Natural selection from egg to live birth
Humans produce a good amount of abnormal eggs, which contain one or several chromosomes more or less than normal. Having even one chromosome less or more will have a huge negative impact on the egg’s and later embryo’s chance to develop to live birth. The incidence of such eggs grows with the age of a woman.
The process of natural selection eliminates such abnormal embryos — they just stop at some point due to their internal reasons. There will be always more eggs than embryos on day 3, and more embryos on day 3 than on day 5. At the same time the percentage of abnormal embryos will decrease from day 1 to day 5. Still 1 in 4 morphologically good embryos is abnormal even on day 5 in the age around 30 and 3 in 4 morphologically good blastocysts are abnormal in the age around 40.
A chromosomally abnormal embryo may look under the microscope very good even at the age of 5 days after long natural selection, even better than all other embryos in the cohort and be selected for the embryo transfer.
The earlier embryonic stage the more intensive the natural selection process is. Still as you see at least 1 in 4 blastocysts is still abnormal and can be chosen for embryo transfer. Hence the natural selection process will continue in-utero and may result in early miscarriage or no implantation at all, if the embryo had one or several chromosomes less or more than needed.
The majority of embryos with incorrect chromosome number will stop in development even before pregnancy can be diagnosed by blood test. Some abnormal embryos, if having been transferred in the uterus, will result in positive pregnancy test (biochemical pregnancy test) but will not be found by an ultrasound examination (will not turn into a clinical pregnancy. Only very low percentage of abnormal embryos can make it to clinical pregnancy and almost all of them will be miscarried before 12 weeks. The nature has a very powerful and strong control over humans as species and does not allow the species to deviate from the 46 chromosome pattern. Only selected types of chromosome mistakes are compatible with development throughout the whole pregnancy and may result in life birth of a baby with certain health problems (for example Down syndrome (trisomy 21) or Turner syndrome (45, XO).
Conclusion: Abnormal chromosome number impairs embryonic developmental processes and is the major reason for implantation failure or early pregnancy loss