Prenatal Genetics: All You Need to Know

In modern obstetrics, the standard of care includes various prenatal screening and tests that can produce a range of results. Together with your patient and family history, certain test results can furthermore trigger a need for genetic testing of you, your partner, and/or your fetus. Genetic testing can provide you with information regarding the presence or absence of various genetic disorders, which are disorders resulting from abnormalities in genes, or in the structure of chromosomes that hold the genes.

Virtually all mothers-to-be are offered prenatal screening tests, in the form of ultrasound and blood testing. Subsequently, if needed, you will be offered prenatal diagnostic tests to determine whether a condition that a screening tests has suggested might be present actually is present. Such testing is performed on cells originating either in the fetus or the placenta. Such cells are obtained by way of amniocentesis or chorionic villus sampling (CVS), performed sometime in the middle of pregnancy. Generally, amniocentesis (the sampling, via a needle, of amniotic fluid and cells contained within it) is performed at or subsequent to the 15th gestational week up to the 20th week. CVS, which is the sampling of tissue from the placenta, can be performed somewhat earlier, as early as the 10th week.

One condition that genetic analysis of fetal or placental cells can reveal is aneuploidy, meaning the absence of chromosomes or the presence of extra chromosomes. One example of aneuploidy is trisomy, in which there are three copies of a particular chromosome instead of two. Monosomy, in contrast, is the absence of a chromosome, such that a particular chromosome is present only as one copy.

In addition to chromosomal abnormalities, genetic testing also can reveal the presence of inherited mutations of particular genes. Mutations may cause a recessive disease when the mutation is present in both copies (maternal and paternal) of a gene. Such conditions include sickle cell disease (SCD), Tay-Sachs disease, cystic fibrosis and phenylketonuria (PKU). These are recessive diseases, because typically they result from the lack of normal gene to produce a particular substance, such as an enzyme (a chemical that enables chemical reactions), whereas the presence of just one normal gene produces enough of the substance for normal function. In some cases, however, a recessive disease is not completely recessive, because people who have just one normal gene can suffer mild symptoms. With the sickle cell gene, for instance, people who are heterozygous (have one normal gene and one abnormal gene and therefore are carriers for the condition) can suffer a sickle cell crisis, if they are put under severe stress, as happens during extreme exercise, high altitude, or dehydration. In such cases, people who are heterozygous, in addition to being carriers, are said to have the ‘trait’ for the condition. Sickle cell trait (SCT) is such an example. People with SCT are usually not as sick as people with SCD, but SCT can be a problem in athletes. Consequently, SCT screening tests are conducted for children who participate in sports.

In addition to testing during pregnancy, genetic analysis also can be conducted on embryos prior to pregnancy –that is in settings of in vitro fertilization (IVF), in which embyros are created outside of the womb, tests for abnormal genetic sequences and abnormal chromosomes can be conducted on the embryos prior to implantation into the would-be mother. In such cases, embryos can be selected for implantation based on their genetic health. In recent years, this capability has been subject to criticism, because, in addition to revealing potential disease, genetic testing also can reveal desirable traits, and gender, opening the possibility that parents will collaborate with physicians to create designer babies. Although current regulations prohibit genetic tampering with healthy human embryos that are destined for implantation in a mother, there is no framework for preventing the selection of embryos based on non-disease factors. The process of IVF creates more embryos than can be made into babies for any couple, and thus some level of selection is inevitable.

As for the decisions regarding which tests should be conducted on whom, there are a range of risk factors associated with each particular condition. SCD and SCT are much more common in people whose ancestors came from regions where malaria is endemic. That’s because having SCD actually protects a person from malaria. Tay-Sachs disease notoriously afflicts Ashkenazi Jews, but it also affects certain French Canadians and some other genetic groups, as it can result from a variety of mutations within the particular gene whose abnormalities lead to the condition. Given the complexity of all of this, genetic counselors play a major role in the process of determining which couples may need particular tests. Based on your family, and on your own history, your obstetrician may refer you to genetic counseling.

David Warmflash
Dr. David Warmflash is a science communicator and physician with a research background in astrobiology and space medicine. He has completed research fellowships at NASA Johnson Space Center, the University of Pennsylvania, and Brandeis University. Since 2002, he has been collaborating with The Planetary Society on experiments helping us to understand the effects of deep space radiation on life forms, and since 2011 has worked nearly full time in medical writing and science journalism. His focus area includes the emergence of new biotechnologies and their impact on biomedicine, public health, and society.

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