Objective: This study assessed fetal outcomes for pregnancies identified at increased risk for Down syndrome by first-trimester combined ultrasound examination and maternal serum biochemistry screening.
Methods: First-trimester combined screening data were obtained from ultrasound clinics across Western Australia between August 2001 and October 2003. Prenatal screening data were linked with pregnancy outcome information held in state health database registers using probabilistic record-linkage techniques.
Results: In 50 of the 60 pregnancies affected by Down syndrome, the adjusted risk was greater than 1 in 300, providing a detection rate of 83% (95% confidence interval [CI] 74-93%). Among all women screened (n = 22,280), 827 had increased risk results but did not have a Down syndrome pregnancy, representing a false-positive rate of 3.7% (95% CI 3.5-3.9%). Ten cases of Down syndrome were detected among women considered not at increased risk, consistent with a false-negative rate of 1 in 2,227. First-trimester combined screening reduced the number of Down syndrome births by 50 in 22,280 (2.24 cases per 1,000 births), which represents the detection of one case of fetal Down syndrome for every 446 women screened. Furthermore, 25% of pregnancies with other birth defects occurred among those identified at increased risk of Down syndrome, and 1 in 8 pregnancies at increased risk were found to have a significant chromosomal or structural defect.
Conclusion: First-trimester combined screening in Western Australia detected 83% (95% CI 74-93%) of Down syndrome pregnancies at a 3.7% (95% CI 3.5-3.9%) false-positive rate.
Level of evidence: II-2.