A matched exposure cohort design was utilized. Specifically, infants whose birth certificates noted the use of infertility treatment were conceptualized as ‘exposed’ infants (n=4,024) and subsequently frequency-matched on maternal geographic residence and plurality of birth at a ratio of ≈1:3 to ‘unexposed’ infants, or those without any notation of infertility treatment (n=14,555). For twin pregnancies, one infant was randomly selected for inclusion in the primary cohort. However, the twin sibling of the index child was included in a secondary cohort so that mothers were able to report on both infants. Another cohort also included all higher order births (n=126 triplets, n=8 quadruplets) and was followed similarly to the primary cohort to answer more specific questions pertaining to multiples in subsequent publications. This paper focuses on the primary cohort only.
Following establishment of the primary cohort by the New York State Department of Health, the recruitment process commenced in September 2008 and continued through December 2010 with the mass mailing of introductory letters and study brochures to the targeted study population (n=18,479), whose infants were approximately 2–4 months of age by September 2008. Introductory letters stated that the Study was interested in women’s pregnancy histories along with childhood growth and development during the first three years of life. Approximately two weeks after each monthly mailing, follow-up telephone calls were undertaken to screen for eligibility: mother’s residence at birth and enrollment within the specified catchment area; ability to communicate in English or Spanish; and the index infant or its twin was currently alive. Interested mothers were sent a study packet, as were all mothers for whom no telephone contact could be made after a maximum of four attempts. Periodic postcard reminders and Facebook© or email messages were used to encourage the return of data collection instruments within the study timeframe for each developmental assessment. Telephone reminder calls were placed if study data collection instruments were outstanding ≥10 days. All materials were offered in English and Spanish. Enrollment was defined as receipt of signed parental consent.