We assessed validity and reliability of data on four serious pregnancy
complications and gestational age in two national registers, the Medi
cal Birth Register (MBR) and the National Register of Hospital Dischar
ges (NRHD). From a cohort of all women in Denmark who gave birth to th
eir first and second singleton infant in 1982-1987, a review was made
of a selected sample of 1662 medical records. Regarding registration o
f pregnancy complications, there was good agreement (kappa above 0.6)
between medical records and the registers, and between the registers.
However, there was a tendency toward understatement evidenced;hy, low
sensitivity of three of four pregnancy complications. The level df agr
eement (43%) for length of gestation was disappointing. The number of
systematic and nonsystematic errors indicate that there was about 52%
more singleton preterm deliveries in Denmark in 1982 than previously r
eported (6.9% instead of 4.5%). It is concluded that the validity of t
he Danish birth registers should be improved by explicit definitions,
increased use of raw data, and data collection by motivated profession
als at birth.