Porth's Essentials of Pathophysiology, 4e

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Cell and Tissue Function

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in 1958, it has been used during pregnancy to deter- mine the number of fetuses, fetal size and position, the amount of amniotic fluid, and placental location. It also is possible to assess fetal movement, breathing move- ments, and heart pattern. There is also good evidence that early ultrasonography (i.e., before 14 weeks) accu- rately determines gestational age. Improved resolution and real-time units have enhanced the ability of ultrasound scanners to detect congenital anomalies. With this more sophisticated equipment, it is possible to obtain information such as measurements of hourly urine output in a high-risk fetus. Ultrasonography makes possible the in-utero diagnosis of hydrocephalus, spina bifida, facial defects, congenital heart defects, congenital diaphragmatic her- nias, disorders of the gastrointestinal tract, and skeletal anomalies. Cardiovascular abnormalities are the most commonly missed malformation. A four-chamber view of the fetal heart improves the detection of cardiac mal- formations. Intrauterine diagnosis of congenital abnor- malities permits planning of surgical correction shortly after birth, preterm delivery for early correction, selec- tion of cesarean section to reduce fetal injury, and, in some cases, intrauterine therapy. When a congenital abnormality is suspected, a diagnosis made using ultra- sonography usually can be obtained by weeks 16 to 18 of gestation. Maternal Serum Markers Maternal blood testing began in the early 1980s with the test for α -fetoprotein (AFP). Since that time, many serum factors have been studied as screening tests for fetal anomalies. Current maternal testing uses three distinct tests (AFP, human chorionic gonadotropin [hCG], and unconjugated estriol) to screen for trisomy syndromes in low-risk women while incorporating the detection of neural tube defects. 61 The combined use of the three maternal serum markers between 15 and 22 weeks of pregnancy has been shown to detect as many as 60% of Down syndrome pregnancies. 1 The use of ultrasound to verify fetal age can reduce the number of false-positive tests with this screening method. α -Fetoprotein is a major fetal plasma protein and has a structure similar to the albumin that is found in postnatal life. AFP is made initially by the yolk sac, gastrointestinal tract, and liver. Fetal plasma levels peak at approximately 10 to 13 weeks’ gesta- tion and then decline progressively until term, while maternal levels peak in the third trimester. 47 Maternal and amniotic fluid levels of AFP are elevated in preg- nancies where the fetus has a neural tube defect (i.e., anencephaly and open spina bifida) or certain other malformations such as an anterior abdominal wall defect in which the fetal integument is not intact. Screening of maternal blood samples usually is done between weeks 16 and 18 of gestation. 1,61 Although neural tube defects have been associated with elevated levels of AFP, decreased levels have been associated with Down syndrome. A complex glycoprotein, hCG is produced exclu- sively by the outer layer of the trophoblast shortly after

SUMMARY CONCEPTS (continued)

Prenatal Screening and Diagnosis The purpose of prenatal screening and diagnosis is not just to detect fetal abnormalities. Rather, it has the fol- lowing objectives: to provide parents with information needed to make an informed choice about having a child with an abnormality; to provide reassurance and reduce anxiety among high-risk groups; and to allow parents at risk for having a child with a specific defect, who might otherwise forgo having a child, to begin a pregnancy with the assurance that knowledge about the presence or absence of the disorder in the fetus can be confirmed by testing. 1 Prenatal screening cannot be used to rule out all possible fetal abnormalities. It is limited to determin- ing whether the fetus has (or probably has) designated conditions indicated by late maternal age, family his- tory, or well-defined risk factors. Screening and Diagnostic Methods Amongthemethodsusedforprenatalscreeninganddiagno- sis are ultrasonography, maternal serum (blood) screening tests, amniocentesis, chorionic villus sampling, and per- cutaneous umbilical fetal blood sampling 1,33,60 (Fig. 6-15). Amniocentesis, chorionic villus sampling, percutaneous umbilical cord blood sampling, and fetal biopsy are inva- sive procedures that carry a small risk for the fetus. Ultrasonography Ultrasonography is a non-invasive diagnostic method that uses reflections of high-frequency sound waves to visualize soft tissue structures. Since its introduction ■■ A number of environmental agents can be damaging to the unborn child, including radiation, environmental pollutants such as mercury, medications and illicit drugs, alcohol, and infectious agents. Because many of these agents have the potential for causing fetal abnormalities, often at an early stage of pregnancy, it is recommended that women of childbearing age avoid all unnecessary use of drugs and abstain from alcohol. ■■ The acronymTORCH stands for toxoplasmosis, other, rubella, cytomegalovirus, and herpes, which are the infectious agents most frequently implicated in fetal anomalies. ■■ It also has been shown that maternal folic acid deficiency can contribute to neural tube defects, and iodine deficiency can cause congenital hypothyroidism and impaired neurological development (cretinism).