sample with current plans to start with a probability-based geographic area sample, though other probability-based options are under consideration. The optimal balance between number of PSUs, number of births per PSU, and environmental variety is not yet known. Different contract teams are working on scenarios, but this will not be resolved prior to the workshop and therefore will not be presented or discussed. The important point for the remaining discussion is that the NCS is currently planning an area probability sample of PSUs that is expected to generate 100,000 live births for participation in the NCS.

The Birth Cohort consists of births collected from a sample of hospitals and birthing centers and a subsample of women giving birth at those selected centers. This is tentatively planned to be about 45,000 participants of the overall sample. A 2-year initial recruitment period is proposed.

Within the current proposal, for each selected geographic PSU, a list of all hospitals and birthing centers will be prepared as a sampling frame for the birth cohort. Based on data from 2006, roughly 98 percent of all births in the United States take place at hospitals or birthing centers. A random sample of hospitals and birthing centers will be selected, with probability proportional to the number of births, and recruited to participate in the study. All women who give birth at the selected hospitals and birthing centers during specific times within the planned 2-year initial recruitment period will be eligible to be sampled while at the hospital, regardless of whether they live within the selected PSU or not. A systematic sample of women giving birth will be selected.

The NCS has documented multiple studies that recruit new mothers (and fathers) in the hospital and some that collect specimens (the Fragile Families Study is one of these). The acceptance rate is high and in some cases over 90 percent. The NCS has several strategies for collecting the relatively few specimens of interest (maternal blood and urine, cord blood, placenta, and perhaps an infant second dried blood spot following newborn screening), including collection from all sampled women during the recruitment windows and then discarding specimens from women who do not consent. The NCS is also piloting a few methods in the Provider-Based Sampling Vanguard sites to give some empirical data on acceptance, logistics, and costs. It is also possible in the birth cohort to attempt to collect medical records not only from the hospital or birthing center, but also from the sample member’s prenatal care provider (if any).

The birth cohort will be a nationally representative sample of births in the United States. It can include stillbirths as well as live births.

While the recruitment of a relatively unbiased sample at acceptable cost is attractive, a knowledge gap that needs to be addressed is prenatal ex-



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