program attrition and lack of program specificity with regards to family planning interventions. Across home visiting programs nationally, heterogeneity of services is well documented, with most participants not receiving the recommended number of visits31. Our current findings provide further insight into the impact of variable dose of the intended intervention on this important outcome. Results suggest not only the need for specific content on pregnancy spacing, but also to maximize delivery of this content to all participants, including those only enrolled briefly.
Finally, we found that very high program utilization was associated with increased maternal risk, including hypertension, diabetes, obesity, and smoking. For this subgroup, pregnancy outcomes were not statistically different from those of very low program utilizers (and suggestive of a trend towards worse outcomes). This is consistent with prior findings that very high prenatal care utilization is associated with greater maternal pregnancy risk33, as well as recent findings from Holland et al. that increasing use of home visiting over time is predictive of poor outcomes34. This non-linear association between maternal risk and dosage of preventive services likely reflects the dynamic process of evaluation by both providers and participants, such that higher needs can be a driver of more frequent visits. Within home visiting, recommended visit schedules are not empirically based but rather derived a priori; although general wisdom suggests “more is better,” our findings suggest that the extent of a home visiting dose-effect relationship may depend on maternal characteristics and on the outcome being evaluated. Further research is needed not only on approaches to strengthen maternal motivation to participate and adhere to recommended visits, but also to optimize program dose based on their risk characteristics and outcomes35.
Strengths include a large regional cohort with 3 years of follow up data and details of service use for this community-based program. We have attempted to minimize selection bias associated with referral and decision to enroll, which may be considerable among the eligible population27. However, selection bias remains a significant limitation of this study design. There may be unmeasured, systematic differences between women who received high and low levels of service. Bias toward overstating the results would occur if, for example, women with subsequent pregnancies were less able to adhere to the home visiting schedule, or if those with adequate participation were especially motivated with regards to family planning. Alternatively, prior data from our program suggests there may be bias in retention such that participants with higher acute needs are more likely to remain engaged in home visiting36.
Furthermore, given limitations of the data we could not quantify either time spent on pregnancy spacing counselling, the quality of such information, or the percentage of women who initiated postpartum contraception. Ascertainment bias should also be considered, because miscarriages, therapeutic abortions, and births outside of Ohio were not counted as events. We have no reason to believe that miscarriages and abortion might be systematically different between women with lower and higher program participation. However, it is possible that women exiting the program earlier were also more likely to move out of the state and therefore be less likely to have a record of subsequent pregnancy, biasing our estimates towards the null. Finally, generalizability may also be concern, given our analysis was confined to one regional program. However, rates of subsequent pregnancy for women in our sample were consistent with rates among an eligible, at-risk population in Ohio.
Among women enrolled in home visiting, short IPI is a risk factor for preterm birth. Greater program specificity with regards to pregnancy spacing may promote long term benefits. Programmatic effects on pregnancy spacing are demonstrated among those with at least moderate program participation. Although findings suggest that increasing delivery of recommended home visits may increase observable program benefits, future efforts to develop and implement empirically-based visit schedules should take into account maternal risk characteristics as well as outcomes of interest.
We acknowledge the participation and support of the United Way of Greater Cincinnati and Ohio Help Me Grow. We also thank Dr. Louis Muglia for his critical review of the manuscript.
Funding: Dr. Goyal was supported by the Building Interdisciplinary Research Careers in Women’s Health program (5K12HD051953-07), co-funded by the Office of Research on Women’s Health (ORWH) and the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), administered by the Center for Clinical and Translational Science and Training at University of Cincinnati. The content (including study design, data collection, analysis and interpretation of data; writing of the report, and decision to submit the article for publication) is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.
CONFLICT OF INTEREST
The authors declare no conflict of interest.