In the case of mosaic DS the data was controversial. Richard, 1974 analyzed maternal age at the time of mosaic DS births and estimated that in 20% of cases, the extra chromosome arose through a mitotic non-disjunction in an euploid embryo soon after fertilization (11). On the other hand, Pangalos et al., 1994, found that a meiotic error led to trisomy 21 in 10 out of 17 cases of mosaic DS. The mean maternal age was 31.4 years. A mitotic error was the most likely mechanism in the remaining cases, which had lower mean maternal age (27.4 years). This mitotic origin might support offering the same recurrence risk and genetic counseling as full trisomy 21 (12).

Other maternal factors include higher socioeconomic status, which is due in part to maternal age (13). Mothers of Down syndrome children had more significant illnesses before conception, particularly psychological illness, and more medication ingestion in the year before conception. These remained statistically significant when adjusted for each other and for maternal age. Unfortunately, specific medications were not identified in this study (14). Families with histories of Alzheimer's disease are more likely to have Down syndrome offspring (15). However, statistical power may have been lacking in most of them , and non of these factors were present in our study (16).