1. Department of EpidemiologyUniversity of Iowa, 145 N. Riverside Drive Iowa City IA 52242‐3535 USA
2. Department of PediatricsUniversity of California San Diego, 9500 Gilman Drive La Jolla CA 92093‐5004 USA
3. California Preterm Birth InitiativeUniversity of California San Francisco, 550 16th Street San Francisco CA 94158‐2545 USA
4. Department of BiostatisticsUniversity of Iowa, 145N. Riverside Drive Iowa City IA 52242‐3535 USA
5. Department of Epidemiology and BiostatisticsUniversity of California San Francisco, 550 16th Street San Francisco CA 94158‐2545 USA
6. Department of PediatricsUniversity of Iowa, 200 Hawkins Drive Iowa City IA 52242‐1009 USA