Birth cohorts in many different geographic areas have greatly added to our understanding about the early life origins of asthma. Investigators have followed a number of birth cohorts since the latter part of the 20th century to try to define the causes for the increase in asthma, including at least 18 cohorts in Europe alone . In the U.S. asthma-related birth cohort studies have been or are being conducted in Arizona , Wisconsin , Detroit , and Massachusetts , among others. Some studies have been population-based, others have focused on high-risk children . To our knowledge, no previous asthma birth cohort study has focused on high-risk, urban, low-income children in a variety of locations.
The URECA study is uniquely poised to add new information about how environmental exposures and lifestyles that are specific to children in low-income areas of the inner city affect immunologic development and the risk of developing recurrent wheezing and ultimately asthma. Recruitment is now completed, and compared to other birth cohorts, the participants have distinct demographic features that reflect the urban population. The children are predominately minorities, and the socioeconomic status of their families is very low compared to the U.S. as a whole. In 2006, 13.4% of U.S. households reported incomes less than $15,000 , compared to about 2/3 of the URECA families. Furthermore, over 80% of the URECA women were not married at enrollment and 42% of the women in the allergic families cohort had less than a high school education, whereas nationwide in 2004, 36% of births were to unmarried women and 78% of women who gave birth had completed 12 or more years of school . Nationally, about 10% of U.S. women report smoking during pregnancy ; the rates are slightly higher for both URECA cohorts.
The URECA study excluded children born earlier than 34 weeks of gestation, to avoid the problem of trying to separate asthma from chronic lung disease associated with prematurity. In spite of the exclusion of very premature babies, the average birthweight of the URECA children (Table 4) was less than the national average of 3316 grams for singleton births in the U.S. in 2004 . This is likely related to the low socioeconomic status and overrepresentation of minorities in the URECA population. The low birthweight rate for black mothers in the U.S. is twice as high as for non-Hispanic white mothers, even for term births . Other characteristics of the URECA children were similar to children in the U.S. a whole. For example, the percentage of U.S. children with 5-minute Apgar scores of 9 or 10 was 88.8% in 2004 compared to 89.6% and 93.9% of the URECA children in the allergic and non-allergic families cohorts, respectively . Furthermore, the percentage of U.S. children born by Cesarean section was 29.1% in 2004 compared to 31.2% and 28.5% of the URECA children in the allergic and non-allergic families cohorts, respectively .
Children in the URECA cohorts are at increased risk of developing asthma from a number of perspectives. Both minority and low socioeconomic status are associated with an increased risk of asthma, which likely is a consequence of harmful environmental exposures. The risk of asthma is further increased for the allergic families cohort by having at least one parent with allergic disease . Several scientific studies indicate that children of atopic parents can have skewed immunologic responses in early childhood, even in the absence of clinical manifestations of atopy [36, 46, 47]. It is possible that this skewing reflects an increased likelihood for allergy and asthma later in childhood. Furthermore, children of atopic and non-atopic parents could differ in other ways, including the number and severity of viral respiratory infections or environmental exposures to microbes, allergens, or indoor pollutants. The two birth cohorts will allow exploration of differences between children of atopic and non-atopic parents.
Strengths of the URECA study include its unique population, large sample size, and prospective collection of data related to urban environmental factors, immune development, and atopic outcomes. Challenges include working with families that may have inconsistent access to transportation and telephone service and may relocate during the study period. In addition, the multicenter nature of this study has required careful attention to the development of data collection and laboratory procedures that can be standardized across multiple centers. Despite the challenges, 87% of the enrolled families are still active in the study.
The study is a high-risk cohort, and findings must be interpreted accordingly. Comparisons with the non-atopic group should provide valuable insights into the relationship of study findings to the general population. In addition, the study focuses on the maternal/child axis, and there is limited, voluntary, participation of fathers. The majority of the mothers in the study are single, and we felt that requiring paternal involvement would severely restrict study enrollment. A final limitation of the study is that the inclusion of detailed immunologic measurements and viral sampling limited the feasible sample size, and as a result the power for genetic analysis is modest. Nonetheless, the longitudinal immunologic and asthma phenotyping will provide precise outcomes for analysis.