Non-invasive refers to non-invasive DNA testing. Noninvasive for twin fetuses can initially determine whether the fetus has any genetic abnormality, but there is a certain false-negative rate for twin fetuses due to the mixing of genes in twin fetuses. Non-invasive DNA can determine whether the fetus has trisomy 18 or trisomy 21 by monitoring fetal DNA fragments in maternal peripheral blood. If a genetic abnormality is detected, the pregnancy can be terminated early so that the baby will not be born with a serious genetic disorder. The rate of non-invasive DNA testing for monozygotic twins is the same as that for monozygotic twins, but in the case of dizygotic twins if the amount of DNA free from the normal fetus exceeds that of the abnormal fetus, this may result in a false negative. Mothers with twin pregnancies who want to determine whether their fetus has a genetic abnormality are advised to visit a prenatal diagnostic center for amniocentesis or cord blood testing.