Williams Manual of Pregnancy Complications, 23 ed.

CHAPTER 4. Procedures Used in Prenatal Diagnosis

Common techniques used for prenatal diagnosis include second-trimester amniocentesis, early or first-trimester amniocentesis, chorionic villus sampling, and fetal blood sampling. The reader is referred to Williams Obstetrics, 23rd edition for discussion of more specialized and investigational procedures such as fetal tissue biopsy, preimplantation genetic diagnosis, and analysis of fetal cells in the maternal circulation.

SECOND-TRIMESTER AMNIOCENTESIS

Amniocentesis is a safe and accurate method of genetic diagnosis and is usually performed between 15 and 20 weeks’ gestation. As shown in Figure 4-1, sonographic guidance is used to pass a 20- to 22-gauge spinal needle into the amnionic sac while avoiding the placenta, umbilical cord, and fetus. The initial aspirate of 1 to 2 mL of fluid is discarded to decrease the chance of maternal cell contamination, approximately 20 mL of fluid is collected for karyotype analysis, and the needle is removed. The uterine puncture site is observed sonographically for bleeding, and fetal cardiac motion is documented at the end of the procedure. The procedure-related fetal loss rate is approximately 1 in 300 to 500. Minor complications are infrequent and include transient vaginal spotting or amnionic fluid leakage in 1 to 2 percent and chorioamnionitis in less than 0.1 percent. Needle injuries to the fetus are rare.

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FIGURE 4-1 Amniocentesis. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)

EARLY (FIRST-TRIMESTER) AMNIOCENTESIS

Amniocentesis is termed early if performed between 11 and 14 weeks. The technique is the same as for traditional amniocentesis, although puncture of the sac may be more challenging due to lack of membrane fusion to the uterine wall. Less fluid is typically withdrawn, approximately 1 mL for each week of gestation. Early amniocentesis has higher rates of postprocedural complications than traditional amniocentesis. Rates of fetal loss, clubfoot (talipes equinovarus), amnionic fluid leakage, and cell culture failure are all higher. For these reasons, early amniocentesis is not recommended.

CHORIONIC VILLUS SAMPLING

The primary advantage of chorionic villus sampling (CVS) is that results are available earlier in pregnancy, which lessens parental anxiety when results are normal and, allows earlier and safer methods of pregnancy termination when they are abnormal. Biopsy of chorionic villi is generally performed at 10 to 13 weeks. Samples may be obtained transcervically or transabdominally, depending on which route allows easiest access to the placenta (Figure 4-2). The indications for CVS and amniocentesis are essentially the same, except for a few analyses that specifically require either amnionic fluid or placental tissue.

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FIGURE 4-2 Transcervical chorionic villus sampling (CVS). (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)

Complications of CVS are similar to those for amniocentesis. The incidence of amnionic fluid leakage or infection is less than 0.5 percent. Early reports suggested an association between CVS and limb-reduction defects and oromandibular limb hypogenesis. However, when the procedure is performed by an experienced operator after 10 weeks, the incidence of these defects is not increased above the background rate.

FETAL BLOOD SAMPLING

Fetal blood sampling, also called percutaneous umbilical blood sampling or cordocentesis, is performed primarily for assessment and treatment of confirmed red cell or platelet alloimmunization and in the evaluation of nonimmune hydrops. Often when severe fetal anemia is suspected, Doppler evaluation of the fetal middle cerebral artery peak systolic velocity is first performed (see Chapter 9, p. 86). Fetal blood sampling is also used to obtain fetal blood cells for genetic analysis when CVS or amniocentesis results are confusing or when rapid diagnosis is necessary, as karyotyping usually can be accomplished within 24 to 48 hours. Blood can be analyzed for metabolic and hematological studies, acid–base analysis, viral and bacterial cultures, polymerase chain reaction and other genetic techniques, and immunological studies.

Under direct sonographic guidance, the operator uses a 22-gauge spinal needle to puncture the umbilical vein, usually at or near its placental origin, and blood is withdrawn (Figure 4-3). Arterial puncture is avoided because it may result in vasospasm and fetal bradycardia. Complications may include cord vessel bleeding or hematoma, fetal-maternal hemorrhage, and fetal bradycardia. The overall procedure-related fetal death rate is cited to be 1.4 percent, but varies according to the indication as well a fetal status.

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FIGURE 4-3 Fetal blood sampling. Access to the umbilical vein varies depending on placental location and cord position. With an anterior placenta, the needle may traverse the placenta. Inset: With posterior placentation, the needle passes through amnionic fluid before penetrating the umbilical vein. Alternately, a free loop of cord may be accessed. (Reproduced, with permission, from Cunningham FG, Leveno KJ, Bloom SL, et al (eds). Williams Obstetrics. 23rd ed. New York, NY: McGraw-Hill; 2010.)


For further reading in Williams Obstetrics, 23rd ed.,

see Chapter 13, “Prenatal Diagnosis and Fetal Therapy.”