Second-trimester genetic sonogram for detection of fetal chromosomal abnormalities in a community-based antenatal testing unit.

OBJECTIVE: To evaluate the efficacy of the second-trimester genetic sonogram for the detection of Down syndrome and other chromosomal abnormalities in a community-based antenatal testing unit.

METHODS: This was a retrospective study using data from two community hospital antenatal ultrasound units. Six hundred and sixty fetal ultrasound examinations in both at-risk (n = 581) and low-risk (n = 79) pregnancies were performed from 15 + 0 to 22 + 6 weeks' gestation and all cases were verified for outcome data. The sonographic detection of a major congenital anomaly or a sonographic marker (increased nuchal skinfold, short humerus, short femur, echogenic bowel, pyelectasis, echogenic intracardiac focus, absence or hypoplasia of fifth mid phalanx or choroid plexus cyst) was recorded. The entire group of 660 ultrasound examinations as well as subgroups with and without non-ultrasound risk factors for a fetal chromosomal abnormality were analyzed to determine the sensitivity, specificity, positive and negative predictive values and positive likelihood ratio for the detection of Down syndrome and other fetal chromosomal abnormalities.

RESULTS: There were 32 (4.85%) chromosomal abnormalities in our study population. Twelve (3.75%) of these were Down syndrome, of which eight (66.6%) had a positive ultrasound examination in the second trimester. Six of seven (85.7%) of the trisomy 18 fetuses, 2/2 of the trisomy 13 fetuses and 2/3 of the non-mosaic 45,X fetuses had positive sonograms. The overall detection rate for chromosomal abnormalities was 20/32 (sensitivity, 62.5%; specificity, 80.7%; negative predictive value, 97.7%; positive predictive value, 14.2%; positive likelihood ratio, 3.24). Major structural defects and sonographic markers, excluding hypoplastic fifth digit and choroid plexus cyst, occurred significantly more frequently in Down syndrome cases compared with normal ones.

CONCLUSIONS: In a community-based antenatal testing unit we have demonstrated a detection rate for fetal Down syndrome with the second-trimester genetic sonogram that is comparable to the range of sensitivities reported by larger centers involving primarily high-risk patients. However, only 12 of the 32 fetal chromosomal abnormalities that we encountered were Down syndrome.