We experienced a case of advanced maternal age in which a fetus was found to be positive for trisomy 18 at re-examination following indeterminate non-invasive prenatal genetic testing (NIPT), the amniotic fluid chromosomal test revealed a normal karyotype, and confined placental mosaicism (CPM) was observed in an SNP microarray analysis of the placenta. The child was born with no defects or complications. In the present case, the result of the original NIPT at week 15 of pregnancy was indeterminate and the subsequent re-examination result was positive; since the definitive normal diagnosis was not reported until the latter half of week 21, the pregnant patient was subjected to psychological stress for a long period of time. The problem with NIPT is that most of the fetus-derived cell-free DNA in the maternal blood is not derived directly from the fetus but from the villus cells of the placenta, leading to indefinite diagnoses; for that reason, the pregnant patient was subjected to psychological stress for a long period of time. Of the 18,251 cases undergoing NIPT in the past 2 years in Japan, 51 had indeterminate results; this was the second case in which a subsequent re-examination gave a positive result for trisomy 18.