Objective: To assess and compare the sensitivity for detecting fetal anomalies and chromosomal aberrations by routine ultrasound examination performed in the second trimester with results from an examination performed at 11-14 weeks gestation.
Design: Observational study.
Setting: Five centers in the southeast region of Sweden.
Population: A total of 21,189 unselected pregnant women.
Methods: The scan was performed at one center in the first trimester and at the remaining four centers in the second trimester. Outcome measures resulting from first trimester scanning were compared with those from the second trimester scanning.
Main outcome measures: Detection rates of fetal structural anomalies and chromosomal aberrations. Results. At the first trimester scan 13% of all anomalies were detected, and at the second trimester scan 29% were detected. Lethal anomalies were detected at a high level at both times: 88% in the first, 92% in the second. The percentage of chromosomal aberrations discovered at the early scan was 71%, in the later 42%. The percentage of heart malformations detected was surprisingly low.
Conclusion: The results showed the advantages of the later scan in discovering anomalies of the heart, urinary tract and CNS, and of the early scan in discovering chromosomal aberrations. Lethal malformations were detected at a high level in both groups, but detection of heart malformations needs improvement.