Page 18 - Sample Moderate prematurity, socioeconomic status, and neurodevelopment in early childhood

Chapter 2
Child Health Care (PCH) centers from across the country, covering urban and rural areas. Together, the thirteen centers monitored 45 446 children (25% of all four-year-old children monitored by Dutch PCH centers). In the Netherlands, 90% to 95% of children are seen regularly and free of charge by PCH doctors of well-child care from birth up to four years.11 PCH professionals monitor mental and physical development through structured interviews with parents, general physical examinations, and standardized screening procedures, all of which are documented in PCH files. Lollipop was approved by the local institutional review board, and written informed consent was obtained from all parents. Lollipop is registered with www.controlled-trials.com (ISRCTN 80622320).
Participants
Each PCH center provided a sample of all preterm children born during a single year, either from January 2002 to January 2003 or from June 2002 to June 2003. Term-born children were sampled from the same PCH centers and belonged to the same age-range as the preterm children. After the file of each second preterm child had been selected, the file of the next term-born child served as a control. In >95% of cases, GA was calculated by using the last date of menstruation, and confirmed by early ultrasound measurements. In the Netherlands, it is routine practice to assess GA with early ultrasound measurements between 100 and 126 weeks of gestation. Children were excluded if they had a congenital malformation or syndrome, if the GA could not be verified or was beyond the set range, or if families moved between sampling and inclusion. An overview of the sampling procedures of Lollipop has previously been provided by Kerstjens et al.6 In short, of the 1145 eligible MP children, 995 (86.9%) parents participated in the long- term follow-up part of the study and of the 674 eligible term-born children, 577 (85.6%) parents participated. Response rates for the developmental questionnaire were 81.0% for MP children and 80.7% for term-born children. The total number of children in this study is 1470 (926 MP and 544 term-born children).
Assessment of SES
We determined SES on the basis of the three most frequently used measures: education, income, and occupation.12,13 Data on the highest completed educational level of both parents were collected by a questionnaire when the children were four years old. The following categories were defined: primary school or less, low-level technical and vocational training (< 12 years of education), high school or medium-
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