Obstetric Issues, Labor, and Delivery

Published on 03/06/2015 by admin

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Chapter 3

Obstetric Issues, Labor, and Delivery

Amniocentesis is a procedure that involves the aspiration of amniotic fluid from the amniotic sac during pregnancy. It is generally carried out with a spinal needle (20–22 gauge) in a transabdominal approach, using a sterile technique under continuous ultrasound guidance. ∗

Amniocentesis can be classified by the time in the pregnancy when it is done and by its indication. In the second trimester amniocentesis is most often performed for genetic indications. Before 15 weeks’ gestation the procedure is called “early” amniocentesis, and 1 milliliter of amniotic fluid per week of gestation is obtained. However, early amniocentesis is gradually being abandoned because it is associated with a high rate of subsequent amniotic fluid leakage (premature rupture of membranes). The majority of amniocenteses for prenatal diagnosis are done between 15 and 20 weeks’ gestation.

In the third trimester amniocentesis is most often performed for fetal lung maturity testing. In the setting of preterm labor or preterm rupture of the membranes, amniocentesis can be used to evaluate possible intraamniotic infection or inflammation. It can also be done for special diagnostic procedures such as polymerase chain reaction for cytomegalovirus in the setting of intrauterine growth restriction (IUGR) or hydrops. Amniocentesis can be helpful in reducing amniotic fluid volume in the setting of polyhydramnios with either premature labor or maternal respiratory difficulty. It is also used for twin-twin transfusion associated with polyhydramnios in one fetus. This type of amniocentesis is often called reduction amniocentesis.

In Rh and other blood group isoimmunizations, amniocentesis has traditionally been used for bilirubin assessment using ΔOD 450, but it is being done far less frequently now because middle cerebral artery Doppler has been found to be extremely accurate in predicting the degree of fetal anemia. In this setting amniocentesis is used to determine whether the fetus is Rh positive or positive for the sensitized antigen so that testing can be avoided if the fetus is not at risk. ∗

Immediate and preliminary (1- to 3-day) results can be obtained for cytogenetics using fluorescence in situ hybridization. Definitive chromosome studies require cultured amniocytes (cells from amniotic fluid) and therefore usually require 10 to 14 days.

5. What options, aside from amniocentesis, are available for prenatal diagnosis?

image Available tests for fetal chromosome evaluation are classified as “diagnostic” (the result is a definitive karyotype) or “screening” (the result quantifies the risk of aneuploidy).

image Chorionic villus sampling: This is the only alternative to amniocentesis that is considered “diagnostic”; it is performed in the first trimester (9 to 12 weeks). This procedure involves either transvaginal or transabdominal ultrasound-guided needle aspiration of a small amount of placental tissue and can be used for cytogenetic, biochemical, or DNA testing. The procedure-related loss rate is 0.8%.

image Preimplantation genetic diagnosis: This is an adjunct to in vitro fertilization. One or more cells are removed from the developing embryo 2 to 4 days after fertilization and then analyzed. Only normal embryos are selected for implantation. When the parents are carriers of an adverse genetic trait, it may obviate the need for testing during pregnancy. It is not considered “diagnostic” for karyotype, however, because of the high rate of mosaicism.

image Fetal free DNA screening from maternal blood: This is considered an “advanced screening test” because of very high sensitivity and specificity (>99%) for trisomy 21 and other common aneuploidies. Introduced in late 2011, this testing is currently very expensive and recommended only for women who have one or more risk factors for aneuploidy (based on maternal serum screening, ultrasound screening, advanced maternal age, family history). In women without risk factors, the positive predictive value is not yet known.

image Second-trimester ultrasound: Many structural fetal defects (e.g., anencephaly, omphalocele) can routinely be seen in patients who undergo ultrasound scanning during the second trimester. Other defects, such as major cardiac defects, can be seen most of the time depending on the sophistication of the center, type of equipment, patient body habitus, and other factors. In addition, many fetuses with chromosome abnormalities including trisomy 13, 18, and 21 syndromes will have findings that will lead to subsequent amniocentesis to confirm the diagnosis.

image Combinations of first-trimester ultrasound and first-trimester or second-trimester maternal serum screening: These screening tests involve ultrasound evaluation of nuchal translucency at 11 to 14 weeks’ gestation, maternal serum levels of human chorionic gonadotropin (hCG) and pregnancy-associated plasma protein A (PAPP-A) at 10 to 14 weeks’ gestation, and “triple

image KEY POINTS: AMNIOCENTESIS AND GENETIC DIAGNOSIS

marker” (alpha-fetoprotein (AFP), hCG, estriol) or “quadruple marker” (triple marker plus inhibin-A) at 15 to 20 weeks’ gestation. Depending on which combination of tests is performed, detection of Down syndrome is 60% to 95% with a 5% screen positive rate. Reasonable detection rates are also achieved for trisomy 18 and open neural tube defects. ∗†‡§image∗∗††

Third-trimester hemorrhage refers to any bleeding from the genital tract during the third trimester of pregnancy. In practice, it refers to any bleeding that occurs from the time of viability, (i.e., 23 to 24 weeks’ gestation). The common causes are classified as placenta previa (7%), placental abruption (13%), and other bleeding (80%), including local lesions of the lower genital tract, vasa previa, early labor, trauma, neoplasia, and marginal placental separation. Such bleeding complicates about 6% of pregnancies. ∗

Ultrasound visualization is the method of choice for diagnosis of placenta previa. Multiple reports show a transvaginal approach to be safe and superior in its accuracy compared with transabdominal ultrasound. ∗

Digital vaginal examination is not recommended when bleeding occurs until placenta previa is excluded by performing an ultrasound examination.

Pearls

If ultrasound is not available in late pregnancy, a useful approach is the double setup examination in which two teams prepared to administer anesthesia are in the operating room. A vaginal examination is performed. If bleeding results from a placenta previa, an emergency cesarean section is performed by the second team.

Placenta previa occurs in 1 in 200 deliveries at term. Complete placenta previa is detected in 5% of second-trimester gestations, with 90% resolving by term; partial placenta previa is seen in 45% of second-trimester gestations and resolves in more than 95% of cases. This apparent resolution is most likely related to the growth of the lower uterine segment in late pregnancy, so the placenta appears to move away from the os.

Placental abruption is the separation of the normally implanted placenta before the birth of the fetus. It results from bleeding from a small arterial vessel into the decidua basalis. It is termed a revealed abruption when vaginal bleeding is present (90%) and a concealed abruption if no bleeding is visible (10%). It is uniquely dangerous to the fetus and the mother because of its serious pathophysiologic sequelae. The incidence varies but averages about 0.83% or 1 in 120 deliveries. Abruption severe enough to cause fetal death is less common (approximately 1 in 420 deliveries). ∗†

In subsequent pregnancies the recurrence risk of placental abruption is between 6% and 16%; after two consecutive abruptions the risk is 25%. Women who have a placental abruption severe enough to cause fetal death have a 7% risk of a similar outcome in a subsequent pregnancy.

Maternal complications

Fetal and neonatal complications

Fetomaternal hemorrhage is caused by a disruption of the normal barrier at the placental-decidual interface. It may occur with abruptio placentae; however, it occurs more commonly with abruptio placentae associated with maternal trauma, with maternal trauma without abruptio placentae, or spontaneously without an apparent precipitating event. Approximately 5% of stillbirths without apparent cause are the result of fetomaternal hemorrhage. The diagnosis is made by performing a Kleihauer–Betke test on maternal blood, which allows quantification of fetal cells in maternal serum. In patients with spontaneous fetomaternal hemorrhage, the presenting symptom is decreased fetal movement. If the fetus is still alive and the hemorrhage is severe enough, the diagnosis is often made because of a sinusoidal fetal heart rate (FHR) tracing. Treatment can consist of immediate delivery if the fetus is near term or intrauterine transfusion if the fetus is premature and no abruption is apparent. ∗

image KEY POINTS: THIRD-TRIMESTER HEMORRHAGE

The Centers for Disease Control and Prevention (CDC) protocol, as well as the American Academy of Pediatrics (AAP) and American College of Obstetricians and Gynecologists (ACOG) guidelines, recommend prophylactic treatment with penicillin or ampicillin for women in labor who are positive for Group B streptococcus (GBS). There has been a 50% reduction in the rate of neonatal GBS infection in institutions since the CDC protocols were introduced, which underscores the usefulness of appropriate chemoprophylaxis. There is some evidence, however, that part of this reduction in GBS disease has been offset by an increase in gram-negative perinatal infections.

For pencillin-allergic women with high risk of anaphylaxis (as in the present case), the CDC’s 2010 guidelines recommend susceptibility testing against both erythromycin and clindamycin. Prophylaxis with erythromycin is no longer recommended, even if sensitivity is documented. Prophylaxis with clindamycin is recommended if GBS is proved sensitive to both clindamycin and erythromycin and if there is no inducible resistance to clindamycin using D-zone testing. If sensitivity is unknown, or if all these requirements are not met, vancomycin is recommended. ∗†‡

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