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Loss of a pregnancy, which means the fertilized egg, embryo or fetus dies and will typically be expelled. This is very common in pregnancy's early stages.
This article is pending medical review.
Written by Julian Zeegers
Reviewed by Britte Megens and Sophie Oppelt
Edited by Juliëtte Gossens
10-15% of clinically recognized pregnancies are lost at some point during pregnancy. Additionally, 5% of carriers undergo a second pregnancy loss and a further 1% even more losses. This phenomenon is also known as ‘recurrent pregnancy loss’ (1). Pregnancy loss especially occurs during the first trimester, and the odds of miscarriage decline over the weeks after. Moreover, many pregnancies are lost without carriers even knowing about them. Carriers might confuse pregnancy loss with regular menstruation. As a result, these pregnancies are not clinically recognized. Below, we discuss some common causes and risk factors of miscarriage, and possible further steps if miscarriages recur.
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Causes of miscarriage
Reasons for recurrent pregnancy loss could be chromosome abnormalities. Chromosomes are very large molecules inside the cell that carry all the genetic information of an individual. Sometimes, there are errors in these molecules, causing them to behave differently or not functionally behave at all. An estimated 65-70% of miscarriages are due to these chromosome errors (1).
Every human cell contains 46 chromosomes, found in 23 pairs, each consisting of a maternal chromosome and a paternal chromosome. The first 22 pairs are called autosomes, which are numbered from large to small. These autosomes contain genetic information about all traits that are not linked to sex. The last pair, the 23rd pair, consists of the sex chromosomes, which determine the biologically acquired sex. Most males have sex chromosomes X and Y, and most females have two X chromosomes.
The most frequent errors that lead to miscarriages are trisomy 16 and trisomy 22 (1). These terms mean that there are three copies of chromosome 16 and 22, respectively, while there should only be two (read more about trisomies here). Monosomy is the most frequent chromosome error of the sex chromosomes. This means that there is only one chromosome present instead of two, for example only X or only Y.
Risk factors for miscarriage
There are several risk factors for miscarriage. For instance, when the pregnant parent is older, this is associated with an increased risk for miscarriage. This is mainly due to the accumulation of cell division errors, causing the previously discussed chromosomes to be not properly separated. This can lead to the aforementioned trisomies and monosomies (1).
Other risk factors on the parental side include smoking, consumption of alcohol, and obesity. Another major risk factor that is oftentimes not well known, is the procedure of invasive prenatal testing: chorionic villus sampling and amniocentesis are examples of invasive prenatal testing and pose a small risk of miscarriage. Read more about invasive prenatal testing here.
I have experienced several miscarriages. What now?
Future parents who have experienced several miscarriages in the past, might understandably be worried about their chances to give birth to a healthy new-born. They might want to know what the exact cause is of their miscarriages and how to solve this problem.
There are several methods for miscarriage chromosome testing. One method to examine chromosomes is conventional cytogenetics. This method isolates several cells from the placenta that contain DNA of the future new-born. These cells are duplicated in the laboratory and visualized using a karyotype (1).
A karyotype is a picture that visualizes every chromosome inside a cell in the correct order and paired. Based on this overview, the trisomies and monosomies can be identified. Another phenomenon that can be extracted from karyotypes is chromosomal translocations. A chromosomal translocation occurs when two chromosomes from different pairs exchange pieces of genetic information with each other. Since translocations can lead to severe chromosome abnormalities, these could be very helpful in finding the cause for recurrent pregnancy loss (1).
Another method to investigate chromosomes is Single Nucleotide Polymorphism (SNP) microarray. Essentially, SNPs are very tiny locations on chromosomes that are a little bit different between individuals, causing the genome products (the proteins produced according to the genetic code) to be slightly altered. This may result in functional changes later on. By using the SNP microarray, doctors can identify the DNA sequence of the fetus to then compare it with the carrier’s sequence. This allows for the identification of the parental origin of the chromosomal abnormality (1). In other words, this is useful to find out which parent the chromosomal abnormality comes from.
Miscarriages can be very burdensome for prospective parents and cause emotional hardship. Genetic testing might give some clarity about the cause of these miscarriages. Additionally, prognosis counseling increases the chances of future pregnancy successes (1). That is, if the cause of the miscarriages is known, actions could be taken to prevent future occurrences. Parents are thus given a chance to carry a baby to term.
McQueen DB, Lathi RB. Miscarriage chromosome testing: Indications, benefits and methodologies. Semin Perinatol. 2019;43(2):101-4. DOI: 10.1053/j.semperi.2018.12.007
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