Materials and Methods
All infants included in the study had mothers who were participants in the California Prenatal Screening Program administered by the Genetic Disease Screening Program (GDSP) within the California Department of Public Health, and had NT measured when fetal CRL was 45-84 mm. NT practitioners who submit measurements to the GDSP are credentialed by the Nuchal Translucency Quality Review Program or Fetal Medicine Foundation. The GDSP works with both credentialing agencies to monitor NT data and maintain quality assurance for all accredited practitioners. All NT measurements are submitted to the California Prenatal Screening Program as part of routine first- and second-trimester screening for aneuploidies.
Study participants were drawn from a sample of all singleton infants with NT results who had estimated dates of delivery from July 2009 through December 2010 based on CRL (n = 163,899). The sample was restricted to pregnancies for which NT measurements were done by clinicians who had practitioner-specific NT medians (n = 114,475). Clinicians who had practitioner-specific medians are those who had a minimum of 75 examinations and for whom the slope of their NT measurements increased by at least 11% across gestational weeks. In California, practitioner-specific medians have been shown to help control for less-experienced practitioners tending towards smaller NT measurements. The sample was further restricted to infants born in 2009 through 2010 who had linked newborn screening records maintained by GDSP, birth certificates, and hospital discharge records from the birth cohort files maintained by the Office of Statewide Health Planning and Development (n = 76,286). Given that our study was focused on euploid infants without CCHDs, we also excluded all infants with chromosomal defects (n = 197) and CCHDs (n = 200) resulting in a final sample of 75,899 infants ( Figure ).
