When an egg meets a sperm (or gametes), a delicate symphony of genetic information is fused. Half of that genetic information is given by the maternal parent and the other half is given by the paternal half. The gametes must be as perfect as possible to create a healthy, viable baby. That means each gamete (egg and sperm) should possess twenty-three chromosomes for a grand total of forty-six. Any more or any less is disastrous to the fetus.
A recent Yale University study took a look at the genetic make-up of fetuses that were miscarried before reaching a viable birthing age. Then, they analyzed the genetic karyotype, (which counts the number of chromosomes) of the fetuses to determine the causes of miscarriage if possible. The study revealed what is fairly well known; first trimester miscarriages are usually the result of a chromosomal abnormality. The most common abnormality was the presence of an extra chromosome, known as a trisomy. The other common chromosomal problem is a missing chromosome giving the genetic genotype of 45X (the individual is missing an X or Y chromosome).
The most common trisomy conditions that affect the ability of a fetus to survive are: Trisomy 2 (Mosaicism), Trisomy 13 (Patau Syndrome), Trisomy 15, Trisomy 16 (the most common of all the trisomy syndromes to cause miscarriage in the first trimester), Trisomy 18 (Edward’s Syndrome), and Turner’s Syndrome. The reason for the variance in what trisomies are lethal and which are not depends on the chromosome. Some additions cause extraordinary failure in fetal development or even placental development. Turner Syndrome however, is a missing chromosome, specifically a sex chromosome (X or Y) and is not a trisomy.
These chromosomal abnormalities can be identified pre-birth through PGD (pre-genetic determination) or amniocentesis. In PGD, the embryo is test for genetic defects prior to being returned to the mother’s uterus. In this way, the embryos that are used are those without genetic defects. This process is used for families who have suffered losses or knowingly carry severe genetic disorders and would like to have an unaffected child. Amniocentesis takes place after a pregnancy has occurred. A sample of the amniotic fluid is sampled and tested for genetic defects.
Most fatal genetic anomalies occur at conception. A large portion of them will miscarry in the first trimester and will never come to term. Some of the above mentioned disorders do not miscarry in the first trimester. Many of them will be born early through pre-term labor. Sadly, some will also be born and will only live a short period of time. In either case, a chromosomal abnormality can have serious effects on the fetus.