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Department of Obstetrics, Stanton Territorial Hospital, Yellowknife NTSchool of Population and Public Health, University of British Columbia, Vancouver BC
A belief that prolonged head-to-body delivery interval endangers the newborn underpins the common obstetrical practice of delivering the baby’s trunk immediately after the head is born. Without intervention, however, birth typically occurs in two steps: once the fetal head is delivered there is usually a pause, and the rest of the infant is born with the next contraction. Allowing a two-step delivery does not increase the risk of fetal harm, and may lower the incidence of shoulder dystocia. A two-step approach to delivery should be considered physiologically normal. This has implications for the definition of shoulder dystocia.
Résumé
L’opinion selon laquelle la vie du nouveau-né est mise en danger lorsque l’intervalle entre l’accouchement de la tête fœtale et celui du reste du corps est prolongé est à l’origine de la pratique obstétricale courante qui cherche à accoucher le tronc fœtal immédiatement après l’accouchement de la tête. Toutefois, en l’absence d’intervention, l’accouchement se déroule généralement en deux étapes : une pause suit habituellement l’accouchement de la tête, puis le reste du corps est accouché au moment de la contraction suivante. Le fait de permettre un tel accouchement en deux étapes ne donne pas lieu à une hausse des risques auxquels le fœtus est exposé et pourrait même abaisser l’incidence de la dystocie de l’épaule. L’approche en deux étapes envers l’accouchement devrait être considérée comme étant physiologiquement normale; il en résulte donc des répercussions pour ce qui est de la définition de la dystocie de l’épaule.
There is a common belief, reinforced in obstetrical textbooks, that a prolonged head-to-body interval during delivery endangers the infant. “Most often, the shoulders appear at the vulva just after external rotation and are born spontaneously. If delayed, immediate extraction may appear advisable. The sides of the head are grasped with two hands, and gentle downward traction is applied until the anterior shoulder appears under the pubic arch.”
In the second study, 22 women with low-risk pregnancies were randomized to “normal” delivery or “fast” delivery that was “hastened by an early episiotomy, speeding delivery technique, encouraging pushing and by the use of Wrigley’s forceps in the case of delay.”
In both studies, a mathematically significant but clinically insignificant decrease in cord pH was observed with increasing duration for delivery of the fetal trunk. It was concluded that anupper time limit of … 40 seconds for delivery of the trunk [is] ideal … Unless the obstetrician can be certain that the fetus is in good condition, it may die or suffer brain damage from added asphyxia as a result of delay during normal birth.
Some medical texts acknowledge that this delay is physiologic, and it is common midwifery practice to allow a pause after delivery of the head to await the next contraction.
Once crowned, the head is born by extension. … During the resting phase before the next contraction, the midwife may check that the cord is not around the baby’s neck … Restitution and external rotation of the head maximizes the smooth birth of the shoulders …
Is there evidence supporting one approach over another? There is great variation in what can occur between delivery of the fetal head and delivery of the body; and fetal status appears to be influenced not only by the duration of the head-to-body interval, but by what happens during that interval.
The imperative to deliver the trunk quickly also stems from concerns about shoulder dystocia, in which a prolonged head-to-body delivery interval is associated with fetal harm. The accepted definition of shoulder dystocia—the need for additional obstetric manoeuvres beyond gentle traction to deliver the fetal shoulders—is subjective, although a head-to-body delivery interval of more than 60 seconds has also been proposed.
However, delay during shoulder dystocia is caused by impaction of the fetal shoulder against the maternal symphysis pubis or the sacral promontory, and this is fundamentally different from a physiological delay between contractions. During shoulder dystocia, uterine contractions, maternal pushing efforts, and reduction manoeuvres invariably occur. In contrast, during the physiological delay in spontaneous birth, the uterus and maternal abdomen are relaxed, the fetal head and shoulders often restitute, and delivery usually occurs spontaneously with the next contraction. Expediting delivery with traction before the next contraction may cause harm. First, it may not be effective, which can lead to an unwarranted diagnosis of shoulder dystocia, increased traction, and ancillary manoeuvres which have a risk of fetal and maternal harm. Second, traction may impair normal restitution of the fetal shoulders and cause shoulder dystocia.
Feared complications of shoulder dystocia are permanent fetal brachial plexus injury and hypoxic ischemic encephalopathy (HIE). Stretching of the brachial plexus nerve roots can lead to transient or permanent nerve damage. This damage is caused by intrinsic impaction of the fetal shoulder and/or traction on the fetal head while the shoulders are impacted. Despite the admonition not to pull during shoulder dystocia, significant traction often occurs.
However, there is no mechanism by which shoulder dystocia itself causes cord compression. If the fetal heart rate (FHR) has been abnormal before delivery, fetal acidosis may be worsened by further delay. Leung et al. examined cord pH in relation to head-to-body delivery interval in 200 cases of shoulder dystocia.
A drop in cord arterial pH observed with increasing head-to-body delivery interval was mostly related to the presence of an abnormal FHR prior to delivery. Prolongation of the head-to-body delivery interval itself was associated with a clinically insignificant drop in cord arterial pH of 0.01 per minute (Figure 1). Of note, the average arterial cord pH in five cases of HIE caused by shoulder dystocia was 7.18—not different from cases of shoulder dystocia without HIE (7.22) (Table).
Figure 1Change in cord arterial pH with head-to-body delivery interval in 200 cases of shoulder dystocia.
Similarly, in a study of over 8000 births, Stallings et al. observed no correlation between head-to-body delivery interval and cord pH in 134 cases of shoulder dystocia.
Among cases lasting three minutes or more, the mean pH was 7.26; the pH remained normal when two or more manoeuvres were required and when neonatal brachial plexus injury occurred. “The implication of these findings is that a ‘rush’ or ‘crash’ delivery of the fetus with shoulder dystocia is unnecessary.”
The mechanism of HIE caused by shoulder dystocia appears different from that of HIE caused by other intrapartum factors. Shoulder dystocia can cause HIE very rapidly. In three of five cases in the series of Leung et al., the head-to-body delivery interval was five minutes or less.
This is consistent with the findings in a United Kingdom confidential enquiry, in which 21 of 56 fetal deaths due to shoulder dystocia occurred with a head-to-body delivery interval of less than five minutes.
In a normally oxygenated fetus, the initial acid generated is predominantly due to accumulation of CO2 (respiratory acidosis); the pH will drop to 7.0 in approximately eight minutes, but the base deficit rises only modestly. Once ventilation is established after birth, excess CO2 is rapidly eliminated and acidemia quickly resolves. Predominantly respiratory acidosis poses little risk of HIE; however, prolonged occlusion leads to metabolic acidosis and HIE.
How, then, does shoulder dystocia cause HIE in less than five minutes and leave the neonate with near-normal umbilical cord blood gases? Invasive testing of the fetus during shoulder dystocia is impractical and unethical.
Since there are no animal models of shoulder dystocia, the mechanism is difficult to identify. Physiologically, however, the discrepancy between cord gases and HIE seems best explained by changes in fetal hemodynamics and cerebral perfusion after the fetal head is born, while the fetal body remains within the mother.
Once delivered, the fetal head is exposed to atmospheric pressure. While the uterus is flaccid and the mother is not pushing, intrauterine pressure is 10 to 15 mmHg above atmospheric, allowing venous return from the fetal head to the fetal thorax. With a contraction, intrauterine pressure rises to 60 to 80 mmHg and with maternal effort, to more than 100 mmHg above atmospheric.
At this pressure, venous blood cannot return from the fetal head to thorax, and cerebral perfusion ceases. Local brain hypoxia and metabolic acidemia occur rapidly, even though circulation and oxygenation of the rest of the body remains normal. During shoulder dystocia, then, limiting maternal pushing efforts between contractions (while manoeuvres are performed) preserves cerebral perfusion as much as possible.
By inference, a prolonged head-to-body delivery interval without uterine contraction or maternal pushing effort is not harmful to the fetus. In 2011, Locatelli et al. published an observational study of a “two-step” approach to delivery in 1231 vaginal births.
the two-step approach involvedwaiting for restitution of the fetal head following its delivery without manipulation of the presenting part, and waiting for uterine contractions to accomplish spontaneous delivery of the shoulders and the whole body.
A dedicated observer measured head-to-body delivery interval, and umbilical cord blood gases were recorded. A prophylactic McRoberts manoeuvre was used if a “turtle sign” was observed, although this did not constitute a diagnosis of shoulder dystocia.
Again, a clinically insignificant drop in cord arterial pH of 0.0078 per minute of head-to-body delivery interval was noted (Figure 2). The proportion of cases with fetal cord pH<7.1 and/or base deficit>12 was similar whether the head-to-body delivery interval was more or less than 60 seconds. The fetal shoulders delivered during the same contraction as the head in less than 20% of cases. Shoulder dystocia was diagnosed if ancillary measures were required to deliver the shoulders with a subsequent contraction. The mean head-to-body delivery interval was 83±57 seconds in multiparas and 93±65 seconds in nulliparas (P=0.02). Although a “turtle sign” occurred in 15 cases, there were only three cases of shoulder dystocia (0.24%), two in precipitous births. There were no cases of brachial plexus injury or HIE.
This very low incidence of shoulder dystocia is possibly related to three factors. First, fetal shoulders that do not deliver with gentle traction between contractions may deliver spontaneously with maternal pushing efforts during the next contraction. Such births were considered normal in the series of Locatelli et al., but would be counted as cases of shoulder dystocia in many conventional settings. Second, the pause between contractions observed in most of the parturients in the study of Locatelli et al. may have allowed more complete restitution of the fetal head and shoulders and prevented true shoulder dystocia. Third, the prophylactic McRoberts manoeuvre employed while waiting for the next contraction may have prevented shoulder dystocia.
After delivery of the head, long intervals between contractions are uncommon. The longest head-to-body delivery interval in the study of Locatelli et al. was six minutes, and only 15 women experienced an interval longer than four minutes.
Provided intrauterine pressure is low, fetal cerebral perfusion continues; but for how long is it safe to wait? In cases of shoulder dystocia, the risk of HIE increases as the head-to body delivery interval increases beyond five minutes; however, the uterine contractions and maternal Valsalva efforts during this interval impair fetal cerebral circulation.
In a two-step approach to delivery, we conclude that a head-to-body delivery interval of up to four minutes between contractions is common, is safe, and may reduce the incidence of shoulder dystocia. After delivery of the head, it would seem that shoulder dystocia should be diagnosed only if delivery of the shoulders does not occur with maternal pushing efforts (and possibly gentle traction) that accompany the next contraction.
Once shoulder dystocia is diagnosed, standard management algorithms should be employed.
However, attention to maternal relaxation between contractions may be helpful in two ways. First, by reducing pressure on the impacted shoulder, relaxation assists the clinician’s efforts to disimpact it; and secondly, by reducing venous back-pressure in the fetal brain, relaxation helps preserve fetal cerebral circulation during the head-to-body interval.
Finally, if fetal monitoring indicates fetal compromise, expedited delivery of both the fetal head and body would be indicated rather than a two-step approach.
REFERENCES
Cunningham F.
Leveno K.
Bloom S.
Hauth J.
Rouse D.
Spong C.
William’s obstetrics.
Normal labor and delivery. 23rd ed. McGraw-Hill,
New York2009 (Chapter 17)
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