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

氏综合症和 18 三综合征是两种传疾病，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，这被称为三；或者少一条，称为单。

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号三胎儿不能存活，要想存活就必须在胚胎早期丢失一条，这一过程称为三自救。