Early dating by ultrasound and perinatal outcome - a cohort study

Background. This study aims to evaluate the impact of routine ultrasound in early pregnancy on pregnancy outcome and perinatal mortality. Methods. Using the Swedish Medical Birth Registry, a two-stage study was designed. First, an ecological study that included data from 45 hospitals, with 1,280,746 children born during the years 1974-1991, was designed by differentiating hospitals on the basis of ultrasound usage. Second, a historical cohort study (n = 60,864) was designed for the years 1982-84, including a treatment group (having had routine ultrasound in early pregnancy) and a reference group (having had no routine ultrasound). Results. In hospitals practicing early dating by ultrasound, post-term births were reduced from 13.3% to 7.4%, while pre-term births were increased from 5.7% to 6.8%. For the treatment group, the cohort study showed: 1) fewer post-term births, RR 0.7 (95% CL 0.7-0.8), 2) lower post-term perinatal mortality, RR 0.3 (95% CL 0.1-1.0), 3) fewer twin births postterm, RR 0.1 (95%CL 0.1-0.7), 4) fewer acute growth retarded among term, RR 0.7 (95% CL 0.6-0.9) and post-term, RR 0.3 (95% CL 0.1-1.1), and 5) a tendency towards less asphyxia among post-term births. Conclusion. The study shows that early dating with ultrasound increases the possibilities of clinical judgment at the end of the pregnancy and also indicates that the procedure could contribute to a positive perinatal outcome.