Amniocentesis

The removal of amniotic fluid by using a needle through the maternal gravid abdomen has been done since 1877. Initially, it was reported as a therapeutic modality to release amniotic fluid from fetuses with polyhydramnios or to help in the management of fetuses affected by rhesus isoimmunization. Subsequently, following the discovery that cultured amniotic fluid cells could be used to obtain fetal karyotype, amniocentesis was and continues to be used for genetic diagnosis. Additionally, it is used for fetal treatment in cases such as polyhydramnios.[1]

Multiple indications for amniocentesis exist. The most common use of amniocentesis is to diagnose fetal chromosomal anomalies after other tests such as ultrasound or biophysical markers have determined a significant likelihood that the fetus will be affected with a chromosomal problem. Other diagnostic tests performed using amniocentesis are determining fetal lung maturity and evaluating alloimmunization. Amniocentesis can also be used to obtain samples to rule out chorioamnionitis in cases in which the clinical picture is unclear, as well as to deliver intra-amniotic dye in cases in which premature rupture of membranes is suspected.[2, 3, 4, 5]

Therapeutic indications for an amniocentesis may include the direct delivery of medications to the unborn fetus and to release intrauterine pressure in the presence of polyhydramnios.

Contraindications

For the most part, amniocentesis is a safe procedure for the mother. As with any procedure involving the skin, an active skin infection on the proposed needle placement site is a fetal and maternal contraindication. Contractions of the uterus are a relative contraindication. Decreased amniotic fluid or anterior placental position may hamper the success of an amniocentesis.[3, 5]

Best Practices

Sterility of the procedure is important because skin contaminants can enter the amniotic fluid and create a fetal infection. Before and after the procedure, fetal heart tones should be obtained to demonstrate fetal viability. The value of the procedure should always be weighed against the risks before an amniocentesis is performed.

Procedure Planning

Consent regarding testing, expectations, complications, and limitations should be clear to the mother, including the possibility of fetal puncture with the needle, preterm labor, infection, and miscarriage. Ideally, genetic amniocentesis should be performed at a gestational age early enough to give the patient the choice of termination of pregnancy legally—usually less than 20 weeks—and late enough in the pregnancy to reduce the chances of miscarriage and foot deformities. Therefore, this procedure is safest between 15 and 18 weeks gestation.[6] Amniocentesis for other indications may be performed later in pregnancy.

Complication Prevention

Real-time ultrasound guidance is generally used for amniocentesis. By using ultrasound, the operator can direct the needle directly toward the amniotic fluid and avoid other structures, such as maternal intraperitoneal organs (bowel, bladder), fetus, or placenta. Avoiding passing the needle through the placenta is preferable because this may create bleeding or placental separation. If the clinician cannot avoid placental insertion, take care to avoid the umbilical cord insertion site, placental edges, and large placental blood vessels.

Miscarriage after amniocentesis has been traditionally reported as 1 in 200, but most recent studies report the risk to be 1 in 1,000. Amniocentesis before 15 weeks gestation, use of large needles, multiple attempts, and unrecognized chorioamnionitis postprocedure are risk factors for miscarriage. The risk of talipes equinovarus is elevated if the amniocentesis is performed prior to 15 weeks gestation.[5, 7]

Additionally, maternal viral infections may be transmitted to the fetus via this procedure. The rate of transmission of HIV is associated with the mother not being on antiretrovirals,[8] and the risk of hepatitis B is associated with the maternal viral load at the time of the procedure.[9] Therefore, counseling regarding risk-benefit analysis should occur with those patients before the procedure.

The goal is to obtain a “clear tap,” indicated by return of clear amniotic fluid during the procedure, without inserting the needle through viable maternal and fetal organs, avoiding preterm contractions, and avoiding a miscarriage.

Patient Instructions

Patients need to be aware of the description of the procedure and the risks, including but not limited to fetal and or maternal infection, fetal damage, preterm labor, puncture of maternal organs, separation of the placenta, and possible miscarriage, which occurs between 1 case in 200 and 1 case in 1000 for genetic amniocentesis.

Additionally, genetic amniocentesis can detect all chromosomal anomalies and some but not all genetic diseases. Therefore, the patient needs to understand the limitations of this procedure and testing.

The patient does not need to be in a fasting state, although it may be recommended if the amniocentesis is performed at a gestational age when the fetus is viable, in the event that a rapid intervention such as a cesarean needs to be performed.

Amniocentesis is usually performed at the physician’s office ultrasound room or in the labor and delivery unit of a hospital equipped to manage complications. The mother’s blood type and group, number of fetuses, and a gross evaluation for anatomical anomalies should be performed beforehand. Fetal heart tones, placental position, and distribution of the amniotic fluid should be evaluated right before the procedure.

Real-time ultrasound with a transducer of 3.5 MHZ is usually used during obstetrical ultrasounds. A skin disinfectant (such as Betadine), a spinal needle (20-22 gauge), and sterile syringes to collect amniotic fluid are also needed. An 18-gauge spinal needle can be used if lots of amniotic fluid is being removed; a therapeutic amniocentesis done to treat polyhydramnios may involve removing 2-3 liters of fluid. Collection tubes should be readily available depending on the test(s) requested.

Anesthesia

Anesthesia is usually not necessarily during amniocentesis because the needle used has a small diameter; although, in some situations, a patient may benefit from the use of anesthesia or anxiolytics. Some discomfort may be experienced by the patient with the initial insertion of the needle into the skin.

Positioning

The patient usually lies in a dorsal lithotomy position to expose the skin of the abdomen to the operator. Sometimes the patient is asked to rotate her body to her left or right side, depending on the position of the placenta, fetus, and the distribution of amniotic fluid in the uterine cavity.

After the procedure is completed, the presence of fetal heart tones by ultrasound should be obtained and documented; also document bleeding through the needle insertion site, abdominal pain, or contractions. If the patient is Rh negative and nonsensitized, she will require 300 micrograms of Rhogam to prevent the generation of anti-D antibodies.

The most common complication after an amniocentesis is cramping; it usually lasts for less than 2 hours. Chorioamnionitis is rare but may have a fatal fetal outcome. Vaginal bleeding or leakage of fluid occurs in about 2% of cases, and most of the time this is self-limited.

Miscarriage after amniocentesis has been traditionally reported as 1 in 200, but most recent studies report the risk to be 1 in 1,000. Amniocentesis before 15 weeks gestation, use of large needles, multiple attempts, and unrecognized chorioamnionitis postprocedure are risk factors for miscarriage. The risk of talipes equinovarus is elevated if the amniocentesis is performed prior to 15 weeks gestation.[5, 7]

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]

The goals of pharmacotherapy are to reduce morbidity and prevent complications.

Class Summary

These agents are used when the patient is Rh negative and nonsensitized.

Rho(D) Immune Globulin (RhoGam UF Plus, Rhophylac, HyperRHO, WinRho SDF)

This agent suppresses the immune response and antibody formation to Rho(D) positive red blood cells by Rho(D) negative individuals, thereby preventing isoimmunization. The patient will require 300 micrograms of Rhogam to prevent the generation of anti-D antibodies.