Chromosome imbalance (aneuploidy) is the major cause of pregnancy loss and congenital disorders in humans. Analyses of small biopsies from human embryos suggest that aneuploidy commonly originates during early divisions, resulting in mosaicism. However, the developmental potential of mosaic embryos remains unclear. We followed the distribution of aneuploid chromosomes across 73 unselected preimplantation embryos and 365 biopsies, sampled from four multifocal trophectoderm (TE) samples and the inner cell mass (ICM). When mosaicism impacted fewer than 50% of cells in one TE biopsy (low-medium mosaicism), only 1% of aneuploidies affected other portions of the embryo. A double-blinded prospective non-selection trial (NCT03673592) showed equivalent live-birth rates and miscarriage rates across 484 euploid, 282 low-grade mosaic, and 131 medium-grade mosaic embryos. No instances of mosaicism or uniparental disomy were detected in the ensuing pregnancies or newborns, and obstetrical and neonatal outcomes were similar between the study groups. Thus, low-medium mosaicism in the trophectoderm mostly arises after TE and ICM differentiation, and such embryos have equivalent developmental potential as fully euploid ones.

染色体失衡（非整倍性）是人类流产和先天性疾病的主要原因。对人类胚胎小活检组织的分析表明，非整倍体通常起源于早期分裂，导致嵌合体。然而，马赛克胚胎的发育潜力仍不清楚。我们跟踪了 73 个未选择的植入前胚胎和 365 个活检样本中非整倍体染色体的分布，这些样本取自四个多灶性滋养外胚层 (TE) 样本和内细胞团 (ICM)。当嵌合体在一次 TE 活检（中低嵌合体）中影响不到 50% 的细胞时，只有 1% 的非整倍体影响胚胎的其他部分。一项双盲前瞻性非选择试验 (NCT03673592) 显示，484 个整倍体、282 个低级别马赛克的活产率和流产率相当，和 131 个中级镶嵌胚胎。在随后的妊娠或新生儿中未检测到嵌合体或单亲二体的情况，研究组之间的产科和新生儿结局相似。因此，滋养外胚层中的中低嵌合体主要出现在 TE 和 ICM 分化后，此类胚胎具有与全整倍体胚胎相当的发育潜力。