Amniocentesis Technique

After appropriate consent has been obtained, the patient is placed on an examination bed in the dorsal lithotomy position. If the patient cannot tolerate that position, the head of the bed may be raised for patient comfort. A bedside ultrasound is done to demonstrate fetal number, placental location, fetal heart tones, and amniotic fluid location.

At that moment, the recommendation is that the clinician generate a plan in which a straight needle can be inserted from the maternal skin to a sizeable pocket of amniotic fluid while avoiding the fetus and umbilical cord. In the case of multiples, pay careful attention to the existence of different gestational sacs and the location of placental membranes.

If the flow of amniotic fluid stops when aspirating with a syringe, or if the amniotic fluid turns bloody, re-evaluation of the needle tip location should occur by ultrasound. The clinician may change the position of the needle accordingly. ^{[2, 5, 7]}

Some clinicians may mark the area they intend to insert the amniocentesis needle with a surgical pen if a free-hand technique is to be used. The ultrasound transducer is removed and covered with a sterile glove or plastic wrap, and the patient’s skin is prepared with an antiseptic solution and draped with sterile towels. Using sterile gloves and maintaining sterile techniques, the clinician places the ultrasound transducer, now covered with a sterile glove, on the patient’s skin surface. Sterile gel or antiseptic solution may be used to help with the sound transmission from the transducer.

The clinician may choose to use a needle guide to help with the needle insertion toward a clear pocket of amniotic fluid. Alternatively, the clinician may use the free-hand technique, by lining up the center of the transducer with the amniotic fluid pocket from where the sample is obtained. A 20-gauge to 22-gauge spinal needle with adequate length is placed in one side of the transducer at an angle almost parallel to the transducer. The clinician inserts the needle under direct ultrasound visualization until the tip of the needle is at the center of an amniotic fluid pocket. The clinician removes the guide of the needle and places a syringe on top of it, making sure the needle does not move during that step. Aspirating and discarding the first 2-4 mL of amniotic fluid (because it may be contaminated with maternal cells) is recommended.

After the syringe is full, or when the total amount of fluid needed is obtained, the syringe can be removed from the needle, and the amniotic fluid can be poured into containers depending on the test desired, or continued to be drained if the purpose is to decrease amniotic fluid volume in cases of polyhydramnios. When all the fluid that is desired has been obtained, the needle is pulled out in a rapid way to reduce discomfort, and fetal heart tones are assessed again at this time.

The correct analysis of fetal DNA, RNA, chromosomes, or metabolites obtained via amniocentesis can determine with accuracy the presence or absence of multiple genetic disorders. The most common lethal chromosomal disorders include trisomy 13 and trisomy 18. Other chromosomal and genetic diseases can be diagnosed accurately by genetic amniocentesis. This rapidly changing field has evolved to a point that new markers are found in a rapid way. Therefore, the clinician should verify the services available at that time from the reference laboratory before testing. A good reference can be found in the web page of the National Center for Biotechnology Information, part of the NIH.

Fluorescence in situ hybridization (FISH) can be used in uncultured cells for rapid detection of specific chromosomal monosomies, some trisomies, and triploidism. However, it cannot detect mosaics, translocation, or rare aneuploidies. The advantage is that it gives rapid information regarding certain chromosomal monosomy or triploidism, such as 13, 18, and 21, X or Y. ^[7]

In some cases, early delivery may be offered to a patient, but the decision may involve testing for fetal maturity prior to delivery. Amniocentesis is used to obtain amniotic fluid for lecithin/sphingomyelin ratio (L/S), phosphatidylglycerol, lamellar body counts, or fluorescence polarization, as these are ways to determine fetal lung maturity by amniotic fluid. ^[10]

The clinical value of amniocentesis to evaluate amniotic fluid for the presence of bacteria (with a gram stain, culture, or glucose level) remains unclear. ^[3]