A miscarriage, or spontaneous abortion, is defined as a pregnancy loss that occurs without outside intervention before the 20th week of pregnancy.
Pregnancy is so complex that there are many different ways for the process to get off course and for a miscarriage to occur.
For example, if there’s a chromosomal abnormality in the sperm or egg then the resulting zygote will have a problem.
One of these problems is called aneuploidy, which is when there are missing chromosomes or additional chromosomes.
For example, if there’s one member of a chromosome pair missing, that results in 45 chromosomes total, instead of the normal 46, it’s called a monosomy.
And if there’s one extra chromosome joining a pair, that results in 47 chromosomes total, and it’s called a trisomy.
Some types of aneuploidy are viable like Turner’s syndrome or Down syndrome, whereas many are not and lead to a miscarriage.
Another abnormality is polyploidy, and that’s when a zygote receives more than one set of 23 chromosomes from either the sperm or egg, resulting in three sets, totaling 69 chromosomes, or even four sets, totaling 92 chromosomes.
Polyploidy is generally not viable and leads to a miscarriage.
One more abnormality is a translocation.
It can either be balanced, where two nonhomologous chromosomes essentially trade equal amounts of DNA, or unbalanced, where the chromosomes exchange unequal amounts of DNA, resulting in either too many or too few copies of certain genes on the involved chromosomes.
Now even if a parent carries a balanced translocation, the sperm or egg may end up with an unbalanced translocation, and the zygote won’t have a normal number of genes.
Some translocations are viable, whereas many are not, and can lead to a miscarriage.
Now, let’s say that there are a normal number of chromosomes present, and that the zygote becomes a blastocyst and tries to implant, there are still many ways in which a miscarriage can occur.
First if the blastocyst fails to implant into the endometrial lining of the uterus, then it won’t find a blood supply and stops growing and get reabsorbed.