Background As the survival of very preterm and low-birthweight infants increases, so does the importance of monitoring the birth prevalence of childhood impairments; disease registers provide a means to do so for these rare conditions. High levels of ascertainment for disease research registers have become increasingly difficult to achieve in the face of additional challenges posed by consent and confidentiality issues. 4Child – Four Counties Database of Cerebral Palsy, Vision Loss and Hearing Loss in Children has been collecting data and monitoring these three major childhood impairments since 1984.
Methods This study used capture–recapture and related techniques to identify areas which are particularly affected by low ascertainment, to estimate the magnitude of missing cases on the 4Child register and to provide birth prevalence estimates of cerebral palsy which allow for these missing cases.
Results Estimates suggest that while overall around 27% of cerebral palsy cases were not reported to 4Child, ascertainment for severely motor-impaired children (93% complete) and those born in two of the four counties was good (Oxfordshire: 90%, Northamptonshire: 94%). After allowing for missing cases, adjusted estimates of cerebral palsy birth prevalence for 1984–1993 were 3.0 per 1000 live births versus 2.5 per 1000 live births in 1994–2003.
Conclusions Capture–recapture techniques can identify areas of poor ascertainment and add to information around the provision of cerebral palsy birth prevalence estimates. Despite variation in ascertainment over time, capture–recapture estimates supported a decline in cerebral palsy birth prevalence between the earlier and later study periods in the four English counties of the geographical area covered by 4Child.