Is a Routine Ultrasound in the Third Trimester Justified? - Additional Fetal Anomalies Diagnosed After Two Previous Unremarkable Ultrasound Examinations

Purpose: The detection of fetal structural abnormalities is a routine part of antenatal care. Ultrasound examination policies vary among European countries. An additional ultrasound for fetal structural anomalies in the 3rd trimester seems important for many reasons. Some abnormalities develop or first become apparent later in pregnancy. The aim of the study was to analyze the rate of previously undetected structural abnormalities in the 3rd trimester. Materials and Methods: 8074 fetuses were examined in this prospective study between 1998 and 2008. All received an examination at 11 - 14 weeks and 79% (6378/8074) of these fetuses also received an ultrasound at 20 - 24 weeks. If both of the examinations did not detect structural abnormalities, the fetuses were included in the study group and received an ultrasound at 28 - 32 weeks (n = 5044). New structural abnormalities in the 3rd trimester were analyzed. The outcome was available in 96% of the cases. Results: The overall incidence of fetal malformations was 3.6% (n = 289), of which 40% (116/289) were diagnosed at 11 - 14 weeks. The following 2nd trimester scan revealed 102 (35%) new fetal malformations. An additional 44 (15%) structural abnormalities were found in the 3rd trimester. These were mainly abnormalities of the urogenital system (n = 18). There were also anomalies of the cardiovascular system (n = 9), the gastrointestinal system (n = 6) and the central nervous system (n = 4). One of the cases had a previously undiagnosed abnormal karyotype. Conclusion: The ultrasound examination in the 3rd trimester is of additional benefit and can detect previously unknown structural abnormalities. These findings are relevant for perinatal management and postnatal follow-up.