Invasive Diagnostic Procedures

Technique of Amniocentesis

After a thorough ultrasonographic examination, a needle insertion site is selected. Under continuous real-time ultrasound guidance, the needle is inserted to the optimal pocket of amniotic fluid (AF) while avoiding the fetus. Avoiding needle insertion through the placenta is desirable but not mandatory. Although Tabor and coworkers reported that transplacental needle insertion increased the risk for the procedure, this has not been confirmed by others.

Local anaesthetic is typically not needed. It is uncertain if the site of needle placement affects the level of pain. Counselling before amniocentesis should emphasise that the actual pain and anxiety experienced during the procedure are significantly lower than expected.

The maternal skin is cleansed with an iodine- or alcohol-based solution; sterile drapes are then placed around the needle insertion site to help maintain an aseptic field. A disposable 22-gauge spinal needle with stylet is most frequently used. During the entire procedure, the needle tip should be continuously visualised using two-dimensional real-time ultrasound monitoring. In view of the extensive anterior placenta, this procedure was performed transplacentally. Use of four-dimensional ultrasound guidance has been suggested, but there are no objective data to indicate improved outcomes ( Fig. 23.1 and Video 23.1a, Video 23.1b ).

After confirming that the needle is in its proper location, the stylet is removed, and a 10- or 20-cc syringe is attached to the hub of the needle. The initial 1- to 2-mL sample is usually discarded. Ten to 20 mL of AF is usually aspirated into sterile disposable plastic syringes, although as little as 3 to 5 mL of AF has been shown to suffice for reliable prenatal cytogenetic results. Maternal cell contamination appears to occur more frequently in genetic amniocentesis samples that are obtained by physicians who perform fewer than 50 genetic amniocentesis annually. Investigators have described using a vacuum container aspiration technique for amniocentesis, but there appears to be little, if any, advantage over using a syringe technique.

Amniocentesis had been reported to be unsuccessful with rates as high as 5.9% to 10.6%, especially in the era when concurrent ultrasound guidance was not the norm. Because real-time ultrasound guidance has become routine, failure to obtain AF occurs far less. However, this is much more problematic with EA. When performed at 15 to 16 weeks of gestation by experienced practitioners, failure to obtain AF should occur in fewer than 1% of cases.

Training in performing amniocentesis has traditionally been by trainees observing experienced operators followed by the trainees performing the procedure under direct supervision of the mentors. Modern high-fidelity simulator-based models might be useful for teaching amniocentesis because trainees’ performance has been shown to improve with experience on the simulator.

Amniocentesis in Twins and Higher-Order Multiple Pregnancies

Amniocentesis can be performed successfully in most twin pregnancies perhaps with no increased risk compared with singleton gestations undergoing amniocentesis, although the risk is difficult to quantify accurately because of the lack of randomised studies.

Separate amniocentesis of each sac is used in most centres in the United States to assess individual fetuses irrespective of the chorionicity. Each amniotic sac may be identified if the clinician injects a dye (indigo carmine) immediately after aspiration of the first AF sample but before withdrawal of the needle. After completion of the first amniocentesis, a second amniocentesis is performed in the ultrasonographically located area of the other fetus. Aspiration of clear AF indicates that the second sac was successfully entered; aspiration of blue-tinged AF indicates that the original sac was reentered. However, this is not necessary to perform routinely because visualisation of the membranes separating the sacs is generally possible. Methylene blue dye is proscribed because it has been associated with high risk for small intestine atresia and fetal death.

Single-needle insertion under ultrasound guidance to sample both sacs in twins has been reported. The main concern is that the single-puncture technique could lead to cross-contamination between sacs, resulting in diagnostic inaccuracy. The technique described by Jeanty and colleagues uses a single myometrial needle puncture into the first amniotic sac and then through the membranous septum into the second sac. This technique has been validated by Sebire and coworkers, with no increase in cell contamination between twin fetuses or increased risk for pregnancy loss.

Using the above techniques, experienced investigators have been successful in obtaining information regarding both fetuses in more than 90% to 95%. The reported loss rates after amniocentesis in twins varies from 0.6% to 2.7%.

Amniocentesis has been performed in several triplet pregnancies, with successful aspiration of fluid from all gestational sacs. Still, data are insufficient to make any statement regarding risks of amniocentesis in triplet gestations.

Maternal Risks of Amniocentesis

Life-threatening maternal risks are extremely rare. Amnionitis occurs in approximately 1 per 1000 women who undergo amniocentesis. This may lead to fetal loss but is seldom life threatening to the mother.

Minor maternal problems, however, are not rare. Approximately 2% to 3% of women experience transient vaginal spotting or leakage of AF after amniocentesis. Although almost always limited in amount and duration, AF leakage could persist and lead to oligohydramnios and pregnancy loss. Oligohydramnios is a well-known cause of fetal deformation and pulmonary hypoplasia.

Uterine contractions or cramping immediately after amniocentesis are not rare. Again, expectant management and reassurance are generally all that is required.

Fetal Risks of Amniocentesis

Potential fetal risks include spontaneous abortion, injuries caused by needle puncture, placental separation, chorioamnionitis, premature labour and injury caused by the withdrawal of AF (e.g., amniotic bands). Rare but reported direct needle injuries include ileocutaneous fistula, peritoneoparietal fistula, gangrene of an arm, ocular trauma, ileal atresia, porencephalic cysts, patellar disruption, brain injuries, peripheral nerve injury and umbilical cord haematoma. Some of these problems are more logically attributed to amniocentesis than others, and all except a few of these case reports are from the era before concurrent use of real-time ultrasound guidance.

HIV Transmission After Amniocentesis

Amniocentesis has been associated with an increase in the rate of vertical transmission of human immunodeficiency virus (HIV) type 1. However, with the advent of retroviral chemoprophylaxis, the risk for transmission as a result of amniocentesis has been markedly reduced. Bucceri and coworkers reported nine HIV-infected women who underwent amniocentesis between 16 and 20 weeks of gestation. Six of these women were on chemoprophylaxis, and none of 10 infants born to these women were infected. The International Perinatal HIV Group reported that five of nine HIV-infected women not on chemoprophylaxis undergoing amniocentesis delivered infected infants, but none of five infants born to women taking zidovudine were infected. Recently, a 4-year report from an Italian registry including selected HIV-infected pregnant women receiving combined antiretroviral prophylaxis found no cases of vertical transmission after amniocentesis or CVS. It appears from this limited evidence that HIV-infected women electing to have amniocentesis may benefit from chemoprophylaxis.