Why Do Chromosomal Abnormalities Cause Miscarriage and Stillbirth?

Many miscarriages are caused by chromosomal abnormalities

Chromosomal abnormalities are a common culprit in miscarriage and stillbirth. Given that many babies are born with genetic conditions such as Down Syndrome and other trisomies, why is it that some chromosomal abnormalities lead to miscarriage?

Potential Chromosome-Related Causes of Miscarriage

Of all miscarriage causes, chromosomal abnormalities are considered to be the most frequent explanation for why miscarriages happen.

Estimates suggest that anywhere between 40 and 75 percent of all miscarriages are caused by random genetic problems in the developing baby.

In most instances, scientists do not know the exact reason why chromosomal abnormalities lead to miscarriage. One theory is that the mother's immune system recognizes a problem in the developing baby's genes and thus ends the pregnancy. 

Another theory is that the developing baby ultimately reaches a point where the specific genetic problem causes the baby to stop growing. Certain genes might be missing that are necessary for continued development, or extra copies of certain genes might cause the baby or placenta to grow improperly. This could explain why some certain types of chromosomal abnormalities lead to miscarriage while others do not.

The simplest answer is that "it just happens." Cell division is a complex process with a lot of things that can go wrong, so it follows that sometimes things do go wrong.

A sperm or egg cell may end up with the wrong number of chromosomes or with chromosomes with missing or extra pieces, which ultimately go on to cause problems such as miscarriage, stillbirth, or genetic disorders.

Risk Factors Related to Chromosomal Abnormalities 

Most of the time, women who have one pregnancy affected by chromosomal abnormalities will go on to have a normal pregnancy since chromosomal issues are somewhat random in nature.

Chromosomal abnormalities typically do not recur unless one or both parents has a balanced translocation or similar genetic issue.

Parental age is one risk factor for having pregnancies affected by chromosomal abnormalities. In couples where the mother is over the age of 35, the risk of miscarriage grows and the frequency of chromosomal abnormalities appears to be higher. For men, the age at which miscarriage rates increase is unclear but is likely to be over 40 years old.

Researchers are investigating other risk factors for chromosomal abnormalities, but the data are not conclusive. For example, exposure to toxic chemicals may increase your risk of chromosomal abnormalities, but the exact relationship is not well understood.

If you are trying to get pregnant and have experienced multiple miscarriages get a referral for a fertility specialist or reproductive endocrinologist. Further genetic screening can help determine any underlying issues you or your partner may have. If there is a problem in the genetic makeup of your sperm or eggs, assistive reproductive technologies, such as screening the embryo before implantation, can help you achieve a viable pregnancy. 

Sources:

Genuis, Stephen J. "Health issues and the environment—an emerging paradigm for providers of obstetrical and gynaecological health care." Human Reproduction June 2006 2201-08. Accessed 25 Nov 2007.

Hanke, W. and J. Jurewicz. "The risk of adverse reproductive and developmental disorders due to occupational pesticide exposure: an overview of current epidemiological evidence." International Journal of Occupational and Medical Environmental Health 2004 223-43. Accessed 25 Nov 2007.

Slama, Remy, Jean Bouyer, Gayle Windham, Laura Fenster, and Axel Werwatz and Shanna H. Swan. "Influence of Paternal Age on the Risk of Spontaneous Abortion." American Journal of Epidemiology 2005 816-23. Accessed 25 Nov 2007.

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