ABSTRACT: Screening for fetal structural and chromosomal abnormalities has become a routine part of antenatal care. First-trimester fetal structural assessment is reported to have anomaly detection rates (DRs) ranging from 12.5% to 83.7%. First-trimester assessment of the fetal heart using an approach similar to that of the second-trimester anomaly scan is feasible, with similar sensitivity for detecting congenital heart disease (CHD). This 2-year prospective study was designed to screen a low-risk unselected population, using an extended first-trimester morphologic protocol, to determine whether DRs comparable to those of the second-trimester structural assessment for major fetal abnormalities were feasible.
The first-trimester ultrasound examination was performed at 12+0 to 13+6 weeks’ gestation and assessed the genetic and morphologic parameters suitable for evaluation at this gestation. Color Doppler cardiac sweep and contingent markers for early detection of major structural abnormalities were included in the protocol. All fetuses underwent a routine detailed second-trimester anomaly scan at 16 to 26 weeks’ gestation. Data included abnormalities diagnosed prenatally by ultrasound and those identified on postnatal neonatal and pathological examination. Increased nuchal translucency (NT) in itself was not considered an abnormality. Abnormalities were considered major if incompatible with life or associated with severe immediate or long-term morbidity. Moderate or minor abnormalities were those associated with short- or long-term morbidity of minor/moderate severity.
A total of 5472 consecutive pregnancies were evaluated. The median maternal age was 28 years, and the prevalence of lethal and severe malformations was 1.4% (76/5472). Fifty-eight (76.3%) of the 76 major congenital anomalies were diagnosed during the first trimester. Previously undetected major fetal abnormalities were discovered at later ultrasound examinations in 0.31% (16/5109) of the population scanned and in 0.04% (2/4744) of the population examined postpartum. Follow-up evaluations rather than the first-trimester scan detected 80 of 89 of the minor/medium anomalies (89.9%). The first-trimester scan identified 40.6% (n = 67) of the 165 cases detected. For severity of malformations, 86.57% and 18.37% of cases detected in the first trimester and at follow-up, respectively, had severe or lethal abnormalities. The first-trimester DRs were 90% (27/30) for major CHD and 69.5% (16/23) for major CNS anomalies. Major chromosomal abnormalities were diagnosed in 21 cases (0.38%), 20 during the first-trimester evaluation. A total of 288 fetuses (5.26% of the first-trimester study group) had NT (≥95th percentile); the prevalence of major anomalies was 8.68% (25/288), and their first-trimester DR was 96% (24/25). In 5184 fetuses with normal NT, the prevalence of major anomalies was 0.98%, and the first-trimester DR was 66.7%. The differences in DRs for major CHD between the increased versus normal NT groups were not significant. The first-trimester examination took a median of 34 minutes, about 10 minutes longer than usually allocated for this examination.
Although the first-trimester scan detected only ~40% of anomalies overall, its efficiency in identifying 76% of major malformations is one benefit. Before termination of abnormal fetuses, a detailed first-trimester protocol may provide information that would prove useful in counseling, particularly for future pregnancies.