Genetic Counseling in Pregnancy

Pregnant woman on counseling appointment

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Every human being has genes. These determine hair color, eye color, and other traits, and are packed into 46 chromosomes inside our cells. The majority of cells in the human body have 23 pairs of chromosomes, but eggs only have 23 unmatched chromosomes each, because they are waiting to match up with sperm (which also have only 23 unmatched chromosomes).

When pregnancy begins and the egg and sperm are joined, you start with a brand new cell, with 46 chromosomes. Each gene is either dominant (meaning it takes only one copy of the gene to have the disease or trait) or recessive (it takes two copies of the gene to have the disease or trait).

Dominant Disorders

Examples of dominant disorders are Marfan syndrome, Huntington disease, breast and ovarian cancer (the BRCA gene), and neurofibromatosis type 1. If a parent has a dominant gene for a certain condition, there is a 50% chance that each child would have the condition.

In an X-linked dominant disorder, the mutated gene is on the X chromosome, one of the two sex chromosomes in each cell. Males only have one X chromosome, but because the trait is dominant, it can cause the disorder. Females can also have the disorder with only one copy of the mutated gene. An example of an X-linked dominant disorder is fragile X syndrome.

Recessive Disorders

If only one parent has a recessive gene, then the child will not have the condition or trait. If both parents are carriers of a recessive gene, there is a one in four chance that each child would inherit the condition (one-fourth of offspring will not have the gene; one half will be carriers, like the parents; one fourth will be affected with the condition). Examples of recessive disorders include sickle cell anemia, Tay-Sachs, and phenylketonuria (PKU).

There are also X-linked recessive disorders, in which the mutated gene is on the X chromosome, but since it is recessive, females would need to have two copies in order to have the condition. This is less common than males having the condition, because males only need one copy of the gene to have the condition. An example of an X-linked recessive disorder is hemophilia.

Types of Testing

Noninvasive prenatal testing (NIPT): This test is run on maternal blood and can look for many common genetic disorders, including Down syndrome. It can be done as early as 10 weeks into pregnancy and does not pose a risk of harm to the baby or pregnancy. However, it has a high rate of being inconclusive, depending on when the test is done, whether it is a multiple pregnancy or whether a donor egg was used.

Quad screen: This is a blood test done between 15 and 20 weeks of pregnancy. There is no risk to the baby during this screening. The mother's blood is screened for one or more substances (alpha-fetoprotein, hCG, estradiol). Higher than normal levels may indicate a neural tube defect while lower values may indicate certain chromosomal disorders, usually Down syndrome.

Ultrasound: A mid-pregnancy ultrasound screening (usually performed between 18 and 22 weeks) checks the baby's anatomy for issues such as kidney problems, heart defects, and limb defects. This procedure does not detect all defects and has not been shown to be helpful in determining Down syndrome in a fetus. A good ultrasound does not indicate that you will not have a baby with a defect, just decreases the likelihood.

Amniocentesis: This test will screen for all known chromosomal defects by sampling fetal cells in the amniotic fluid. It is done with the placement of a needle, guided by ultrasound, into the uterus to collect the fluid. It is usually done between 15 and 18 weeks of gestation. It normally takes two weeks to receive the results.

The results can be very accurate, however, they cannot tell you the severity of a present defect. There is also a risk to the baby from this procedure. About 1 in 700 women will miscarry after the amniocentesis, even if the baby was unaffected, and a tiny percentage may experience uterine infection. 

Chorionic villus sampling (CVS): CVS can be done earlier in pregnancy. A small needle is used to perform a targeted placental biopsy. This test is less accurate than amniocentesis and the rates of complications are higher. The rate of miscarriage following CVS is between 2% and 3%.

Who Should Be Tested?

According to the March of Dimes, anyone who has had unanswered questions about diseases or traits in their family should consider genetic counseling and screening. Particularly:

  • Those who have a family history of birth defects.
  • Women who are pregnant or planning to become pregnant after the age of 35.
  • Couples who already have a child with an inherited disorder or a birth defect.
  • Couples who have a newborn diagnosed with a genetic disease through routine screening.
  • Women who have had three or more miscarriages or babies who died in infancy.
  • People who are concerned that their lifestyles, jobs, or medical history may pose a risk to pregnancy, including exposure to drugs, radiation, chemicals, infection, or medications.
  • Couples who would like testing or more information about a genetic defect that occurs more frequently in their ethnic group.
  • Couples who are first cousins or other blood relatives.
  • Pregnant women who, based on screening tests, have been told their pregnancy may be at increased risks for complications or birth defects.

Ultimately, the decision is yours. There is a wide range of genetic testing, from non-invasive to maximally invasive. Results can be troubling and require you to make very difficult decisions regarding continuing a pregnancy. A genetic counselor can help you sort out all the data and make informed, loving choices.

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Article Sources
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  2. U.S. National Library of Medicine. What are the different ways in which a genetic condition can be inherited?. Updated May 26, 2020.

  3. Grinshpun-Cohen J, Miron-Shatz T, Ries-Levavi L, Pras E. Factors that affect the decision to undergo amniocentesis in women with normal Down syndrome screening results: it is all about the ageHealth Expect. 2015;18(6):2306-2317. doi:10.1111/hex.12200

  4. Bakker M, Birnie E, Robles de Medina P, Sollie KM, Pajkrt E, Bilardo CM. Total pregnancy loss after chorionic villus sampling and amniocentesis: a cohort study. Ultrasound Obstet Gynecol. 2017;49(5):599-606. doi:10.1002/uog.15986

  5. March of Dimes. Prenatal tests. Updated May 2017.

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