Numerator
Number of infants born before 37 completed weeks of gestation
Methodology
Depending upon county population size, the period county percent for the most recent 3, 5, or 10 years is reported.
References
Goldenberg RL, Culhane JF, Iams JD, Romero R. Epidemiology and causes of preterm birth. The Lancet, Volume 371, Issue 9606, 5 January 2008-11 January 2008, Pages 75-84.