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Most of our knowledge of CPM stems from cases based on CVS. CPM is detected in 1–2 percent of pregnancies undergoing CVS, most commonly in the form of trisomy mosaicism.^{57, 66–68} Low levels of trisomy confined to the placenta typically do not have a significant effect on fetal growth and development. Although high levels of trisomy will generally affect placental function, chromosomal abnormalities that may be lethal to the fetus are often tolerated to some degree when confined to the placenta. For example, trisomy 16 can be present in very high levels in the placenta as long as the fetus is entirely (or mostly) diploid.⁶⁹ Placentas associated with CPM16 are small and there is almost always FGR, as well as an increased risk for malformations, maternal PE, and PTB.^{69, 70} Importantly though, the babies born in conjunction with CPM16 typically do quite well after birth once separated from the abnormal placenta.^{71, 72}

The level and distribution of the abnormal cells depends, in part, on how and when the mosaicism arose in development.⁷³ Selection may also favor contribution of diploid cells to the embryo.⁷⁴ Furthermore, some trisomies may be tolerated in the inner core of the villi, but not in the trophoblast component (e.g. trisomy 8). The fact that trisomy may be present at low levels or patchy in its distribution^{55, 65} (due to independent clonal development of each of the ∼50–70 villous trees) can lead to apparent “false‐positive” and “false‐negative” diagnostic results using CVS. CPM has been identified using NIPT testing. The level of trisomy detected by NIPT, compared with CVS, should represent more of an average across the placental trophoblast.^{75, 76} Surprisingly though, NIPT has also missed CPM even when high levels of placental trisomy were present.^{77, 78} This is possibly explained by trisomy‐specific differences in trophoblast shedding into maternal circulation⁷⁶ or by absence of the trisomy from incorporation into the syncytiotrophoblast and EVTs, from which the “fetal” DNA likely originates.