It is common for pregnant women to have a screening sonogram at 18 weeks’ gestation. An anatomy scan is done to look for congenital anomalies. Brain, heart, kidneys, lungs, and spine are examined.
The presence of either acute or chronic maternal illness during pregnancy may lead to complications in the developing fetus. Chronic conditions may expose the fetus to maternal medications that are potentially teratogenic. Acute illnesses such as varicella, Lyme disease, and cytomegalovirus expose the fetus to infectious agents that may cause birth defects. Other factors, such as maternal smoking, alcohol use, and maternal exposure to radiation or chemicals, also may necessitate genetic counseling.
Two percent to four percent of newborns have a genetic abnormality or birth defect. This broad definition of a birth defect includes not only a visible malformation but also functional defects that might not be apparent at birth.
Birth defects have a significant impact on childhood morbidity and mortality. Almost 11% of childhood deaths can be traced to a genetic cause. If contributing genetic factors related to childhood deaths are considered, this increases to almost 25%. Consultation with a geneticist for a newborn or infant may be prompted by many different findings, including the presence of a malformation, abnormal results on a routine newborn screening test, abnormalities in growth (e.g., failure to gain weight, increase in length, or abnormal head growth), developmental delay, blindness or deafness, and the knowledge of a family history of a genetic disorder or chromosomal abnormality or (as a result of prenatal testing) the presence of a genetic disorder or chromosomal abnormality in the infant.
Adolescents and adults may be seen by a geneticist for evaluation of a genetic disorder that has late onset. Some neurodegenerative disorders, such as Huntington disease and adult-onset spinal muscular atrophy, present later in life. Some forms of hereditary blindness (retinal degenerative diseases) and deafness (Usher syndrome, neurofibromatosis type 2) may not show significant symptoms until adolescence or early adulthood. Genetic consultation also may be prompted for a known family history of a hereditary cancer syndrome(breast, thyroid, colon, and ovarian cancers). Individuals may wish to have testing done to determine if they carry a mutation for these syndromes and would be at risk for developing certain types of cancers. A known family history or personal history of a genetic disorder or chromosomal abnormality might prompt testing in anticipation of pregnancy planning.
A pedigree usually is drawn to help visualize various inheritance patterns. Answers to questions about the family help determine if there is an autosomal dominant, AR, X-linked, or sporadic disorder. When a child is affected with the new onset of an AD disorder, it is necessary to closely examine the parents to check for the presence of manifestations. If the parents are unaffected, the child’s condition is most likely the result of a new mutation; the risk of recurrence is extremely low (although not 0, because of the possibility of gonadal mosaicism in one of the parents). When one parent is affected (even mildly so due to varying penetrance), the recurrence risk rises to 50%. With X-linked disorders, the focus is on the maternal family history to determine if there is a significant enough risk to warrant testing.
Questions about the couple’s age are important to ascertain the risk related to maternal age for chromosome abnormalities and paternal age for new mutations leading to AD and X-linked disorders. A history of more than two spontaneous abortions increases the risk that one of the parents has a balanced translocation and the spontaneous abortions are due to chromosomal abnormalities in the fetus.