The study analyzes the effects of prenatal care utilization on birth outcomes using an infant sample from Brazil and alternative prenatal care and birth outcome measures. The analytical approach accounts for self-selection into prenatal care through adjusting for several theoretically relevant covariates and evaluates the heterogeneity in prenatal care effectiveness by applying quantile regression and estimating effectiveness by birth defect status. Large positive effects of prenatal care visits on BW were observed for infants both with and without birth defects. However, when adjusting for gestational age, prenatal care visits had significant effects only in the group without birth defects. This suggests that prenatal care increases BW through both increasing gestational age as well as through improving fetal growth rate in pregnancies that are uncomplicated with birth defects, but that it does not improve fetal growth rate in pregnancies that are complicated with birth defects. When gestational age was not adjusted for, the estimated effects of prenatal care visits on BW were likely biased by the reverse effects of gestational age on the number of prenatal visits (longer pregnancies are more likely to have more visits). This suggests that prenatal visits are likely more effective for pregnancies that are uncomplicated with birth defects. This is supported by the significant effects of prenatal care delay on preterm birth and of inadequate care on BW in the group without birth defects and the lack of such effects in the group with birth defects.
The lack of significant effects of prenatal care delay on BW in both infant groups can be due to different factors, including measurement/recall error, self-selection into prenatal care initiation, or the relative ineffectiveness of timing of prenatal care initiation compared with the intensity of prenatal care use (i.e. number of prenatal visits). Adverse self-selection regarding timing of prenatal care initiation, which had been supported in previous studies, suggests that women at higher risk for adverse birth outcomes will initiate prenatal care earlier than those at lower risk. Adverse self-selection can also affect the other prenatal care use measures. This will result in underestimation of the effects of prenatal care use when unaccounted for. The effects of certain risk factors, such as number of live births, number of miscarriages/stillbirths and chronic illnesses, on prenatal care use and on the birth outcomes, and the change in the estimated effects of prenatal visits and the Kessner indicators when adjusted for the model covariates, provide support for the adverse self-selection hypothesis. For instance, previous number of live births reduced the number of prenatal visits but also reduced LBW risk.
Adverse self-selection can also explain the increased risks of LBW with adequate care compared with intermediate care in the group with birth defects, and the lack of significant effects of adequate care on BW in the group without birth defects. This is because high risk pregnancies are likely to get more adequate care (as defined by the Kessner index), inducing a positive bias into the estimate of effects of adequate care on LBW and a negative bias into the effects on BW. Adding indicators for the type of birth defect and whether the birth defect was diagnosed prenatally slightly reduced this bias for the group with birth defects, but the persistence of the positive effect of adequate care on LBW in this group is likely due to the role of unmeasured risks (such as certain maternal health risks) in adverse self-selection in this group.
We attempted to account for self-selection using a well-specified regression model that accounts for several theoretically relevant covariates and confounders. Due to the role of unobservable (unmeasured) risks in self-selection, it is likely that the bias in estimating prenatal care effectiveness persisted using the classical regression model. Instrumental variables (IV) analysis can be used to explicitly account for unmeasured confounders, but this requires the availability of appropriate instruments. We had no access to theoretically appealing instruments such as distance to prenatal care providers or other measures of accessibility to prenatal care.13 Further studies are needed with datasets that provide data on instruments such as availability and price of prenatal care in order to estimate prenatal care effects using an IV model. Due to the possibility of incomplete adjustment for adverse self-selection, we consider the estimated effects of the various prenatal care measures to be lower bound estimates (i.e. prenatal care use will likely be found more effective when adjusting further for self-selection).
The quantile regression analyses suggest that pregnancies with higher fetal health risks that are correlated with lower BW quantiles will benefit more from increasing the number of prenatal visits than those with lower fetal health risks. In the group without birth defects, the larger increase in BW in the higher risk group is likely due to larger increases in both fetal growth rate and gestational age, compared with the lower risk group. In the group with birth defects, it is unlikely that prenatal care visits had a larger effect on fetal growth rate among the higher risk group compared with the low risk group.
Switching from inadequate to intermediate care (as defined by the Kessner index) was found to be more beneficial for pregnancies that are uncomplicated compared with those that are complicated with birth defects, and to result in an increase of about 110 gm in BW mean in the earlier group. The quantile regression analysis suggests that for the group without birth defects, switching from inadequate to intermediate care was slightly more beneficial in increasing BW for pregnancies with higher fetal health risks, likely through a larger effect on length of gestation compared with pregnancies with lower fetal health risks (given that this larger benefit was not observed when adjusting for gestational length). The study results also suggest that for pregnancies that are uncomplicated with birth defects, switching from inadequate to intermediate care is more beneficial than switching from adequate to intermediate care. This result is consistent with Joyce (1994
) who found an increase of about 140 to 180 gm in BW mean in African American and Hispanic samples, respectively, from the US with switching from inadequate to intermediate care, but found smaller and generally insignificant effects of switching from intermediate to adequate care.
The heterogeneity in prenatal care effectiveness by quantile order suggests that prenatal care has heterogeneous effects by unmeasured fetal health risks, which include socio-economic, biologic and environmental risks that contribute to being born at lower BW quantiles. This highlights the importance of identifying women at higher risk for delivering babies at low BW quantiles, as they will benefit most from increased prenatal care utilization. Identifying this group is not straightforward but the study results suggest that first-time mothers, women with chronic illnesses or with history of miscarriages and stillbirths, and older women are more likely to belong to this group. Further studies are needed to identify predictors of this group including socio-economic, health and area characteristics. The heterogeneity of prenatal care effectiveness was masked by analysis of prenatal care effects at BW mean (using OLS), which highlights the usefulness of quantile regression as a more flexible and informative approach to estimate the effects of treatments on continuous outcomes than mean effect models, especially when treatment effectiveness is expected to vary by unmeasured risks that are expected to affect the outcome. The quantile regression results are in line overall with Abrevaya (2001
), who found a larger impact of not receiving care at lower versus high BW quantiles using US natality data.
The potential ineffectiveness of prenatal care visits in improving fetal growth rate, and the lack of effects of switching from inadequate to intermediate care on BW in the group with birth defects, might be due to developmental constraints that limit potential benefits of prenatal care. The differences in the effects of measured prenatal factors and other covariates on the birth outcomes between the groups without and with birth defects (such as the effects of numbers of live births and miscarriages/stillbirths, first trimester bleeding, difficulty in conception, tetanus immunization, multivitamin use, exposure to physical shocks, history of birth defects, ancestry, infant's gender, and parental education) support the existence of different developmental processes and risks between the two infant groups. One analytical complication and also a potential contributor to the result in the birth defect group is potential heterogeneity in prenatal care effectiveness by birth defect type and severity. The sample size was relatively small to allow analyses by birth defect type. Adding indicators for the type of birth defect and for prenatal diagnosis of the birth defect had no effect overall on the primary results. Future studies with larger samples that estimate prenatal care effectiveness for each of these birth defects are needed. Another important question for future research relates to evaluating the effects of prenatal diagnosis of the congenital anomaly on birth outcomes and its potential for modifying the effects of prenatal care utilization.
Important results for other prenatal factors and model covariates were observed. The strong beneficial effects of number of previous live births in reducing LBW suggests that the decrease in fertility rates is a potential contributor to the increase in LBW rate in Brazil (and likely in the US). The average number of children per childbearing age woman decreased from about 4.31 to 3.05 in Brazil between 1980 and 1990 (Global Health Council 2006). This implies a 33% increase in LBW rate in this period based on the study estimates (see ), accounting for more than 60% of the LBW rate increase reported in the Southeastern region in Brazil during this period (Goldani et al. 2004a
). The potential effect of multivitamin use on reducing preterm birth risk deserves further evaluation in datasets that allow more specific measures of multivitamin use. First trimester bleeding increased LBW risk significantly in the group with birth defects and should be evaluated in further studies for its potential as a clinical marker for increased risks of LBW among pregnancies complicated with birth defects, in order to improve prenatal care delivery and pregnancy management. The strong positive effects of family history of birth defects in this group are interesting and might suggest larger maternal investment in prenatal health due to perception of larger risks (due to family history of birth defects).14
This is also an important question for future research. More research is also needed to understand the increased LBW risk (in both groups) with older maternal age and how this can be addressed through prenatal care.
Unlike the US, where significantly higher rates of LBW and preterm birth are observed among African-American infants [e.g. 13.3% LBW and 17.5% preterm birth versus 6.8% and 11.1%, respectively, among whites in 2002 (Arias et al. 2003
)], African ancestry, which was reported in more than 40% of the sample, showed some positive effects on BW. This raises interesting questions about the role of environmental, social and behavioral factors in contributing to differences in these health outcomes between white and African-American births in the US. Further research is needed to confirm and better understand these results.
The prenatal care demand results generally support the adverse self-selection hypothesis where women at potentially higher risks (e.g. acute and chronic illnesses and lower fertility) for adverse birth outcomes had more prenatal visits. This suggests that one way to increase prenatal care utilization might involve increasing women's awareness of the potential benefits of more frequent use of prenatal care, perhaps through media campaigns, since women likely respond to perceived pregnancy risks and prenatal care benefits. Prenatal care standards might also need to be modified in order to improve utilization. At least six visits are recommended in Brazil for a term pregnancy (Goldani et al. 2004a
). More visits are recommended in more developed countries—for instance, the American College of Obstetrics and Gynecology (ACOG) recommends 15 visits for a full-term pregnancy and about 8–9 visits are recommended in Sweden for first time pregnant women (Hildingsson et al. 2005
). Increasing the standard of care and average number of prenatal visits to 12 in Brazil is expected to improve the average BW by about 170 gm and the BW of higher risk pregnancies by about 295 gm. These fairly large effects are not surprising given the low utilization of prenatal care. The average number of prenatal visits in the study sample was 7 visits compared with 12 visits in the US, and 60% of women in the study sample initiated prenatal care in the first trimester, compared with more than 80% of women in the United States (Martin et al. 2005
The geographic differences in prenatal care utilization were well correlated with differences in income per capita between the states. The average income per capita in the states of Minas Gerais, Paraiba, Rio Grande do Sul and Santa Catarina was, respectively, 58%, 35%, 70% and 80% of that in the state of Sao Paulo (Institute of Brazil for Geography and Statistics (IBGE), http://www.ibge.gov.br/
, accessed 22 April), suggesting that even with the existence of universal insurance and public ambulatory care clinics, average wealth at the state level still matters for prenatal care access. While there were differences in availability of clinics between the various states, these are unlikely to have accounted for all the differences observed in utilization across the studied states.
One limitation of the study is the lack of data on other potentially relevant covariates, including smoking, alcohol use and marital status. Several previous studies of prenatal care effectiveness have shown that including or excluding smoking and/or drug use in the birth outcome function had no effect on the results for prenatal care effectiveness (e.g. Joyce 1994
; Warner 1998
; Reichman et al. 2006
). The majority of the study sample is expected to consist of mothers who are either married or have a stable relationship, given that about 93% of the mothers reported living with the child's father at the time of birth and that only observations with complete data on father's characteristics were included. Another potential limitation is that the included birth sample may not be fully representative nationally, given that it was selected from a non-random sample of hospitals. The distributions of the outcomes of this sample were comparable with those reported in other studies in Brazil (e.g. Goldani et al. 2004a
; Barros et al. 2005
), suggesting that a large sample selection bias is unlikely.15
The hospitals participating in ECLAMC are located in socio-economically diverse communities, as can be seen from the variation of the socio-economic variables that were included in the study (see ). Further, the hospitals are located in five states and several cities, providing a large geographic representation. Therefore, the sample is considered to be representative of a large proportion of the Brazilian population. While all these factors are acknowledged as limitations, they are unlikely to have had any real impact on the study results.
Finally, we had no data on the quality or content of prenatal care and the study provides results only for increasing the quantity of prenatal care at the average ‘unobserved’ care quality and content levels. This is a common limitation to most observational studies of prenatal care effectiveness that focus on utilization measures due to the lack of data on quality and content of care. It is expected that the estimated average effectiveness of prenatal care utilization will increase as quality and content of care increase, but it is important to also evaluate the substitutability between quantity and quality of prenatal care in improving birth outcomes. Data are needed to evaluate the effects of quality of prenatal care in improving birth outcomes.