To describe the potential impact of using noninvasive prenatal testing (NIPT) as a second‐tier test, on the diagnosis and outcomes of pregnancies identified as high risk through first trimester screening (FTS) in a cohort of real pregnancies.

Materials and Methods
Western Australian FTS and diagnostic data (2007–2009) were linked to pregnancy outcomes. Karyotype results from invasive prenatal testing in high‐risk women were analysed. The outcomes of abnormal results that would not be detected by NIPT, assuming a panel of trisomy 21/18/13 and sex chromosome aneuploidies, and the likelihood of diagnosis in a screening model using NIPT as a second‐tier test are described.

Abnormal karyotype results were reported in 224 of 1488 (15%) women with high‐risk pregnancies having invasive diagnostic testing. NIPT potentially would have identified 85%. The 33 abnormalities unidentifiable by NIPT were triploidies (n = 7, 21%), balanced (n = 8, 24%) and unbalanced rearrangements (n = 10, 30%) and level III mosaicisms (n = 8, 24%). For conditions not identifiable by NIPT, fetal sonographic appearance was likely to have led to invasive testing for 10 of 17 (59%) pathogenic abnormalities. If a policy was adopted recommending invasive testing for FTS risk >1:50 and/or ultrasound detected abnormality, the residual risk of an unidentified pathogenic chromosomal abnormality in those without a diagnosis would have been 0.33% (95% CI 0.01–0.65%).

A screening model with NIPT as a second‐tier for high‐risk pregnancies would be unlikely to have changed the outcome for the majority of pregnancies. Optimising the diagnosis of rare pathogenic abnormalities requires clear indicators for invasive testing over NIPT.