Objective: The accuracy of cell-free DNA aneuploidy screening varies by the chromosome assessed. The positive predictive value is consistently low for monosomy X (MX), at less than 30%. This study aims to investigate maternal age and other possible predictors of false-positive MX screening results in order to guide pre-test and post-test counselling. Methods: A total of 52 499 NIPT samples were tested over 69 months, across three specialist obstetric services. Outcome data were available for 96 out of 107 cases high risk for MX. Cytogenetic outcomes were compared to clinical and demographic data to look for trends that may indicate higher likelihood of a false-positive NIPT result. Results: The likelihood of a false-positive MX result significantly increased with the absence of ultrasound features suggestive of MX and with lower PAPP-A levels. Non-significant trends towards false-positive results were identified with increased maternal age, increased body mass index and Caucasian ethnicity. Conclusion: Maternal age is not a reliable predictor of a false-positive result. Assessment of ultrasound findings and placental serology in the first trimester is important for appropriate post-test counselling and should continue to be a part of screening even when NIPT is used as a first-tier screening test.