VI LABOR, DELIVERY, AND POSTPARTUM LABOR AND VAGINAL DELIVERY Kelly A. Best, MD CHAPTER 61 1. What is the definition of labor? Labor begins when uterine contractions of sufficient frequency, intensity, and duration are attained to bring about effacement and progressive dilation of the cervix. 2. What two steps are theorized to be crucial to the initiation of labor in human pregnancy? 1. Retreat from pregnancy maintenance 2. Uterotonic induction Despite extensive investigation into the associated physiologic and biochemical changes, the physiologic processes in human pregnancy that result in the onset of labor are still not defined. 3. In which patients is induction of labor considered? Awaiting the onset of normal labor may not be an option in certain circumstances. At preterm gestations, indications for labor induction include severe preeclampsia, fetal growth restriction with abnormal antepartum surveillance or other evidence of fetal compromise, and deterioration of maternal disease to the point that continuation of pregnancy is believed to be detrimental. In cases of rupture of membranes without labor (i.e., premature rupture of membranes) at term (37 to 42 weeks) or postterm (≥42 weeks), induction of labor is often performed. 4. What is a Bishop score, and how is it used? A Bishop score is a quantifiable method to assess the likelihood of a successful induction. Elements include dilation, effacement, station, consistency, and position of the cervix (Table 61-1). A score of 6 or less trans- lates into a need to ripen the cervix and is associated with less successful inductions. A score 8 or greater generally means the cervix does not need ripening and induction is more likely to be successful. Table 61-1. Bishop Scoring System for Assessment of Inducibility DILATION EFFACEMENT STATION CERVICAL CERVICAL SCORE (cm) (%) (-3 TO +3 SCALE) CONSISTENCY POSITION 0 Closed 0-30 −3 Firm Posterior 1 1-2 40-50 −2 Medium Midposition 2 3-4 60-70 −1, 0 Soft Anterior 3 ≥5 ≥80 +1, +2 — — 5. What methods are available for cervical ripening? Mechanical methods provide local pressure and stimulate endogenous release of prostaglandins, which results in cervical ripening. Options include a transcervical Foley catheter (filled with 30 to 60 mL after insertion) or hydroscopic dilators (laminaria or a similar synthetic product). Pharmacologic methods include low-dose oxytocin, prostaglandin E2 (dinoprostone), and prostaglandin E1 (misoprostol). Prostaglandin E1 can be administered intravaginally or orally, and prostaglandin E2 can be applied within the vagina in either gel or suppository form. 6. When is cervical ripening contraindicated? Contraindications to labor induction are the same as those for spontaneous labor and vaginal delivery. They include—but are not limited to—vasa previa, placenta previa, fetal malpresentation (e.g., breech 263 Downloaded for Sean Oldroyd ([email protected]) at KAWEAH DELTA MEDICAL CENTER from ClinicalKey.com by Elsevier on February 09, 2021. For personal use only. No other uses without permission. Copyright ©2021. Elsevier Inc. All rights reserved. 264 LABOR, DELIVERY, AND POSTPARTUM or transverse lie), umbilical cord prolapse, and previous transfundal uterine surgical procedures. Because of an increased risk of uterine rupture during induction of labor with prostaglandins among women with previous low-transverse cesarean sections, the American College of Obstetricians and Gynecologists (ACOG) advises against the use of prostaglandins in these cases. For more information on a trial of labor after cesarean section (TOLAC), see Chapter 63. 7. How is the pelvis assessed clinically? Historically, clinical assessment of the pelvis (known as pelvimetry) was used to predict whether vaginal birth was possible and provide information about a patient’s overall pelvic dimensions and configuration. However, this has been shown not to be reliable; the best way to assess whether a pelvis is adequate is to allow for a trial of labor. • The pelvic inlet is bounded in the anteroposterior dimension (obstetric conjugate) by the sacral promontory and the pubic symphysis. Laterally, it is bound by the linea terminalis. The size of the pelvic inlet can be estimated by palpating the sacral promontory, which provides the measurement for the diagonal conjugate, which is approximately 1.5 to 2 cm greater than the obstetric conjugate. • The pelvic midplane is bounded laterally by the inferior margins of the ischial spines, anteriorly by the lower margin of the symphysis pubis, and posteriorly by the sacrum (usually S4 or S5). • The pelvic outlet consists of two triangular areas that are not in the same plane but share a common base, which is the distance between the two ischial tuberosities. The apex of the posterior triangle is at the tip of the sacrum, and the anterior triangle is formed by the area under the pubic arch (Fig. 61-1). Most patients have an intermediate form of the four classically described pelvic types (Fig. 61-2). 8. What are the three stages of labor? The first stage begins with the onset of labor and ends when the cervix is fully dilated (10 cm). The second stage begins when dilation of the cervix is complete and ends with delivery of the infant. The third stage begins immediately after delivery of the infant and ends with delivery of the placenta. 9. How are uterine contractions monitored? Uterine monitoring can be either external or internal. • External monitoring: performed by securing a displacement transducer to the maternal abdomen. When the uterus contracts, the transducer moves; this movement is translated into an electronic signal and is transcribed as a vertical displacement from the resting uterine tone. This form of monitoring provides information only about the frequency of contractions. Transverse diameter (bituberous) Ant. triangle Sacrotuberous Post. triangle ligt. Figure 61-1. The pelvic outlet. Ant., Anterior; ligt., ligament; post., posterior. (From Hobel CJ, Chang AB. Normal labor, delivery, and postpartum care. In: Hacker NF, Moore JG, Gambone JC, eds. Essentials of Obstetrics and Gynecology. 4th ed. Philadelphia: Saunders; 2004:108.) Downloaded for Sean Oldroyd ([email protected]) at KAWEAH DELTA MEDICAL CENTER from ClinicalKey.com by Elsevier on February 09, 2021. For personal use only. No other uses without permission. Copyright ©2021. Elsevier Inc. All rights reserved. LABOR AND VAGINAL Delivery 265 Anthropoid Platypelloid Gynecoid Android Figure 61-2. The four basic pelvic types (Caldwell-Moloy classification). The dotted line indicates the transverse diameter of the inlet. (From Hobel CJ, Chang AB. Normal labor, delivery, and postpartum care. In: Hacker NF, Moore JG, Gambone JC, eds. Essentials of Obstetrics and Gynecology. 4th ed. Philadelphia: Saunders; 2004:109.) • Internal monitoring: performed by threading a pressure catheter through the cervix, around the presenting part of the fetus, and into the uterine cavity. This method provides information on both the strength and frequency of contractions. 10. What is a normal contraction pattern? A normal contraction pattern during labor consists of three to five contractions in 10 minutes, aver- aged over a 30-minute period. 11. What is an “adequate” contraction pattern? An adequate contraction pattern refers to contractions of sufficient frequency and strength to cause cervical change. Uterine contractions can be measured in Montevideo units. To calculate Montevideo units, total vertical displacement above resting uterine tone is multiplied by contraction frequency over a 10-minute period (Fig. 61-3). An intrauterine pressure catheter must be used. Contractions are considered adequate when they reach 200 Montevideo units or greater. 100 100 12 12 75 10 75 10 8 8 50 50 6 6 4 25 25 2 2 UA 0 mm Hg 0 kPa UA 0 mm Hg 0 kPa Figure 61-3. Calculation of Montevideo units. UA, Uterine activity. (From Bashore RA, Koos BJ. Fetal surveillance during labor. In: Hacker NF, Gambone JG, Hobel CJ, eds. Hacker and Moore’s Essentials of Obstetrics and Gynecology. 5th ed. Philadelphia: Saunders; 2010.) Downloaded for Sean Oldroyd ([email protected]) at KAWEAH DELTA MEDICAL CENTER from ClinicalKey.com by Elsevier on February 09, 2021. For personal use only. No other uses without permission. Copyright ©2021. Elsevier Inc. All rights reserved. 266 LABOR, DELIVERY, AND POSTPARTUM 12. What are the two phases of the first stage of labor? The first stage is divided into a latent phase and an active phase. The duration of the latent phase is variable and has little bearing on the subsequent course of labor, whereas the characteristics of the active phase are usually predictive of the outcome of a particular labor. 13. What is the Friedman curve? In the 1950s, Emanuel A. Friedman popularized the use of an objective measure of labor progres- sion. He constructed a graphic representation of expected cervical dilation against time, with varying expectations for nulliparous and multiparous patients. Used in conjunction with fetal descent, these curves were used to provide clinical feedback about the normalcy of a patient’s labor progress. “Labor curves” have since been updated in light of changing demographics, improved statistical and experimental methods, and more robust study designs. 14. What cervical changes generally take place during the latent phase of labor? Although little cervical dilation occurs during this time, considerable changes take place in the extracellular matrix (collagen and other connective tissue components) of the cervix. Cervical ripening generally includes palpable softening, effacement, and anterior rotation of the cervix in the pelvic axis. 15. When does conversion from the latent to active phase occur? During labor, the transition is characterized by increased regularity and intensity of contractions, accompanied by progressive and predictable cervical change. The active phase of labor begins at 6 cm dilation and accelerates at different rates based on parity. 16. What is the duration of the first stage of labor? • Latent phase: The median duration and 95th percentile duration of the latent phase depend on baseline cervical dilation and are similar between nulliparous and parous women.