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.