Gladys A. Ramos, Thomas R. Moore
As the fetus has become more accessible through technologic advances, the desire to intervene on behalf of the fetus has led to the development of a number of obstetric diagnostic and therapeutic procedures. Any procedure performed during pregnancy carries risk to both mother and fetus, so it is important to counsel the mother regarding the potential benefits and risks of all options before embarking on any intervention.
Prenatal Diagnostic and Therapeutic Procedures
Obstetric transvaginal and transabdominal sonography plays a pivotal role in contemporary obstetric care, with ultrasonic imaging being done in about 70% of pregnancies in the United States today. Human data have shown no adverse fetal effects of ultrasound. Box 17-1 lists common abnormalities that may be identified prenatally with ultrasound.
BOX 17-1 Examples of Fetal Abnormalities Detected by Prenatal Ultrasound
Central Nervous System
• Spina bifida
• Cleft lip and/or palate
• Hypoplasia of the nose
• Cystic hygroma
• Nuchal skin thickening
• Atrial septal defect
• Ventricular septal defect
• Tetralogy of Fallot
• Transposition of the great vessels
• Congenital cystic adenomatoid malformation (CCAM)
• Lung sequestration
• Diaphragmatic hernia
• Bowel atresia or obstruction
• Echogenic bowel
• Renal agenesis
• Polycystic kidney disease
• Posterior urethral valves
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.
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%.
BOX 17-2 Biophysical Profile∗
Fetal breathing—30 seconds of rhythmic movement of the fetal thorax
Fetal movement—at least 3 movements of the fetal body or limb
Fetal tone—one extension and flexion of a limb joint
Amniotic fluid—single deepest vertical pocket of amniotic fluid > 2 cm
In conjunction with a 30-minute nonstress test (NST)
∗ Two points for each time these events are documented on real-time ultrasound and two points for a reactive NST. A score of 8-10 is considered normal.
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.
Amniocentesis, which involves removing a sample of fluid from the amniotic cavity, is the most common invasive prenatal diagnostic procedure. Using direct ultrasonic guidance, a 22-gauge needle is advanced into a clear pocket of amniotic fluid under sterile conditions, taking care to avoid maternal bowel and blood vessels, and the placenta if possible. About 20 mL of amniotic fluid is withdrawn for genetic studies. Rh immune globulin (RhO-GAM) must be given to the Rh-negative gravida because of the small risk for procedure-related isoimmunization.
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.
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.
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.
CHORIONIC VILLUS SAMPLING (CVS)
Another method to access fetal cells for prenatal genetic diagnosis is CVS of the placenta. The indications for CVS are similar to amniocentesis. The advantage of CVS is that it is performed earlier than amniocentesis (typically between the 10th and 12th weeks of gestation), allowing for earlier prenatal diagnosis. Although technically feasible, CVS is not performed before the 9th week because it has been associated with an increased risk for oromandibular and limb dystrophy, presumably from a vascular insult.
CVS may be performed under sterile conditions transcervically or transabdominally. In transcervical CVS, the distal 3 to 5 cm of a catheter is inserted through the cervix and into the placenta under sonographic guidance. A 20-mL syringe with nutrient medium is attached, and negative pressure is applied to obtain fragments of placental villi. Transabdominal CVS uses an 18- to 20-gauge needle inserted into the placenta transabdominally. With either approach, RhO-GAM should be administered to Rh-negative patients. The procedure-related loss rate is less than 1%.
Direct visual inspection of dividing villi cells obtained with CVS allows for detection of chromosomal abnormalities within 3 days, and tissue culture yields cytogenetic results in 6 to 8 days. The diagnostic precision of CVS is somewhat less than the standard amniocentesis owing to a 1% risk for chromosomal mosaicism, which is often due to confined placental mosaicism. A disadvantage of CVS is that amniotic fluid AFP levels cannot be assessed with this technique, and thus patients at risk for neural tube defects must be deferred to amniocentesis in the second trimester.
Cordocentesis (percutaneous umbilical blood sampling) is a procedure in which fetal blood is obtained directly from the umbilical vein at the placental cord insertion site under direct ultrasonic guidance. Confirmation of the fetal origin of the blood specimen is obtained by measuring the fetal mean corpuscular volume (MCV), which is typically greater than 120 fL (maternal MCV is usually less than 100 fL).
Historically, the most common indication for cordocentesis was to determine fetal hematocrit in the hemolytic disease, Rh isoimmunization. With the recent advent of fetal anemia assessment by Doppler of the fetal middle cerebral artery, cordocentesis for this indication is less frequent. Today, cordocentesis is often performed for rapid fetal karyotype evaluation. Unlike amniocytes, fetal leukocytes may be cultured rapidly, and results are typically available in 3 days.
The fetal loss rate is about 1% per procedure. In the case of a hydropic fetus, the risk for fetal loss may approach 7%. The cause of pregnancy loss may be due to chorioamnionitis, rupture of membranes, bleeding from the puncture site, bradycardia, or thrombosis of the umbilical vessel.
Cervical insufficiency or incompetence is defined as the inability of the uterine cervix to retain a pregnancy in the absence of contractions or labor (see Chapter 19). Cervical cerclage, a circumferential suture placed into the cervix, has been proposed as a surgical treatment for this condition. The cerclage is usually placed at 13 to 16 weeks.
The most common procedure, McDonald’s cerclage, entails placement of a simple pursestring monofilament suture near the cervicovaginal junction (Figure 17-1). Shirodkar’s cerclage differs in that the stitch is placed as close to the internal os as possible. The bladder and rectum are dissected off the cervix, the woven tape-like suture is tied, and the mucosa is replaced over the knot. Transabdominal cervicoisthmic cerclage is rarely indicated and is reserved for select patients with previously failed vaginal cerclage, cervical hypoplasia, or a cervix severely scarred from prior lacerations or prior surgery. This type of cerclage entails dissection of the bladder from lower uterine segment through an abdominal incision. For transvaginal cerclage, the suture is typically removed before the onset of labor. For abdominal cerclage, cesarean delivery is performed. Patients must be counseled thoroughly regarding the risks associated with cerclage, which include bleeding, infection, iatrogenic rupture of amniotic membranes, and damage to adjacent organs (bladder and bowel).
FIGURE 17-1 McDonald-type cervical cerclage. Although suture technique may vary somewhat, four sutures are usually placed high into the cervix using a nonabsorbable material such as Mersilene (A) and then tied, providing additional support at the level of the internal cervical os (B). This suture is cut for labor.
(From Gabbe SG, et al: Obstetrics, 5th ed. Philadelphia, Churchill Livingstone, 2007.)
The incidence of an operative obstetric delivery in the United States today is about 35% to 40%, of which 10% to 15% are operative vaginal deliveries using either a forceps or vacuum device. About 25% to 30% of all deliveries are cesarean births. Each operative procedure has inherent benefits and risks.
Forceps are instruments designed to provide traction and rotation of the fetal head when the expulsive efforts of the mother are insufficient to accomplish safe delivery of the fetus. Commonly used forceps are shown in Figure 17-2. There are two classes of obstetric forceps: classic forceps and specialized forceps. Forceps selection depends on the obstetric indication.
FIGURE 17-2 Types of obstetric forceps in use. Simpson forceps are an example of classic or standard forceps. Kielland forceps (for midforceps rotation) are an example of specialized forceps and are used infrequently. Piper forceps are used for breech delivery of the aftercoming head (see Figure 17-4).
Classic or standard forceps are used to effect delivery by applying traction to the fetal skull. The components of each blade are illustrated in Figure 17-3. The blades have a cephalic curve designed to conform to the curvature of the fetal head. Simpson forceps (an example of classic or standard forceps) have a tapered cephalic curve that is designed to fit on a molded fetal head. The pelvic curve of classic forceps approximates the shape of the birth canal.
FIGURE 17-3 Components of classic forceps.
In general, there are four indications for an operative vaginal delivery:
1. Prolonged second stage of labor. In nulliparous women, this is defined as lack of continuing progress for 2 hours without regional anesthesia or 3 hours with regional anesthesia. In multiparous women, it is defined as lack of continuing progress for 1 hour without regional anesthesia or 2 hours with regional anesthesia.
2. Suspicion of immediate or impending fetal compromise
3. To stabilize the aftercoming head during a breech delivery (Figure 17-4)
4. To shorten the second stage of labor for maternal benefit. Maternal conditions such as hypertension, cardiac disorders, or pulmonary disease, in which strenuous pushing in the second stage of labor is considered hazardous, may be indications for forceps delivery. Epidural analgesia, which also decreases strenuous pushing during the second stage of labor, may also be recommended for this purpose.
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:
1. Outlet forceps: Scalp is visible at the introitus without separating the labia, fetal head is at perineum, fetal skull is at pelvic floor, sagittal suture is in anteroposterior or right/left occiput anterior or posterior position, and rotation of the fetal head does not exceed 45 degrees.
2. Low forceps: Leading part of the fetal skull is at station +2 cm or more. Low forceps have two subdivisions: rotation of 45 degrees or less and rotation of more than 45 degrees.
3. Mid forceps: Fetal head is engaged, but the leading point of the skull is above station +2 cm.
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.
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.
The vacuum extractor is an instrument that uses a suction cup that is applied to the fetal head. Because of relative ease of use compared with forceps, vacuum delivery has become more prevalent in the United States. After confirming that no maternal tissue is trapped between the cup and the fetal head, the vacuum seal is obtained using a suction pump. Traction is then applied using similar principles described previously for a forceps delivery. Flexion of the fetal head must be maintained to provide the smallest diameter to the maternal pelvis by placing the posterior edge of the suction cup 3 cm from the anterior fontanelle squarely over the sagittal suture.This is illustrated in Figure 17-5. With the aid of maternal pushing efforts, traction is applied parallel to the axis of the birth canal. Detachment of the suction cup from the fetal head during traction is termed a pop-off. If progress down the birth canal is not obtained with appropriate traction, or if two pop-offs occur, the procedure should be discontinued in favor of a cesarean delivery. The indications for vacuum delivery are the same as for forceps delivery.
FIGURE 17-5 Application of the vacuum extractor. A: Incorrect application, which deflexes the fetal head, thereby increasing the presenting diameter. B: Correct application over the posterior fontanelle, which flexes the fetal head when traction is applied.
The prerequisites for use of the vacuum extractor are also the same as for forceps, with a few exceptions. The vacuum extractor is contraindicated in preterm delivery because the preterm fetal head and scalp are more prone to injury from the suction cup. The vacuum extractor is suitable for all vertex presentations, but unlike forceps, it must never be used for delivery of fetuses presenting by the face or breech.
COMPARISON OF FORCEPS AND VACUUM DELIVERY
Understanding the potential advantages and disadvantages of each operative vaginal delivery instrument allows the operator to counsel the mother appropriately and choose the device that is best suited for the particular clinical situation.
Forceps have a higher overall success rate for vaginal delivery. The failure rate for forceps is 7%, whereas the failure rate for vacuum extraction is 12%. In general, forceps deliveries cause higher rates of maternal injury, and vacuum extraction causes higher rates of fetal morbidity. Forceps have an increased risk for trauma to vaginal and perineal tissues and damage to the maternal anal sphincter. In contrast, neonates delivered by vacuum have more cephalohematomas (accumulation of blood beneath the periosteum) and exclusively have subgaleal hematomas (blood in the space above the periosteum that has a large potential space and can allow significant blood loss). Sequential use of one instrument followed by the other has been associated with a disproportionately high fetal morbidity rate and should be avoided or approached with extreme caution. Long-term retrospective studies of adolescents delivered by normal vaginal delivery, forceps, vacuums, and cesarean delivery have shown little difference in physical and cognitive impairment.
Cesarean delivery is delivery of the fetus through an incision in the maternal abdomen and uterus. Hospitals offering obstetric services must have the personnel and equipment to perform an emergent cesarean delivery within 30 minutes.
Cesarean delivery is the most common major operation performed in the United States today. The rate of cesarean deliveries has increased more than fivefold, from 5% of births in 1970 to nearly 30% of births currently. The dramatic increase in the cesarean delivery rate has been attributed to many factors, including assumed benefit for the fetus, relatively low maternal risk, societal preference, and fear of litigation.
The perinatal benefits of cesarean section are largely based on unquantified and scanty evidence. There has been more than a 10-fold decrease in perinatal mortality in the United States over the past 40 years concurrent with advances in prenatal, intrapartum, and neonatal care. How much of this improvement is due to the increased use of cesarean delivery is debatable, with the exception of management of the term breech delivery. Perinatal and neonatal mortality and significant neonatal morbidity have been shown to improve from 5.0% for those breeches delivered vaginally to 1.6% for those delivered by cesarean.
The overall mortality rate from cesarean delivery is currently less than 1 in 1000, but this is about 5 times greater than that from vaginal delivery. However, recent studies have shown that the maternal mortality rate for an elective cesarean delivery approximates that of vaginal delivery. This is due to advances in surgical techniques, anesthetic care, blood transfusions, and antibiotics.
The maternal morbidity with cesarean delivery is increased compared with vaginal delivery, owing to increased postpartum infections, hemorrhage, and thromboembolism.
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.
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.
American College of Obstetricians and Gynecologists. Cervical insufficiency. In ACOG Practice Bulletin No. 48. Washington, DC: ACOG; 2003.
American College of Obstetricians and Gynecologists Vaginal birth after previous cesarean delivery ACOG Practice Bulletin No. 54. Obstet Gynecol, 104; 2004:203-212.
American College of Obstetricians and Gynecologists Screening for fetal chromosomal abnormalities ACOG Practice Bulletin No. 77. Obstet Gynecol, 109; 2007:217-227.
Fox N.S., Chervenak F.A. Cervical cerclage: A review of the evidence. Obstet Gynecol Surv. 2008;63:58-65.