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Prenatal Genetic Counseling

By

Jeffrey S. Dungan

, MD, Northwestern University, Feinberg School of Medicine

Last full review/revision Oct 2019| Content last modified Oct 2019
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Preconception reproductive options include

Postconception reproductive options include

Preimplantation genetic testing (PGT) is used to identify genetic defects in embryos created through in vitro fertilization before they are implanted. It may be done if a couple has a high risk of certain mendelian disorders or chromosomal abnormalities. There are 3 forms of PGT:

  • PGT-M: Testing for monogenic (ie, single-gene) abnormalities

  • PGT-A: Testing for aneuploidy

  • PGT-SR: Testing for structural rearrangements such as unbalanced translocations

Information presented at genetic counseling should be as simple, nondirective, and jargon-free as possible to help anxious couples understand it. Frequent repetition may be necessary. Couples should be given time alone to formulate questions. Couples can be told about information that is available on the Internet ( www.acog.org) for many common problems, such as advanced maternal age, recurrent spontaneous abortions, previous offspring with neural tube defects, and previous offspring with trisomy (see Risk Factors for Complications During Pregnancy Risk Factors for Complications During Pregnancy Risk factors for complications during pregnancy include Preexisting maternal disorders Physical and social characteristics (eg, age) Problems in previous pregnancies (eg, spontaneous abortions)... read more Risk Factors for Complications During Pregnancy ).

Many couples (eg, those with known or suspected risk factors) benefit from referral to genetic specialists for presentation of information and testing options. If a fetal abnormality is suspected, patients can be referred for ongoing care to a center that specializes in neonatal care.

Risk Factors for Congenital Disorders

Some risk of genetic abnormality exists in all pregnancies. Among live births, incidence is

  • 0.5% for numeric or structural chromosomal disorders

  • 1% for single-gene (mendelian) disorders

  • 1% for multiple-gene (polygenic) disorders

Among stillbirths, rates of abnormalities are higher.

Risk of having a fetus with a chromosomal disorder Overview of Chromosomal Anomalies Chromosomal anomalies cause various disorders. Anomalies that affect autosomes (the 22 paired chromosomes that are alike in males and females) are more common than those that affect sex chromosomes... read more is increased for most couples who have had a previous fetus or infant with a chromosomal disorder (recognized or missed), except for a few specific types (eg, 45,X; triploidy; de novo chromosomal rearrangements). Chromosomal disorders are more likely to be present in the following:

Rarely, a parent has a chromosomal disorder that increases risk of a chromosomal disorder in the fetus. Asymptomatic parental chromosomal disorders (eg, balanced abnormalities such as certain translocations and inversions) may not be suspected. A balanced parental chromosomal rearrangement should be suspected if couples have had recurrent spontaneous abortions, infertility, or a child with a malformation.

The chance of a fetal chromosomal disorder increases as maternal age increases because rates of nondisjunction (failure of chromosomes to separate normally) during meiosis increase. Among live births, the rate is about

  • 0.2% at age 20

  • 0.5% at age 35

  • 1.5% at age 40

  • 14% at age 49

Some chromosomal disorders are submicroscopic and thus not identified by traditional karyotyping. The submicroscopic chromosomal abnormalities, sometimes called copy number variants, occur independently of the age-related nondisjunction mechanisms. The precise incidence of these abnormalities is unclear, but incidence is higher in fetuses with structural abnormalities. A multicenter study sponsored by the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) demonstrated a 1% incidence of clinically relevant copy number variants in fetuses with normal karyotypes independent of indication for testing and a 6% incidence in fetuses with structural abnormalities (1) Risk factors for congenital disorders reference Prenatal genetic counseling is provided for all prospective parents, ideally before conception, to assess risk factors for congenital disorders. Certain precautions to help prevent birth defects... read more .

An autosomal dominant Autosomal Dominant Genetic disorders determined by a single gene (Mendelian disorders) are easiest to analyze and the most well understood. If expression of a trait requires only one copy of a gene (one allele)... read more disorder is suspected if there is a family history in more than one generation; autosomal disorders affect males and females equally. If one parent has an autosomal dominant disorder, risk is 50% that the disorder will be transmitted to an offspring.

For an autosomal recessive Autosomal Recessive Genetic disorders determined by a single gene (Mendelian disorders) are easiest to analyze and the most well understood. If expression of a trait requires only one copy of a gene (one allele)... read more disorder to be expressed, an offspring must receive a pathogenic gene variant for that disorder from both parents. Parents may be heterozygous (carriers) and, if so, are usually clinically normal. If both parents are carriers, offspring (male or female) are at a 25% risk of being homozygous for the pathogenic gene variant and thus affected, 50% are likely to be heterozygous, and 25% are likely to be genetically normal. If only siblings and no other relatives are affected, an autosomal recessive disorder should be suspected. Likelihood that both parents carry the same autosomal recessive trait is increased if they are consanguineous.

Because females have two X chromosomes and males have only one, X-linked recessive disorders X-Linked Recessive Genetic disorders determined by a single gene (Mendelian disorders) are easiest to analyze and the most well understood. If expression of a trait requires only one copy of a gene (one allele)... read more are expressed in all males who carry the pathogenic gene variant. Such disorders are usually transmitted through clinically normal, heterozygous (carrier) females. Thus, for each son of a carrier female, risk of having the disorder is 50%, and for each daughter, risk of being a carrier is 50%. Affected males do not transmit the gene to their sons, but they transmit it to all their daughters, who thus are carriers. Unaffected males do not transmit the gene.

Risk factors for congenital disorders reference

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Prenatal Genetic Counseling and Evaluation
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