rates. Bracken said most of the provider examples that he has worked with are from convenience samples, but, since he had 100 percent acceptance, he finds it hard to believe that a random model would result in large numbers of defections.

McLanahan asked about the importance of eight-week first-trimester measures. If the provider sample can do as well on response rates, this provider cohort might be preferable because it would result in more data on prenatal care. But if it turns out that the most important data are in the first six weeks of pregnancy, then there is a question about whether it is worth the extra cost to get very early data. Information from the scientific community would help to make the decision about how to allocate the sample. Bracken said that the importance of getting data at eight weeks entirely depends on the hypothesis. Some exposures, such as cigarette smoking and the outcome of low birth weight, exert a lot of their effects in the third trimester, so third-trimester exposures are very important, but it is crucial to be able to measure early exposures as well. He stated restricting this massive study to look only at late trimester exposure is unnecessary. According to his data and estimates, he said, if the study is conducted efficiently, they can expect about 30,000 women to be recruited by eight weeks of pregnancy in the all-provider cohort.

McLanahan said Fragile Families found that most mothers received prenatal care but not always in the first trimester. She observed, however, that there is a big difference in access to early prenatal care by race and ethnic minorities. She urged NCS to consider the ability of the design to address disparities. She asked whether starting with a provider sample would produce consistency across race and ethnic groups, income groups, and other subpopulations, in terms of representativeness and response rates.



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