Registration of Down syndrome in the Medical Birth Registry of Norway: validity and time trends.

Objective: To validate Down syndrome registration in the Medical Birth Registry of Norway (MBRN), 2001-2005, and study time trends and geographical differences in Down syndrome prevalence,1967-2005.

Design/setting: Population-based cohort study, Norway.

Population: 2.3 million pregnancies and births registered in the MBRN, 1967-2005.

Methods: We linked data from the MBRN during 2001-2005 with data from Norway's four laboratories of medical genetics. We calculated sensitivity and positive predictive values (PPV) of the MBRN registration overall, and by background variables. Prevalence rates from 1967 to 2005, overall and regional, were presented graphically as smoothed lowess estimates, crude and standardized for maternal age. Time trends were evaluated, adjusting for maternal age by logistic regression.

Main Outcome Measures: Sensitivity, PPV, and prevalence rates.

Results: Five hundred and seventy-six verified cases of Down syndrome gave a prevalence of 2.0 per 1,000 among 288,213 births and terminations in the MBRN during 2001-2005. Of verified cases, 470 (81.6%) were registered with Down syndrome in the MBRN, while 470 (90.2%) of 521 MBRN-registered cases were verified. Sensitivity was higher in the Northern (93.1%; p=0.005) and Middle (90.6%; p=0.02) region relative the Southern (76.3%), higher for mothers > or =35 years (92.9%) than younger ones (86.1%; p=0.01), and higher for live births (88.8%) relative stillbirths (55.6%; p<0.001). When adjusting for maternal age, there were no significant time trends in prevalence rates from 1967 to 2005. Regional differences over time were found, probably representing reporting differences.

Conclusions: Validity of registration in the MBRN was satisfactory during 2001-2005. Increasing prevalence rates over time were explained by increasing maternal age.