Rose MacColl described two philosophies in maternity care: the organics and the mechanics. One philosophy is not designed to be better, nor more correct than the other, but they are very different ways of viewing women’s bodies and how they work during birth.
The origin of mechanical views of birth has been linked to the use of dissection of women who died during pregnancy, labour, or soon after birth as a means to generate new knowledge about how the body operated. The invention of X-rays also played a part, as we could peer into the insides of live bodies. Bones are easy to see on X-ray – and it isn’t difficult to then imagine that birth involves one bony shape (the head of the fetus) passing through another bony shape (the woman’s pelvis).
Some babies simply don’t fit through the woman’s pelvis and her labour becomes obstructed. Safe caesarean section has transformed what was historically a life-threatening complication. At present the only reliable way to determine whether the baby will fit, is for women to labour, and see what happens. Evidence is clear that performing a caesarean section after many hours of labour, when the fetal head is firmly wedged into the bony pelvis is a riskier proposition for both mother and baby (Yang and Sun, 2017). It would be really handy to have a reliable way to determine in advance whether the combination of this particular fetus in this particular woman’s pelvis is going to result in a safe vaginal birth or lead to a difficult caesarean section, so we can offer pre-labour caesarean section to women would benefit from this.
We have long recognised that X-ray imaging during pregnancy carries with it some risk, particularly to the baby. To use routine X-rays of the woman’s pelvis with the fetus in utero is therefore not a viable option. In more recent times, ultrasound has largely replaced X-ray imaging in pregnancy. So, why not use ultrasound to look at the woman’s pelvis to see if it is going to have sufficient room for the baby to pass through?
That’s exactly what a team of Brazilian researchers have done. Neto, et al have recently published their findings in an open access journal (so they are available to anyone who wants to read the paper – linked below). Women who were in labour at term, with a head-first fetus, were asked for permission to have an ultrasound examination performed. This was a trans-perineal scan, meaning the ultrasound probe was placed on the outside of their genital area, enabling the researcher to take measurements of certain features of the bones of their pelvis, and to see the fetal head at the same time. Researchers were particularly interested in measuring the pubic arch angle (there are images in the paper showing what this is) as the fetus has to pass under this angle to exit the pelvis. The results of this measurement were not given to staff who were caring for the woman, so that it wouldn’t impact on the care the woman was offered.
After the women gave birth, the researchers then looked to see if there was a difference in the pubic arch angle for women who gave birth vaginally and those who had a surgical birth (either caesarean section or forceps). You might expect that a smaller angle measurement, and therefore a tighter fit out of the pelvis would mean lower rates of vaginal birth. That’s not what the research showed however, with no significant difference in the pubic arch angle between the two groups of women (see Table 2).
What they did find was that when the fetus was coming through the pelvis in the less common occipitoposterior position (with the back of it’s head in the back part of the woman’s pelvis), the women did have a significantly smaller pubic arch angle (see Table 3). They didn’t go on to examine whether this position of the fetal head predicted surgical birth, but given that only ten fetuses were in this position, and sixty-eight women had surgical births, the study would not have had sufficient numbers to determine this with any statistical accuracy.
So, what do their findings mean? They support the idea that birth is more than simply a mechanical event involving getting one bony object to pass through another bony object. Measuring the size and shape of the woman’s pelvis and the fetus don’t seem to be able to predict how the birth will ultimately unfold.
The authors didn’t specifically discuss the significance of the relationship between narrower angle measurements and occipitoposterior head positions. This could well be that this position, which places the broader occiput on the opposite side of the pelvis from this narrow angle, might be a physiological mechanism to support the passage of the fetus through this particular shape of pelvis. This might be a beneficial arrangement rather than a problem.
The study highlights that even though birth has been an area of scientific study for pretty much the entire time we have had science, there are very many things that we still don’t understand about what is going on. Fortunately, women and babies continue to get on with the task of birth even in the absence of a scientific understanding of precisely how they manage to do it. And that leaves us open to see birth as a bit magical and mysterious, and I must confess that because I’m more of an organic, I like it that way.
Neto, RHC, Viana Jnr, AB, Moron, AF, Araujo Jnr, E, Carvhalho, FH, & Feitosa, HC. (2020). Pubic arch angle measurement by transperineal ultrasonography: A prospective cross-sectional study. Revista Brasileria de Ginecologia e Obstetricia, 42(4), 181-187.
Yang, X-J, & Sun, S-S. (2017). Comparison of maternal and fetal complications in elective and emergency caesarean section: A systematic review and meta-analysis. Archives of Gynecology and Obstetrics, 29, 503-512.
Categories: New research
Tags: caesarean section, mode of birth, occipitoposterior, ultrasound
Birth Small Talk 
Love this article. Fabulous facts and ideas to share. Handy for the birth worker 👍
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Thanks Kristen, I’m pleased you found it useful.
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