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 Exposure to Teratogens 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 , taking supplemental folic acid Diet and supplements Ideally, women who are planning to become pregnant should see a physician before conception; then they can learn about pregnancy risks and ways to reduce risks. As part of preconception care... read more .) are recommended for all women who are planning to become pregnant. Parents with risk factors are advised about possible outcomes and options for genetic evaluation Genetic Evaluation Genetic evaluation is part of routine prenatal care and is ideally done before conception. The extent of genetic evaluation a woman chooses is related to how the woman weighs factors such as... read more . If testing identifies a disorder, reproductive options are discussed.
Preconception reproductive options include
Artificial insemination if the man is a carrier
Oocyte donation if the woman is a carrier
In vitro fertilization In vitro fertilization (IVF) Assisted reproductive techniques (ARTs) involve manipulation of sperm and ova or embryos in vitro with the goal of producing a pregnancy. ARTs may result in multifetal pregnancy, but risk is... read more (IVF) with preimplantation genetic testing of embryos Preimplantation Genetic Testing (PGT) Genetic evaluation is part of routine prenatal care and is ideally done before conception. The extent of genetic evaluation a woman chooses is related to how the woman weighs factors such as... read more
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:
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 ).
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.
(See also page General Principles of Medical Genetics Overview of Genetics A gene, the basic unit of heredity, is a segment of DNA containing all the information necessary to synthesize a polypeptide (protein). Protein synthesis, folding, and tertiary and quaternary... read more .)
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 Overview of Congenital Neurologic Anomalies Congenital brain anomalies usually cause severe neurologic deficits; some may be fatal. Some of the most serious neurologic anomalies (eg, anencephaly, encephalocele, spina bifida) develop in... read more , most congenital heart defects Overview of Congenital Cardiovascular Anomalies Congenital heart disease is the most common congenital anomaly, occurring in almost 1% of live births (1). Among birth defects, congenital heart disease is the leading cause of infant mortality... read more ) result from polygenic or multifactorial (ie, also influenced by environmental factors) inheritance.
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
Most chromosomal disorders due to older maternal age involve an extra chromosome (trisomy), particularly trisomy 21 (Down syndrome Down Syndrome (Trisomy 21) Down syndrome is an anomaly of chromosome 21 that can cause intellectual disability, microcephaly, short stature, and characteristic facies. Diagnosis is suggested by physical anomalies and... read more ). Paternal age > 50 increases risk of some spontaneous dominant pathogenic gene variants (formerly termed mutations), such as achondroplasia Osteochondrodysplasias (Osteochondrodysplastic Dwarfism) Osteochondrodysplasias involve abnormal bone or cartilage growth, leading to skeletal maldevelopment, often short-limbed dwarfism. Diagnosis is by physical examination, x-rays, and, in some... read more , in offspring.
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.