1.2. Development
Within the framework of the follow up program, the Dutch translation of the Bayley Developmental Scales (BOS 2–30) had been administered until the age of 2 years. This standardized instrument assesses both mental and motor development of 2 to 30-month-old children [14]. From December 2003 onwards, a new version of the BOS 2–30 was used: the Bailey Scales of Infant Development-Second Edition-Dutch version (BSID-II-NL) [15]. The BOS 2–30 and the BSID-II-NL share the same background and are substantially related to each other [16]. Both tests provide a mental developmental index (MDI) and a psychomotor developmental index (PDI) with a mean (SD) of the normative population of 100 (15). We clustered the results into normal (N − 1 SD), mild developmental delay (− 2 to − 1 SD), and severe developmental delay (b−2 SD). We identified the children with a developmental delay (mild or severe) at any contact moment, and lack of a score in the normal range at a later contact moment, and clustered them as such.
At 5 years of age, both the short version of the Revised Amsterdam Intelligence Test (RAKIT) and the Movement Assessment Battery for Children (MABC) were adminis- tered, as described previously [17]. For the RAKIT [18], the mean (SD) score of the normative population is 100 (15). The MABC [19] presents a total impairment score (TIS) and three subtest scores: manual dexterity, ball skills, and balance skills. The MABC test results are expressed in percentiles, which we clustered into normal (Np15), borderline (p6-p15), and definite motor problem (≤p5). The maximal exercise capacity was tested with the Bruce treadmill protocol at 5 years of age. This yielded a maximal endurance time which was converted to SDS using recently published Dutch reference values