Down's syndrome and trisomy 18 syndrome are two genetic disorders that are often screened for as part of prenatal genetic testing.

唐氏综合症和 18 三体综合征是两种遗传疾病，通常作为产前基因检测的一部分进行筛查。

Sometimes though, one parent might contribute one chromosome too many, which is called trisomy, or one chromosome less, which is called monosomy.

但是有时候，双亲中的一方可能会贡献多于一条染色体，这被称为三体；或者少一条染色体，称为单体。

Since trisomy 15 isn't compatible with life, the fetus only survives only if they lose one copy of the chromosome in the early embryo, called trisomy rescue.

15号染色体三体胎儿不能存活，要想存活就必须在胚胎早期丢失一条染色体，这一过程称为三染色体自救。