Open access peer-reviewed chapter
Abstract
Obesity is an epidemic worldwide with about half of the population being classified as overweight and obese. Second stage labor arrest may occur more commonly in patients with obesity, leading to a higher number of cesarean deliveries. Second stage arrest is diagnosed based on a fixed time frame without consideration of the total amount of expulsive forces exerted. The beginning of the second stage of labor is determined by an arbitrary parameter that depends on the timing intervals at which patients are examined. In other words, the specific point at which the second stage begins (complete dilation) is influenced by the frequency of examinations conducted during labor. Therefore, it would be reasonable to consider factors such as the cumulative duration of pushing and/or the number of pushes and/or cumulative force generated as clinical measurements to determine the optimal length of the second stage of labor. By redefining the criteria used to define second stage labor dystocia or arrest, we can ensure that patients are given an appropriate amount of time to generate the expulsive forces necessary for a safe vaginal delivery. Here, we will review the evidence to help optimize the management of the second stage.
Keywords
- second stage of labor
- obesity
- maternal body mass index
- cesarean delivery
- abnormal labor
1. Introduction
The second stage of labor is defined as the time from complete dilation of the cervix to delivery of the fetus. Several factors can influence the length of the second stage, including parity, maternal body mass index (BMI), fetal weight, fetal presentation, fetal position, and the use of analgesia [1]. In this chapter, we will focus on how obesity affects the duration of labor as the prevalence of obesity is rising among reproductive-aged individuals.
Obesity, defined as a BMI ≥ 30 kg/m2, poses unique challenges for obstetrical providers throughout pregnancy. It increases the risk of hypertensive disorders, gestational diabetes, macrosomia, intrauterine growth restriction, and infections. Intrapartum care of obese women also presents challenges, including a higher rate of cesarean delivery and associated complications such as postoperative wound infections, thromboembolic events, postpartum hemorrhage, endometritis, and delayed wound healing [1]. One study showed the rate of cesarean delivery during the second stage of labor in obese patients was twice as high as that in leaner individuals, especially for those with a BMI ≥ 35 kg/m2 [2]. Most of these cesarean deliveries were unscheduled due to labor arrest [2, 3].
Currently, the American College of Obstetricians and Gynecologists (ACOG) and Society for Maternal-Fetal Medicine (SMFM) define second stage of labor arrest as at least 2 hours of pushing in multiparous women and at least 3 hours of pushing in nulliparous women, with longer durations for epidural use and fetal malposition if progress is being documented [4]. However, the actual onset and duration of the second stage can be inaccurately recorded as the very definition of the beginning relies on an arbitrary time point when the examiner finds the cervix to be completely dilated. It would be reasonable to consider the cumulative length of time a patient is pushing, the number of pushes, and/or cumulative force generated as a clinical measurement to define the optimal length of the second stage and potentially change the trigger for diagnosis of second stage labor arrest. This would be a similar concept to using Montevideo units for defining adequate uterine contractility during the first stage of labor.
In this chapter, we will review the evidence on optimal management of the second stage of labor, with a focus on achieving vaginal delivery without increasing maternal and neonatal morbidities and mortalities.
2. Pelvic anatomy and physiology of labor
2.1 Anatomy
The maternal pelvis consists of the bony pelvis and surrounding soft tissue structures. The size and shape of the pelvis play an important role in the second stage of labor. Four different pelvic types have been described (Figure 1): gynecoid (41.4%), android (32.5%), anthropoid (23.5%), and platypelloid (2.6%) [5]. The most common pelvic type is gynecoid, which is considered the most favorable shape for fetal descent during the different cardinal movements of labor (engagement, descent, flexion, internal rotation, extension, external rotation). During the second stage of labor, the fetal head is mostly found in an oblique occiput anterior (OA) position [6]. However, certain pelvis types like anthropoid and platypelloid are at a higher risk of fetal malposition, such as occiput posterior (OP) and occiput transverse (OT) position, respectively. Patients with a fetus in the OP position have been found to have a longer second stage of labor, up to 60 minutes longer compared to a fetus in the OA position [7]. A systematic review of randomized controlled trials demonstrated that manual rotation of the occiput was associated with a higher rate of vaginal delivery: 64.9% in the manual rotation group vs. 59.5% in the control group [8]. Manual rotation also may shorten the second stage of labor by 15 to 30 minutes, depending on parity [8]. Manual rotation causes no difference in neonatal outcomes and thus should be attempted before proceeding to operative delivery or cesarean delivery [4].
Figure 1.
Illustrating the four different pelvic types, the gynecoid pelvis is considered the most ideal type for childbirth due to its wide pelvic inlet. The android pelvis, more commonly found in males but occasionally in females, has a heart-shaped pelvic inlet with a narrow opening and a pronounced sacrum. The anthropoid pelvis is characterized by an oval-shaped inlet and is often associated with the OP position of the baby’s head during labor. Lastly, the platypelloid pelvis is wide and flat in shape.
2.2 Endocrinology of labor
Hormonal signals play a crucial role in orchestrating the initiation and progression of labor. Throughout pregnancy, progesterone keeps the uterus in a quiescent state [1, 9, 10]. As labor approaches, progesterone levels decline, while estrogen and corticotropin-releasing hormone (CRH) start to rise [9]. CRH is released from the placenta and stimulates the fetal adrenal glands to produce cortisol which in turn boosts the production of prostaglandins [9]. Estrogen also promotes the synthesis of prostaglandins, increases receptivity to oxytocin, and stimulates the growth of uterine muscle, leading to the initiation of labor [9]. Prostaglandins help soften and ripen the cervix [1, 10]. During this time, oxytocin levels and receptors rise, aiding in the stimulation of the smooth muscle fibers in the myometrium leading to an increase in the frequency, duration, and intensity of uterine contractions [9, 10]. Oxytocin is released from the posterior pituitary gland, and its release is regulated by a positive feedback mechanism during labor, where further descent of the fetus into the pelvis increases oxytocin release [6].
3. Second stage of labor
3.1 Current definition of duration of the second stage of labor
The duration of the second stage of labor can vary among individuals and is influenced by several factors. The second stage of labor begins when the cervix is fully dilated and usually last about 50 minutes in nulliparas and 20 minutes in multiparas, however, is increased in duration in patients with epidural analgesia [10]. In 2014, an obstetric care consensus recommended following a specific maximum length to avoid prematurely performing a cesarean delivery: nulliparous with an epidural up to 4 hours, nulliparous without an epidural up to 3 hours, multiparous with an epidural up to 3 hours, and multiparous without an epidural up to 2 hours [4].
3.2 Prolonged second stage of labor
A prolonged second stage of labor refers to a delay in the progression of labor following complete cervical dilation, which has been redefined by ACOG in recent years to prevent primary cesarean sections. Unfortunately, obesity is recognized as one of the leading causes of a protracted labor course, as studies indicate that obese patients take approximately 1.3 hours longer to progress from 4 to 10 cm [1, 11, 12, 13]. While it is well-established that the first stage of labor is prolonged in obese patients, the duration of the second stage remains a subject of controversy [13]. Some studies suggest a longer duration [14, 15], others indicate equivalence [16, 17], and others report a shorter duration of the second stage [18]. This variability could be attributed to factors such as the increased likelihood of cesarean delivery interrupting the natural course of labor or the wide range of BMI values encompassed within these studies. Here we approach this issue from a different perspective.
Traditionally, the duration of the second stage of labor is measured in length of time. Defining the start of the second stage based on the cervix reaching 10 cm is prone to inaccuracies due to the variability in frequency of pelvic examination during labor. It is entirely possible that many patients are completely dilated well before the examiner detects it. The length of the second stage also depends on the frequency of uterine contractions, the time a patient spent “laboring down”, and any breaks they may take in between or during contractions. Thus, defining a prolonged second stage by the total length of time is confounded by the heterogeneity of the clinical picture. Medical decision-making based on absolute time definitions may even result in patients undergoing cesarean delivery before having an adequate chance or length of time to push. Therefore, we propose using the cumulative length of time a patient is pushing or the number of pushes as a clinical measurement to define the optimal length of the second stage.
Concerns have been raised about the impact of a prolonged second stage of labor on perinatal outcomes. A randomized control trial conducted by Gimovsky et al. demonstrated that an extra hour of pushing reduced the incidence of cesarean delivery from 43.2 to 19.5% without increasing maternal and neonatal morbidity [19]. There have been several studies including a large multicenter randomized control trial, cohort studies, and systematic reviews that have also shown no increase in perinatal outcomes with a prolonged second stage of labor [20, 21, 22]. Therefore, it may be reasonable to extend the second stage pushing time for patients without fetal heart rate abnormalities, especially for obese patients [4].
3.3 Uterine contractility
Uterine contractions are essential to descent and ultimately delivery of the fetus. Contractions exhibit three main properties: frequency, duration, and strength [23]. In the second stage of labor, it is observed that contractions last up to 1 minute and recur at intervals no longer than 90 seconds [10]. Obesity is linked to reduced uterine contractility and irregular contraction patterns [11, 14]. While the exact mechanism for dysfunctional contractions in obese patients is unknown, there are several potential causes.
Despite dysfunctional contractions, obese patients are still capable of generating adequate Montevideo units, but may require higher doses of oxytocin to produce adequate contractions [17]. Obese individuals undergoing labor induction require progressively higher cumulative doses of oxytocin as their BMI class increases [17]. In order to optimize the chances of a vaginal delivery in obese patients, it may be necessary to closely monitor and aggressively adjust oxytocin doses to ensure adequate contractions to facilitate progression of labor.
3.4 Valsalva forces
Expulsive forces, or pushing through cognizant contraction of the striated muscles in the chest and abdomen, have been demonstrated to be necessary for achieving vaginal delivery [24]. Directed pushing, where the patient actively participates and follows verbal instruction, has been associated with a shorter duration of the second stage [6]. It is the cumulative effects of uterine contractions and expulsive forces that are necessary for the fetus to descend through the pelvis, resulting in a vaginal delivery.
We have found that patients with a BMI ≥ 35 kg/m2 generate lower pressure while pushing in the second stage of labor [25]. Lower intrauterine pressure suggests less expulsive force and hence increased risk for second-stage dystocia or arrest. This suggests that obese patients may need a longer second stage of labor to achieve a vaginal birth.
There are several potential explanations for these observations. It is possible that the abdominal musculature in patients with a BMI ≥ 35 kg/m2 is stretched beyond the optimal point of sarcomere overlap and therefore unable to generate adequate expulsive force through muscle contraction. Or perhaps, an increased body weight is also associated with metabolic dysregulation by which hyperlipidemia interferes with calcium influx and leads to fatty infiltration of the muscles, thus impairing abdominal and myometrial contractility. However, further research is needed to explore the exact mechanism.
Since obese patients can not generate sufficient expulsive force per contraction, it may be better to measure the cumulative force of sequential contractions combined with pushing generated during the second stage of labor as a parameter for assessing progress and determining the likelihood of achieving a successful vaginal delivery.
3.5 Immediate versus delay pushing
There are two common practices used in the management of the second stage of labor: (1) delayed pushing, or “laboring down” in which the patient rests and the fetal head passively descends with contractions), and (2) immediate pushing, where pushing with contractions is initiated once the cervix is complete. Delayed pushing was thought to provide several benefits, including reducing maternal fatigue, minimizing the risk of perineal trauma, and allowing time for the baby’s head to mold and adjust to the birth canal [26]. However, a meta-analysis of randomized control trials, involving 5445 patients with epidural analgesia, showed delayed pushing was associated with increased morbidity. Among patients in the delayed pushing group, there was a significantly prolonged duration of labor, higher incidence of chorioamnionitis, and lower umbilical cord pH, with no difference in the mode of delivery [27]. Another large multicenter randomized clinical trial including 2414 patients found similar rates of spontaneous vaginal delivery: 85.6% in the immediate pushing group versus 86.5% in the delayed pushing group. Again, in the delayed pushing group there was a longer second stage of labor, higher rates of chorioamnionitis, increase neonatal acidemia, and also higher rates of postpartum hemorrhage [28]. The trial was stopped prematurely because of concern for excess morbidity in the delayed pushing group. Based on these findings, it is reasonable to prioritize immediate pushing over delayed pushing [28].
3.6 Maternal position
There are varying opinions regarding the optimal maternal position during the second stage when pushing. Maternal position may play a significant role in facilitating fetal descent. A Cochrane review comparing different pushing positions found that an upright position offers several benefits, including reduced fetal heart rate abnormalities, decreased rates of episiotomy, and a shorter duration of the second stage of labor by 6.6 minutes [29]. However, an upright position was associated with a higher risk of postpartum hemorrhage when compared to the supine position [29].
Both upright and lateral pushing techniques are effective in preventing perineal trauma [29]. Pushing in the lateral recumbent position can also aid in rotating a fetus in the OP position. In the epidural and position trial collaborative group study, a higher rate of vaginal delivery was observed when patients pushed in a lateral recumbent position compared to other positions [10, 32]. Nevertheless, due to the variability in findings and individual patient and provider preference, there is no position recommended over others.
ACOG recommends frequent position changes during labor to enhance comfort and promote optimal fetal positioning, as long as these positions are not contraindicated for the patient [30]. It is important for healthcare providers to consider individual circumstances, patient preferences, and safety when determining the most appropriate pushing position.
3.7 Operative vaginal delivery
Over the last 15 years, the rate of operative vaginal deliveries has declined as the rates of cesarean deliveries rise [4]. An operative vaginal delivery involves the use of either a vacuum or forceps to assist in the delivery of the baby. This procedure may be necessary in situations where there is a need to expedite delivery due to maternal or fetal factors. Some specific indications for operative vaginal delivery include a prolonged second stage of labor, non-reassuring fetal heart tones, maternal exhaustion, and maternal cardiopulmonary disorders or brain aneurysms that prevent prolonged maternal pushing [23].
Although there are risks associated with operative vaginal delivery, including maternal perineal trauma, lacerations, fetal scalp and skull injuries, and intracranial hemorrhages, large studies have demonstrated no difference in serious neonatal morbidity between operative vaginal delivery and unplanned cesarean delivery [4]. The safety of the procedure depends on the practitioner’s experience and the station of the presenting part [32]. The overall success rate of operative vaginal delivery has been reported to be between 86 and 91% [33]. However, lower success rates have been found in cases where maternal BMI exceeds 30 kg/m2 and ironically, maternal obesity is known to be a risk factor for operative vaginal deliveries [31, 34].
Because obese patients generate lower pressure during pushing, an operative device may be needed to supplement the force during the second stage of labor.
Overall, the decision to proceed with an operative vaginal delivery is based on individual circumstances and requires careful consideration of the potential benefits and risks for both the mother and the fetus. It is important for healthcare providers to assess each situation on a case-by-case basis to make an informed decision, but operative vaginal deliveries should be considered a safe and acceptable alternative to cesarean delivery [4].
4. Pain control
4.1 Epidural analgesia
Epidural analgesia use in labor is a popular choice for pain relief among many individuals due to its effectiveness. However, the use of epidural analgesia has an impact on the progression of labor. A meta-analysis revealed that epidural anesthesia prolongs the second stage of labor by approximately 15 to 28 minutes [35], which means that patients who opt for epidural analgesia are typically given more time to complete the second stage of labor.
Additionally, studies have explored the impact of the timing of epidural administration by comparing an early versus late epidural placement. Despite some controversies, the collective findings indicate that the timing of epidural placement has minimal impact on the overall progression of labor [36]. However, a recent study of 7647 patients demonstrated an association between early epidural use (< 6 cm) and a decreased risk of a prolonged second stage of labor [37]. Based on these findings, it is reasonable to consider epidural use at any point during labor, taking into account the patient’s personal values and preferences.
It is worth noting that alternative pain control options, such as intravenous and intramuscular opioids, are available but are generally recognized to be less effective than epidural analgesia, and are avoided during the second stage of labor due to concern for respiratory depression of the neonate.
5. Conclusion
Optimizing the management of the second stage of labor involves considering factors such as pelvic shape and size, fetal position and presentation, Montevideo units, Valsalva forces, timing of pushing, maternal position, and considering operative vaginal deliveries when appropriate. Optimal, evidence-based practice recommendations include:
Ensuring adequate Montevideo units, particularly in obese patients who are more prone to dysfunctional labor.
Initiating immediate pushing at the time of completion, irrespective of the use of epidural analgesia or the urge to push.
Attempting manual rotation of a fetus from an occiput posterior or occiput transverse position to an occiput anterior position.
Encouraging patients to follow their instinct to bear down when they feel the urge to do so, in a position they find comfortable
It is important to appreciate that an unplanned and emergent cesarean section during second stage of labor is associated with a significantly increased maternal and perinatal morbidity and mortality [38]. Therefore, every attempt should be made to achieve a safe vaginal birth. To facilitate this goal, it may be necessary to redefine the optimal length of the second stage of labor, considering alternative clinical measurements such as the cumulative length of time a patient spends pushing, the number of pushes, and/or the cumulative force generated.
Lastly, it is important to note that obesity presents numerous challenges throughout pregnancy, and these challenges may become more pronounced in the second stage of labor. Ultimately, the most effective strategy for minimizing complications during pregnancy and delivery is the prevention of obesity itself and achieving adequate weight loss prior to conception.
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