Labor and Delivery

September 15, 2011

The process by which a child is born is both simple and intricate. From the moment of conception, it is inevitable that the growing fetus will need to develop, grow, and eventually pass from its uterine environment either by a vaginal delivery or a cesarean section delivery. The cardinal signs of labor or mechanisms of labor describe the basic stages through which a fetus must pass to be delivered vaginally. The cardinal mechanisms are:

(a) engagement defined as the lowest portion of the presenting part at or below the maternal ischial spines; (b) descent; (c) flexion; (d) internal rotation; (e) extension; (f) external rotation; and (g) expulsion. These seven stages vary slightly from patient to patient and labor to labor.

Labor can be both difficult to diagnose, difficult to initiate, and difficult to stop. Labor is classically defined as regular uterine contractions resulting in cervical change. However, labor can start with amniorhexis or ruptured membranes (breaking the bag of water). Cervical change can also occur prior to or without labor as in the case of prelabor cervical changes or incompetent cervix. In fact, the diagnosis of the initiation of labor is often made retrospectively.

Labor is divided into three or four stages. The first stage is defined as the period of time from the beginning of labor until complete cervical dilation. The second stage of labor is defined as the period of time from complete dilation of the cervix until delivery of the fetus. The third stage of labor is defined as the period of time from the delivery of the fetus until the delivery of the placenta, and the fourth stage of labor has been defined by some as the 1-hr period of time after the delivery of the placenta.

The first stage of labor is a process that is divided into two phases, latent and active, and results in the eventual complete dilation. It can be difficult to diagnose the initiation of the first stage of labor as well as the transition from the latent to the active stage. This transition is marked by an increase in the rate of cervical dilation. Typically this occurs at approximately 4 cm dilation. The average length of the active phase of labor ranges from 4.6 to 7.7 hr for nulliparous (women having their first babies) and 2.4 to 5.7 hr in multiparous (women having subsequent babies) patients. The second stage of labor (the pushing stage) ranged from 57 to 66 min for primaparous patients and from 17 to 24 min in multiparous patients. The use of epidural anesthesia can increase the length of the pushing stage. These average durations of the different stages of labor are used to help diagnose labor abnormalities and qualify treatments for them.

One of the dystocias (abnormalities) of the first stage of labor is a prolonged latent phase. Typically 20 hr is considered the cutoff in the primaparous patient, and 14 hr is the typical cutoff in the multiparous patient. Therapies for a prolonged latent phase include no intervention, analgesia (pain control), and augmentation of labor (helping the labor progress more efficiently). An example of analgesia commonly used is 10-20 mg of morphine. Oxytocin (more commonly referred to by its trade name Pitocin) is typically the drug of choice for labor augmentation. Oxytocin is an eight-amino-acid peptide hormone that is stored and released from the posterior pituitary.

In addition, a prolonged latent phase, defined by greater than 12 hr for primaparous patients and greater than 6 hr for multiparous patients, has been associated with an increased risk of cesarean section, increased rate of more serious vaginal and perineal lacerations, and increased fever and bleeding.

Abnormalities of the active phase of labor are protraction and arrest disorders. A protraction disorder is defined by rates of dilation at less than the expected rates for nulliparous and multiparous patients. There is a usual expectation that a rate of 1.2 cm/hr change for nulliparas and a rate of 1.5 cm/hr change for multiparas are the outer limit of normal. There are, of course, many exceptions to these rules. Treatment is oxytocin augmentation.

Arrest of active phase is defined by the absence of dilation over a 2-hr period with adequate uterine contractile force. Assuming that oxytocin has already been administered for the preceding protraction disorder, the treatment for the arrest of active phase is cesarean delivery.

Dystocias or abnormalities of the second stage of labor include: protraction or arrest of descent of the presenting fetal part. Treatments for these disorders again are oxytocin and operative vaginal delivery or cesarean section delivery. These disorders were associated with a higher rate of cephalopelvic disproportion (CPD—baby will not fit through the birth canal), macrosomia (a large baby), and cesarean delivery. CPD, however, is a difficult diagnosis to make and is often an overused diagnosis. In general, it implies that either the fetus is too large or the maternal pelvis is too small to accommodate one another. Of course position or presentation can also affect this diagnosis and therefore each labor is unique regardless of any absolute measurement of the maternal pelvis or fetal size. In fact, after a cesarean for the diagnosis of CPD, the chance that a woman will be successful in having a vaginal birth after cesarean (VBAC) is approximately 83% after one prior cesarean and 75% after two prior cesarean deliveries.

MALPRESENTATION

Ninety-five percent of deliveries occur with a fetus in the vertex (head down) presentation. Malpresentation defines a presentation that is not in an occiput anterior (the baby is facing toward the mother’s back) vertex presentation. This includes occiput posterior (the baby is “sunny-side up,” i.e., facing upward as it delivers) and transverse presentation (looking sideways), breech (buttocks or legs presenting), transverse (the baby’s side is facing down), or oblique lie. Although breech presentation is not an absolute contraindication to a vaginal delivery, meaning that it does not absolutely prohibit a vaginal delivery, it has fallen out of favor recently after the publication of a large multicenter European study that reported an increased risk of neonatal morbidity after breech vaginal birth. The delivery of a second twin that is in the breech presentation, however, is still a safe and acceptable mode of delivering a second twin in the right circumstances.

Abnormalities of the third stage of labor include issues involved in placental separation from the uterus. Removal of the placenta can be spontaneous (comes out on its own with just some gentle traction by the obstetrician or midwife) or manual (the obstetrician or midwife reaches in to the uterus to remove it). Spontaneous expulsion of the placenta results in fewer infections, less risk of uterine inversion (when the uterus turns inside out), and less blood loss. However, manual extraction may be necessary, with the proper anesthesia, in cases of cord evulsion (when the umbilical cord tears off the placenta), hemorrhage, or retained placenta.

Induction of labor (IOL) is a common procedure in the practice of obstetrics and can be challenging. It has the potential to be extremely successful and beneficial to the patient, but alternatively can increase the risk of a cesarean delivery. In general there are three things to consider when planning an induction: the indication, the chance of success, and the mode of induction. Indications for induction include both maternal and fetal.

Clear maternal indications for induction include most cases of severe preeclampsia, chorioamnionitis (infection of the fetal membranes), and intrauterine fetal demise (death). Fetal indications include prolonged pregnancy greater than 42 weeks, poorly controlled diabetes mellitus, severe intrauterine growth restriction (fetus is not growing as we hope and expect it to), premature rupture of membranes (the bag of water breaks before labor begins), and other conditions that would compromise fetal health and not benefit from prolongation of in utero existence.

The success of induction depends on many factors including the gestational age (how far along in pregnancy the patient is), parity (how many children the patient has had), and the cervical “ripeness.” Inductions are more likely to be successful in the term pregnancy and in a multiparous patient. The term “cervical ripeness” is used to describe the cervical receptivity to induction agents. The classically described five cervical characteristics (dilation, effacement, station, consistency, and position) are combined to comprise what is now referred to as the Bishop score. A multipara with a Bishop score of greater than or equal to nine has a 100% chance of a successful induction with oxytocin and/or amniotomy.

The two common agents used are oxytocin and prostaglandins. Additional agents are also used for cervical ripening and/or dilation such as laminaria or Foley balloon catheters placed within the cervical os. High doses or rapid infusions of oxytocin have potential side affects including hypotension (low blood pressure) and water intoxication.

Prostaglandins are used for IOL as well. These medications can be of particular use in the induction of a patient with a poor Bishop’s score or “uninducible” cervix on exam. The mode of action is both at the level of the cervical collagen as well as uterine muscle receptors.

Hyperstimulation syndrome is a disorder of labor that can occur during spontaneous or induced labors. Hyperstimulation syndrome occurs when contractions are too frequent and/or too long in duration. As a result of these frequent contractions, the fetus may experience distress and exhibit fetal heart rate abnormalities. Treatment is aimed at improving fetoplacental perfusion and decreasing uterine tone. Typical maneuvers used to increase oxygen supply to the fetus include repositioning the mother onto her left side to increase cardiac output, oxygen administration, giving medications that will slow contractions, and, on occasion, urgent cesarean delivery is required.

Cesarean delivery is the transabdominal surgical removal of the fetus from the uterus. The primary cesarean rate in the United States ranges from 8% to 18% on average and the total cesarean rate (including repeat cesarean sections) ranges from 16% to 22%. Typically, a Pfanensteil or vertical skin incision is used to start and the abdomen is opened in layers. The uterine incision is most commonly used in the term gestation is the low transverse incision which is considered to be the strongest in healing and least hemorrhagic. Other potential uterine incisions are the low vertical, the T-incision, the J-incision, and the classical incision. The average blood loss at the time of cesarean delivery is 1,000 ml versus 600 ml for a vaginal birth. There is also a higher incidence of febrile morbidity with cesarean delivery.

Assisted vaginal deliveries or instrumental deliveries include vacuum and forcep deliveries. An assisted delivery may be indicated when there is a prolonged second stage (it is taking too long for the mother to push out the baby), there is suspicion of fetal compromise, or when there is a need to shorten the second stage in instances of certain maternal ill health. Several prerequisites need to be met before an instrumental delivery is begun. Both forceps and vacuum deliveries are associated with an increase in neonatal and maternal morbidities. Some of these morbidities include: damage to the maternal soft tissue; maternal pain; neonatal subgaleal and cephalohematomas (blood clots in or around the brain); neonatal retinal hemorrhages (bleeding in the back of the eye); neonatal hyperbilirubinemia (increase in bilirubin, which causes jaundice); and neonatal facial nerve and/or ocular (eye) damage.

Despite a short-term morbidity of as high as 5%, the long-term morbidities do not appear to be increased.

Labor typically occurs at full term which is defined as a pregnancy at greater that 37 weeks gestation. However, up to 10% of all viable pregnancies are delivered before term. Depending on the exact gestational age at delivery, there can be serious morbidity and mortality rates in the neonate related only to prematurity. Some of the common risks of prematurity include: intracranial hemorrhage, respiratory distress syndrome, sepsis, necrotizing enterocolitis, retinal hemorrhage, auditory dysfunction, and jaundice.

Modern science has yet to understand the natural process for the initiation of labor at term and has only some ideas and associations for what causes preterm labor. The true biochemical process by which labor is initiated has yet to be described, and similarly the cessation of labor is hard to achieve. The cessation of labor or tocolysis has been attempted with several types of medications including: beta mimetics, oxytocin antagonists, nonsteroidal anti-inflammatories, magnesium sulfate, calcium channel blockers, progesterone, and even ethanol. There are varying successes with these drugs and varying side effects. Perhaps the most progress in preventing prematurity has been the use of maternal steroid administration, which may not be without its own side effects. Modern medicine continues to research potential therapies for preterm labor, yet the best medicine may continue to be prevention.

SEE ALSO: Cesarean section, Episiotomy, Pregnancy, Prenatal care, Teen pregnancy, Ultrasound

Suggested Reading

  • Simkin, P. (1989). The birth partner. Boston: The Harvard Common Press.
  • Simkin, P., Whalley, J., & Keppler, A. (1984). Pregnancy, childbirth, and the newborn: A complete guide for expectant parents. New York: Meadowbrook.

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