Miscarriage

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.

Secondly if the blastocyst implants in tissue other than the endometrial lining of the uterus, such as the Fallopian tube, it’s called an ectopic pregnancy.

In the majority of cases, ectopic pregnancies won’t have the space or blood supply necessary to support a developing pregnancy.

As a result, the blastocyst stops growing and is reabsorbed.

If the blastocyst does implant in the endometrium and continues its development, it relies on the corpus luteum to maintain the pregnancy.

And one hypothesis is that a miscarriage can occur if the corpus luteum fails to secrete enough progesterone.

Finally, as the pregnancy continues and the placenta develops, the fetus begins to rely on the placenta to maintain the pregnancy.

If the placenta is unable to secrete sufficient levels of hormones or provide the fetus with a sufficient blood supply at any point, the fetus can stop growing and a miscarriage can occur.

During the embryonic period, which is between the third and eighth week of pregnancy, the fetus is especially vulnerable to damage from teratogens, which refers to anything that disturbs normal development – potentially resulting in a birth defect or death of the fetus.

The list of environmental teratogens is long, and it includes things like medications such as isotretinoin, alcohol, recreational drugs like cocaine, tobacco smoke, and heavy metals like mercury.

Vital organs like the heart and brain are developing quickly during the embryonic period and teratogens can cause them to have severe abnormalities.

Between weeks 10 through 14 of pregnancy, chorionic villus sampling can be performed to assess the growing fetus.

That’s when either a needle or a catheter is used to grab a small sample of the placenta, so that a genetic analysis can be done on fetal cells from the placenta.

Around week 15, amniocentesis can be performed instead of chorionic villus sampling.

That’s when a needle is placed into the amniotic fluid, to pull out some fetal cells.

These procedures are usually well tolerated, but they both have a small risk of causing trauma and introducing an infection - both of which could lead to miscarriage.

Other traumatic causes of miscarriage include motor vehicle accidents and accidental falls.

As the fetus grows throughout the pregnancy, uterine abnormalities can also become an issue.