In their landmark book, The Labor Progress Handbook, Penny Simkin and Ruth Ancheta describe six steps in the progression of labor1:
- The cervix moves from a posterior to an anterior position
- The cervix ripens or softens
- The cervix effaces
- The cervix dilates
- The fetal head rotates, flexes, and molds
- The fetus descends and is born.
|Figure 1 – Fetus within pelvic inlet|
They explain that when the cervix has not undergone the first three steps of anterior movement, ripening, and effacement, significant dilation rarely occurs. “If this progress is ignored, an incorrect diagnosis of dysfunctional labor may be made before the woman is even in labor!”
Cesarean section for the indication of “obstructed labor” has steadily increased over the past 40 years. Simkin and Ancheta illustrate that failure to progress is often a result of fetal malposition, such as persistent asynclitism or occiput posterior.
“If early contractions are painful and irregular with little or no progress in dilation, it makes sense to consider persistent asynclitism or another unfavorable fetal position, such as occiput posterior. Labor normally begins with the fetal head in asynclitism, (the head is angled so that one of the parietal bones, rather than the vertex, presents at the pelvic inlet). This facilitates passage of the fetal head through the pelvic inlet, and then the head usually shifts into synclitism so that the vertex presents as the head descends further. Sometimes the asynclitism persists and, if so, it can keep the fetus from rotating and descending. Without descent, the head may not be well applied to the cervix and contractions often become irregular and ineffective.”
Maternal positions alter the forces of gravity, and midwives have long known that “having the woman lean forward moves the fetus’s center of gravity forward, which encourages its head to pivot into a more favorable position, leading to more regular, more effective contractions.”
While the major cause of failure to progress is well described, it remains unknown how non-progression ocurrs. How does the fetus get stuck? The anatomic reason for persistent asynclitism has not been fully described by either midwifery or obstetrics. However, biomechanical data from orthopedics and observations by gynecologic surgeons fill gaps in understanding of this very common phenomenon.
In their classic text, Vaginal Surgery2, Nichols and Randall describe how gradual the early stages of labor progression really are:
“Normally, and certainly in the labor of the primipara, at full dilation of the cervix the presenting part does not at that time emerge from the cervix and, for the first time, begin to descend into and through the vagina. Rather, the fully engaged presenting part, almost completely covered by thinned, beginning to dilate cervix, has in all probability occupied the upper third to half of the vagina for 2 weeks or more. As a result, distention of the upper vagina, with accommodation of the engaging vertex or breech, has occurred very gradually, so gradually in fact that the patient may not be aware of the descent taking place until she notices a new awareness of heaviness, low backache, and at times rectal pressure, while at the same time breathing becomes somewhat easier, for ‘lightening’ has occurred.”
|Figure 2 – The pelvic “floor”|
For the fetal head to enter the pelvic inlet, the top of the sacrum must move up and out of the pelvic interior, into a position known as “counter-nutation”. When the pelvis is in this position, the lumbar spine is flattened. As described above, the fetus moves very slowly through this early progression. This means the maternal pelvis stays in a counternutated position from the time the fetus enters the pelvis, until it is born. While it is often assumed that maternal lumbar curvature increases throughout pregnancy, it was proven in 1976 that the spine actually flattens during the final weeks before birth3. Even if a fully gravid woman is trying to maintain a full lumbar curvature, her spine remains in counternutation.
Figure 2 shows the correct orientation of the pelvis in the standing body, the entrance into the pelvis, and the pelvic diaphragm with anal opening at the back. Toward the end of pregnancy, the cervix, which has been pointing toward the back all these months, must now rotate anteriorly toward the vaginal opening before labor can progress.
|Figure 3 – Improper sitting position
in late pregnancy
The most likely reason anterior rotation of the cervix becomes stalled is because the fetal head has moved too deeply into the back of the pelvis. In order for the cervix to rotate forward, the fetal head must move forward and into a more favorable position. This is supported by Simkin and Ancheta, who show that forward-leaning maternal positions help labor to progress after persistent asynclitism.
With this understanding, it becomes obvious that the very worst position a woman in the last weeks of pregnancy can spend time in is a full squat with her knees higher than her hips. The pelvic inlet is fully open at this stage and the fetal head has entered the pelvis. Therefore, gravity simply moves the asynclitic head further into the back of the pelvis where it prevents anterior rotation of the cervix and stalls the progression of labor.
Deep squatting is being advised as preparation of the pelvic “floor” for birth. However, no scientific data exists that the pelvic floor needs special preparation outside of normal physical activity. If the pelvic diaphragm were anything but fully supportive, the sharp angle that it makes as it wraps around the anus would widen, and fecal incontinence would ensue.
The pelvis and pelvic diaphragm have a range of motion that is utilized in many functions, including vaginal childbirth. While pelvic nutation and full lumbar curvature represent the position of pelvic, pelvic organ, and spinal stability, during the last weeks of pregnancy the pelvis takes advantage of its full range of motion by remaining in counternutation as the cervix is prepared for birth. Support the subtle and lengthy early stages of labor progression by keeping sitting positions where the knees are lower than the hips.
1 Simkin P Ancheta R The Labor Progress Handbook Wiley-Blackwell 2000
2Nichols D Randall C Vaginal Surgery Williams and Wilkins 1989
3Snijders C et al Change in form of the spine as a consequence of pregnancy. Digest of the 11th International Conference on Medical and Biological Engineering August 1976