* This is the Professional Version. *
Prenatal Genetic Counseling
(See also General Principles of Medical Genetics.)
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 (eg, avoiding teratogens, taking supplemental folate—see page Diet and supplements.) are recommended for all women who are planning to become pregnant. Parents with risk factors are advised about possible outcomes and options for evaluation. If testing identifies a disorder, reproductive options are discussed.
Preconception options include
Postconception options include
Preimplantation genetic diagnosis 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 or has a high risk of certain mendelian disorders or chromosomal abnormalities.
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.marchofdimes.com ) for many common problems (eg, advanced maternal age, recurrent spontaneous abortions, previous offspring with neural tube defects, previous offspring with trisomy). 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.
Some risk of genetic abnormality exists in all pregnancies. Among live births, incidence is
Among stillbirths, rates of abnormalities are higher. Most malformations involving a single organ system (eg, neural tube defects, most congenital heart defects) result from polygenic or multifactorial (ie, also influenced by environmental factors) inheritance.
Risk of having a fetus with a chromosomal disorder 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
Most chromosomal disorders due to older maternal age involve an extra chromosome (trisomy), particularly trisomy 21 (Down syndrome). Paternal age > 50 increases risk of some spontaneous dominant mutations, such as achondroplasia, in offspring.
Some chromosomal disorders are submicroscopic and thus not identified by traditional karyotyping. The submicroscopic chromosomal abnormalities 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.
An autosomal dominant 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 disorder to be expressed, an offspring must receive a mutant gene 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 mutant gene 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 2 X chromosomes and males have only one, X-linked recessive disorders are expressed in all males who carry the mutation. 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.
* This is the Professional Version. *