Transvaginal Ultrasound

Transvaginal ultrasound is useful in the first trimester of pregnancy because the close proximity of the intravaginal ultrasonic transducer allows for high-frequency scanning and thus better resolution of the pelvic organs and developing pregnancy than transabdominal imaging. Transvaginal ultrasound is commonly used in the first trimester to determine accurate dating of the pregnancy as well as fetal location and number. The nuchal translucency measurement (first-trimester screening), a sonographically derived assessment of the subcutaneous fluid collection at the level of the fetal neck, is a screening test for chromosomal and structural abnormalities that is performed between 11 and 14 weeks’ gestation, typically by a transabdominal but also a transvaginal approach (see Figure 7-2, pg 81). First-trimester vaginal ultrasound can also identify structural malformations. Transvaginal sonographic measurement of cervical length in the mid-trimester can be used to identify patients at risk for preterm delivery. The median length of the cervix at 24 to 28 weeks is 3.5 cm. Patients with a cervical length less than 2.0 cm are at significantly increased risk for preterm birth (threefold to fivefold). Finally, transvaginal ultrasonic imaging of the lower uterine segment in the second or third trimester allows for very precise identification of placental location in relation to the internal cervical os. In a patient with vaginal bleeding, excluding placenta previa is important in management.

Transabdominal Ultrasound

After 16 weeks’ gestation, transabdominal ultrasound (second-trimester screening) is used to evaluate the fetus for structural abnormalities, provide a baseline assessment of fetal growth, and provide information regarding fetal well-being. The ability of a second-trimester scan to identify a fetus with an anomaly ranges from 17% to 74%. The reason various studies show such a wide range in sensitivity is probably due to variations in patient population and operator skill. The specificity, or the ability of ultrasound to correctly identify a normal fetus, approaches 100% in all studies. Thus ultrasound is useful in ruling out fetal anomalies, but it is not as reliable in detecting them.

In the third trimester, transabdominal ultrasound is useful in assessing fetal growth. Serial biometric measurements of the fetal head, abdomen, and limbs provide longitudinal information regarding the fetal growth trajectory. Software packages integral to the ultrasonic machines allow calculation of a fetal weight estimate from these measurements; this estimate is often used clinically. However, understanding that these estimates may have an error of ±15% (a variation of ±1 lb or 450 g in a 7-lb or 3400-g fetus) limits the utility of ultrasonic fetal weight, especially in larger (>8 lb or 4000 g) fetuses.

Ultrasonic visualization of aspects of fetal behavior (body movement, breathing) provides highly predictive information regarding fetal oxygenation and well-being. These aspects are combined to determine the biophysical profile (Box 17-2). The risk for fetal death within the week following a biophysical profile score of 8 or more is less than 1%.

Doppler Sonography

Doppler sonography, which can precisely measure the velocity profile of blood flowing through fetal vessels, allows for characterization of vascular impedance. The umbilical artery, which normally has high-velocity flow during cardiac diastole, may have low, absent, or even reversed diastolic flow in a compromised fetus with high-resistance placental vasculature. Similarly, because the peak flow velocity through a blood vessel is inversely proportional to the viscosity of the liquid flowing through it, Doppler studies of the fetal middle cerebral artery are used as a noninvasive estimate of fetal hematocrit. This is useful in management of severe fetal anemia in pregnancies complicated by isoimmunization.

Finally, ultrasound is used to assist in performing invasive obstetric procedures. Amniocentesis, chorionic villus sampling (CVS), and percutaneous umbilical blood sampling (cordocentesis) are examples of procedures that require continuous ultrasonic guidance.

Genetic Diagnosis

Amniocentesis for prenatal diagnosis of chromosomal anomalies is performed at 16 to 20 weeks of gestation. The procedure-related risks are an approximately 0.3% pregnancy loss rate and a 1% postprocedure measurable amniotic fluid leakage rate. Early amniocentesis done before the 15th week of gestation is associated with a higher miscarriage rate (3% to 4%), a higher postprocedure leakage rate (3%), and an additional risk for limb deformities, including clubfoot (1%). Amniotic cells require 1 to 2 weeks of culture before final chromosomal analysis is possible, although fluorescent in situ hybridization (FISH) can be used with chromosome-specific probes (e.g., trisomy 21, 18, and 13) and gives preliminary results in 3 days.

Single gene defects that have been characterized at the molecular level are amenable to prenatal diagnosis through amniocentesis. Using polymerase chain reaction (PCR), fetal DNA in the amniocytes can be amplified rapidly to allow for direct or indirect molecular analysis of genetic disorders. Examples of common prenatally diagnosed genetic disorders include cystic fibrosis, Tay-Sachs disease, sickle cell disease, and fragile X syndrome.

Biochemical Testing

An example of biochemical testing that can be performed on amniotic fluid is determination of the level of alpha-fetoprotein (AFP). AFP is a fetal serum protein that should, under normal circumstances, be detectable in the amniotic fluid in only trace amounts. In the event that the fetal dorsal or ventral wall is open (e.g., neural tube defect or gastroschisis), amniotic fluid AFP will be elevated, allowing detection of these defects even if ultrasonic imaging is equivocal or nondiagnostic.

Diagnosis of Perinatal Infections

In the United States the most commonly encountered prenatal infections with potential sequelae for the fetus include cytomegalovirus (CMV), parvovirus B19, varicella zoster virus (VZV), and toxoplasmosis. Often these are accompanied by ultrasonographic findings on mid-trimester ultrasound, including abdominal, liver, and intracranial calcifications, fetal hydrops, echogenic bowel, ventriculomegaly, and intrauterine growth restriction. These findings can prompt amniotic fluid analysis by culture or PCR to identify the pathogen. In addition, amniotic fluid Gram stain, white blood cell count, glucose level, and culture have been used to diagnose preterm chorioamnionitis, which is a major cause of premature labor.

Other Diagnostic Testing

Amniocentesis is commonly used in the third trimester to determine the risk for neonatal lung immaturity in the case of impending premature birth or before elective delivery. This is performed by measuring the pulmonary phospholipids or lamellar bodies, which enter the amniotic fluid from the fetal lungs. The presence of phosphatidylglycerol and a lecithin-to-sphingomyelin (L/S) ratio greater than 2 are associated with minimal risk for respiratory distress in the neonate. In a case of suspected premature rupture of the membranes when the diagnosis is unclear using standard tests, infusion of 2 to 3 mL of indigo dye into the amniotic fluid may be performed. If the dye is then noted on a vaginally placed tampon, rupture of the membranes is confirmed.

Therapeutic Amniocentesis

The primary role of therapeutic amniocentesis has been in the management of polyhydramnios and twin-twin transfusion syndrome. Polyhydramnios, typically defined as a single deepest vertical pocket of amniotic fluid greater than 8 cm on ultrasound, can cause maternal respiratory embarrassment or premature labor. Excessive amniotic fluid volume may arise from lack of fetal swallowing or from excessive fetal urination. The latter condition occurs in the twin-twin transfusion syndrome (see Chapter 13). Serial amniocenteses to remove large volumes of excessive amniotic fluid from the sac of the recipient twin have been associated with improved perinatal outcome; however, recent data suggest that laser ablation of placental vascular connections between twin placentas with twin-twin transfusion syndrome is significantly better.

Indications

In general, there are four indications for an operative vaginal delivery:

FIGURE 17-4 Delivery of the aftercoming head, using Piper forceps.

Types of Forceps Operations

Forceps application is classified according to the station and position of the presenting part at the time the forceps are applied. The American College of Obstetricians and Gynecologists (ACOG) has proposed the following classification:

Before performing a forceps-assisted vaginal delivery, appropriate consent from the patient regarding potential risks and benefits should be obtained. The indication for the procedure should be clearly outlined to the patient and in the medical record. The cervix must be fully dilated, membranes ruptured, and the fetal head engaged into the pelvis. Clinical assessment to determine the level of the presenting part, estimation of the fetal size, and adequacy of the maternal pelvis is mandatory. There must be no doubt regarding the position of the fetal head. This evaluation is performed by palpation of the sutures and fontanelles in comparison to the maternal pelvis. Anesthesia must be adequate by either pudendal nerve block with local infiltration (for outlet forceps only) or regional anesthesia. The bladder should be emptied to prevent damage to that structure and to provide more room to effect delivery.

Forceps Technique

The forceps blades are inserted sequentially into the vagina such that the sagittal suture of the fetal head is directly between and perpendicular to the shanks. Damage to maternal tissues may be avoided by placing one operator hand into the vagina to guide the toe of the blade along the natural pelvic curve of the birth canal. With the next maternal pushing effort, the forceps are locked, and traction is applied. The direction of pull should be parallel to the axis of the birth canal at that level, such that typically there is downward traction initially, followed by ever-increasing upward traction as delivery of the fetal head occurs. With complete delivery of the head, the shanks are nearly perpendicular to the floor. If progress of the fetal head is not obtained with appropriate traction, the procedure should be abandoned (failed forceps) in favor of a cesarean delivery.

Indications

Four indications account for 90% of the marked increase in cesarean delivery over the past 40 years: dystocia (30%), repeat cesarean delivery (25% to 30%), breech presentation (10% to 15%), and fetal distress (10% to 15%). An absolute indication for a cesarean delivery is a previous full-thickness, nontransverse incision through the myometrium. This occurs in all classic cesarean deliveries and some myomectomy surgeries. All pregnancies complicated by placenta previa should also be delivered by cesarean birth.

Types of Cesarean Deliveries

Cesarean deliveries are classified by the uterine incision (Figure 17-6), not the skin incision. In the low transverse cesarean delivery (LTCD), the uterine incision is made transversely in the lower uterine segment after establishing a bladder flap. The advantages of this approach include decreased rate of rupture of the scar in a subsequent pregnancy and a reduced risk for bleeding, peritonitis, paralytic ileus, and bowel adhesions.

FIGURE 17-6 Types of cesarean delivery incisions.

In the classic cesarean delivery, a vertical incision is made in the upper segment of the uterus through the myometrium of the uterus. A vertical incision may also be made in the lower segment, in which case the procedure is referred to as a low vertical cesarean delivery, although the incision invariably extends into the upper segment of the uterus. The common indications for a classic cesarean delivery include the preterm breech with an undeveloped lower uterine segment, transverse back-down fetal position, poor access to the lower segment due to myomas or adhesions, or a planned cesarean hysterectomy.

The type of uterine incision has important implications regarding risk for uterine rupture in future pregnancies. Uterine rupture, defined as separation of the uterine incision, may cause significant maternal complications due to massive hemorrhage and fetal damage or death. An LTCD incision is associated with less than a 1% risk for symptomatic uterine rupture in the subsequent pregnancy, although this risk may be higher if labor induction or augmentation is carried out. A classic cesarean delivery carries a 4% to 7% risk for uterine rupture. Patients with a classic uterine incision are thus destined to have repeat cesarean deliveries for all subsequent pregnancies.

Prevention

Two clinical interventions have been shown to reduce cesarean delivery rates: external cephalic version (ECV) and vaginal birth after cesarean delivery (VBAC).

ECV converts a malpresenting fetus to the vertex position to avoid a cesarean delivery for breech presentation. This procedure is performed in labor and delivery, after the 36th or 37th week of gestation, under ultrasonic guidance. A tocolytic may be given to decrease uterine tone. Using external manipulation, the fetus is gently guided to the vertex presentation. Fetal risks due to umbilical cord entanglement and placenta abruption are low (<1%).

The success rate of ECV is about 60%. Parity, gestational age, placental location, and dilation or station affect this success rate. An ECV program can decrease the rate of cesarean delivery in this group of patients by more than half and an obstetric service’s overall cesarean birth rate by about 2%.

Women with a prior cesarean delivery represent the second most common overall cause of cesarean delivery, VBAC (25% to 30%). In fact, about 10% to 15% of pregnant women have had a previous cesarean delivery.

A trial of labor may be offered if one or two previous LTCDs were performed, the uterine incision did not extend into the cervix or uterine upper segment, and there is no history of prior uterine rupture. Adequate maternal pelvic dimensions should be noted by clinical examination. Personnel and equipment should be immediately available in case emergent cesarean delivery is required.

The overall success rate of VBAC is about 70%, although this ranges from 60% (dystocia) to 90% (malpresentation), depending on the indication for the previous cesarean delivery. Compared with repeat cesarean delivery, a successful vaginal delivery is associated with less maternal morbidity without an increase in perinatal morbidity. However, if uterine rupture does occur, there may be a 10-fold increase in perinatal mortality as well as substantial maternal morbidity.