A 35-year-old primigravida, pregnant after in-vitro fertilization, was seen because of a trisomy 13/trisomy 18 (T13/T18) risk of 1:55, based on the result of her first-trimester combined test. She elected for non-invasive prenatal testing (NIPT) at 14 + 5 weeks' gestation, which was positive for T13. After counseling, the patient elected to undergo amniocentesis. Quantitative fluorescence polymerase chain reaction (QF-PCR) showed no signs of trisomy, and full karyotyping confirmed a normal 46,XY result. Analysis of the published literature on NIPT for T13 gives an overall detection rate of 91.6%, with a false-positive rate of 0.097%. Based on this detection rate, hypothetical calculations show that the positive predictive value is highly dependent on the prevalence of the disease, resulting in an unfavorable balance between benefit and harm in a general population.