Although most couples are blissfully unaware of the statistics surrounding miscarriage, pregnancy loss is actually quite common, with 10-25% of recognized pregnancies ending in miscarriage. If you have suffered a pregnancy loss or are currently in the process of having a miscarriage, you may be wondering what caused the loss and worry about whether it will happen again. This article aims to answer the following questions:
- What causes miscarriage?
- How common is pregnancy loss?
- What type of genetic testing is available for miscarriage tissue?
- How can chromosome testing help?
Causes of Miscarriage
There are many different reasons why miscarriage occurs, but the most common cause for first trimester miscarriage is a chromosome abnormality. Chromosome abnormalities – extra or missing whole chromosomes, also called “aneuploidy” – occur because of a mis-division of the chromosomes in the egg or sperm involved in a conception. Typically, humans have 46 chromosomes that come in 23 pairs (22 pairs numbered from 1 to 22 and then the sex chromosomes, X and Y). For a baby to develop normally it is essential that it have exactly the right amount of chromosome material; missing or extra material at the time of conception or in an embryo or fetus can cause a woman to either not become pregnant, miscarry, or have a baby with a chromosome syndrome such as Down syndrome.
Over 50% of all first trimester miscarriages are caused by chromosome abnormalities. This number may be closer to 75% or higher for women aged 35 years and over who have experienced recurrent pregnancy loss. Overall, the rate of chromosome abnormalities and the rate of miscarriage both increase with maternal age, with a steep increase in women older than 35.
Pregnancy Loss – How Common is it?
Miscarriage is far more common than most people think. Up to one in every four recognized pregnancies is lost in first trimester miscarriage. The chance of having a miscarriage also increases as a mother gets older.
Most women who experience a miscarriage go on to have a healthy pregnancy and never miscarry again. However, some women seem to be more prone to miscarriage than others. About five percent of fertile couples will experience two or more miscarriages.
Of note, the rate of miscarriage seems to be increasing. One reason for this may be awareness – more women know they are having a miscarriage because home pregnancy tests have improved early pregnancy detection rates over the past decade, whereas in the past the miscarriage would have appeared to be just an unusual period. Another reason may be that more women are conceiving at older ages.
Types of Genetic Testing Helpful for Miscarriages
Genetic testing actually refers to many different types of testing that can be done on the DNA in a cell. For miscarriage tissue, also called products of conception (POC), the most useful type of test to perform is a chromosome analysis. A chromosome analysis (also called chromosome testing) can examine all 23 pairs of chromosomes for the presence of extra or missing chromosome material (aneuploidy). Because so many miscarriages are caused by aneuploidy, chromosome analysis on the miscarriage tissue can often identify the reason for the pregnancy loss.
The most common method of chromosome analysis is called karyotyping. Newer methods include advanced technologies such as microarrays.
Karyotyping analyzes all 23 pairs of chromosome but requires cells from the miscarriage tissue to first be grown in the laboratory, a process called “cell culture”. Because of this requirement, tissue that is passed at home is often unable to be tested with this method. About 20% or more of miscarriage samples fail to grow and thus no results are available. Additionally, karyotyping is unable to tell the difference between cells from the mother (maternal cells) and cells from the fetus. If a normal female result is found, it may be the correct result for the fetus or it may be maternal cell contamination (MCC) in which the result actually comes from testing the mother’s cells present in the pregnancy tissue instead of the fetal cells. MCC appears to occur in about 30% or more of the samples tested by traditional karyotype. Results from karyotyping usually take a few weeks to months to come back from the laboratory.
Microarray testing is a new type of genetic testing done on miscarriage samples; the two most common types of microarray testing are array CGH (comparative genomic hybridization) and chromosome SNP (single-nucleotide polymorphism) microarray. Microarray testing is also able to test all 23 pairs of chromosomes for aneuploidy, but does not require cell culture. Therefore, you are more likely to receive results and the results are typically returned faster when microarray testing is used. Additionally, some laboratories are collecting a sample of the mother’s blood at the same time the miscarriage tissue is sent to enable immediate detection of maternal cell contamination (MCC).
Chromosome Testing – How can it help?
If a chromosome abnormality is identified, the type of abnormality found can be assessed to help answer the question: “Will this happen to me again?”. Most of the time, chromosome abnormalities in an embryo or fetus are not inherited and have a low chance to occur in future pregnancies. Sometimes, a specific chromosome finding in a miscarriage alerts your doctor to do further studies to investigate the possibility of an underlying genetic or chromosome problem in your family that predisposes you to have miscarriages.
Furthermore, if a chromosome abnormality is identified it can prevent the need for other, sometimes quite costly, studies your doctor might consider to investigate the cause of the miscarriage.
Lastly, knowing the reason for a pregnancy loss can help a couple start the emotional healing process, moving past the question of “Why did this happen to me?”.
Chromosome testing can be especially important for patients with repeated miscarriages, as it can either give clues to an underlying chromosomal cause for the miscarriages or rule out chromosome errors as the reason for the miscarriages and allow their doctor to pursue other types of testing. For couples with multiple miscarriages determined to have a chromosomal cause, in vitro fertilization (IVF) with preimplantation genetic diagnosis (PGD) testing may be able to help increase their chances of having a successful healthy pregnancy.
For more information about microarray genetic testing for miscarriage tissue or IVF with PGD visit www.genesecurity.net