Obstetric Procedures - Clinical Tree

Clinical Keys for This Chapter

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Real-time (two-dimensional [2D]) ultrasonic imaging of the uterus, placenta, fetus, and cervix has become important for assessment of almost all pregnancies in the United States. Early in pregnancy, ultrasonic imaging is important for pregnancy dating and for ruling out multiple pregnancies. During mid-gestation, measurement of the biparietal diameter of the fetal head, the abdominal circumference, and the femoral length can determine normal or abnormal fetal growth, and later in pregnancy changes in these parameters can be used to assess the progress of fetal growth and well-being.
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Amniocentesis was the first invasive procedure developed to access amniotic fluid. Real-time 2D ultrasound has significantly reduced complications related to amniocentesis. Access to amniotic fluid has made biochemical testing possible for assessment of fetal genetic and metabolic parameters.
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Placental tissue and fetal cells can be obtained safely from the amnion for the assessment of fetal genetics. Amniotic fluid obtained by amniocentesis provides access to cells shed from the amnion that can be used for genetic studies. Placental tissue obtained by either chorionic villus sampling (CVS) via the cervical route (transvaginal catheter) or by transabdominal needle aspiration may also be used for genetic analysis of the fetus.
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The cervix is an important barrier that protects the fetus from early delivery. The normal length of the cervix is 4 to 5 cm, but, in approximately 5% of pregnancies, it can silently undergo shortening. If identified after mid-pregnancy (20 to 22 weeks), a cerclage can be placed around the cervix to keep it from further shortening and dilating, which would increase the risk of early preterm delivery.
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Forceps application or the use of a vacuum extractor (VE) is sometimes warranted to safely deliver the fetus. When a woman experiences prolonged labor after complete cervical dilation and the fetus has descended to the point where the head is at a station of at least +2 (using a 0 to 5 cm scale), the second stage of labor can be reduced by the use of forceps or a VE. When forceps or VE fails, a cesarean delivery should be performed.

As the fetus has become more accessible for monitoring as a result of technological 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 risks 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 obstetric intervention.

The diagnostic indications for ultrasonic imaging, amniocentesis, chorionic villus sampling (CVS), and cordocentesis are covered in this chapter, as well as the therapeutic indications for cervical cerclage, obstetric forceps, vacuum extraction, and cesarean delivery. The techniques used in these obstetric procedures are also described.

Ultrasound

Two-dimensional (2D) ultrasound has been the standard in sonography for the past 30 years. Obstetric transvaginal and transabdominal sonography play a pivotal role in contemporary obstetric care, with ultrasonic imaging being done in approximately 90% 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

Hydrocephalus
Anencephaly
Arachnoid cyst
Porencephaly
Agenesis of corpus callosum
Spina bifida

Face

Cleft lip and/or palate
Hypoplasia of the nose

Neck

Cystic hygroma
Goiter
Nuchal skin thickening

Heart

Atrial septal defect
Ventricular septal defect
Tetralogy of Fallot
Transposition of the great vessels
Arrhythmias

Lungs

Congenital cystic adenomatoid malformation
Lung sequestration
Diaphragmatic hernia

Abdominal Wall

Gastroschisis
Omphalocele

Gastrointestinal Tract

Bowel atresia or obstruction
Echogenic bowel

Urinary System

Renal agenesis
Polycystic kidney disease
Hydronephrosis
Posterior urethral valves

Skeletal Dysplasia

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 better resolution of the pelvic organs and developing pregnancy than are possible with 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 (see Chapter 7 ). 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. It is performed between 11 and 14 weeks' gestation, most commonly by a transabdominal approach, but also by a transvaginal approach. 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 a three- to fivefold increased risk of preterm birth.

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, exclusion of placenta previa is important in overall obstetric management.

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