Chromosomal abnormalities manifest themselves in two ways: 1)Spontaneous abortions 50%of human pregnancies--> spontaneous abortion or miscarriage nearly all during first trimester of pregnancy,with many during the first month,when pregnancy is recognized only by hormonal assays 50%of spontaneously aborted embryos and fetuses have chromosomal abnormalities Therefore 25%of all human embryos have chromosomal abnormalities.Chromosomal abnormalities manifest themselves in two ways: 1) Spontaneous abortions of human pregnancies --> spontaneous abortion or miscarriage of spontaneously aborted embryos and fetuses have chromosomal abnormalities nearly all during first trimester of pregnancy, with many during the first month, when pregnancy is recognized only by hormonal assays 25% of all human embryos have chromosomal abnormalities. 50% 50% Therefore