Birth from an evolutionary perspective — Part 1 „Obstetric Dilemma“

In Austria, the average birth rate is 30%, depending on the region and hospital. This means that every seventh child in Austria is born by cesarean section (abdominal birth). This can, if medically indicated, save lives, but according to the World Health Organization (WHO) should be between 10-15%. This means that in Austria, on average, twice as many caesarean sections are performed as recommended by the WHO. Media, influencers and unfortunately also science contribute to the fact that birth is often perceived as dangerous and even impossible without medical help.

For me as an evolutionary anthropologist the question arises "If the human birth process has such a high failure rate of 30%, why has this process not adapted in the course of evolution? After all, there is a need to ensure reproduction and survival of the human species."

In the last decades, many of my colleagues have looked into possible explanations for the "risky" birth process in humans. Some of them point to an apparent evolutionary mismatch between the large infant skull (due to a larger growing brain) and the female pelvis adapted to the upright gait (e.g. Washburn, 1960; Rosenberg, 1992; Rosenberg & Trevathan, 1996, 2021; Fischer & Mitteroecker, 2015; Grunstra et al, 2019; Haeusler et al, 2021). The upright gait has made the human pelvis wider and shorter to provide stability during walking. It is also oriented differently than in other primates to better handle the vertical loads of upright walking. This means that the largest diameters of the three pelvic planes are oriented in different directions. The largest diameter of the pelvic inlet is oriented mediolaterally and the largest diameter of the pelvic outlet is oriented anterior-postiorly. Between the pelvic inlet and the pelvic outlet is the narrowest point, also known as the pelvic midline (see figure 1). During birth, the baby must therefore turn its head in different directions in order to pass the narrowest diameter in each of the three pelvic planes. The blue line (see figure 1) shows the orientation of the baby's head in the respective pelvic planes. In the pelvic inlet the infantile head is oriented transversely, in the pelvic middle diagonally and in the pelvic outlet straight (either baby‘s nose at the maternal coccyx or the maternal pubic bone). The fetal head thus performs a 90° rotation during the birth process.

Figure 1: Different pelvic planes of the maternal pelvis and the corresponding orientation of the child's head.

The conflict between the pelvis‘ ability to support walking upright and give birth successfully is known as the "obstetric dilemma (OD)" and for many anthropologists is one of the main factors why birth in humans is so challenging.

A mismatch between the fetal skull and the maternal pelvis described above can also lead to what is called a cephalopelvic disproportion (CPD). However, a tight fit between the infant skull and maternal pelvis does not only occur in humans, as shown in figure 2 (Rosenberg & Trevathan (2002) adapted from Schultz (1969)). The size of the infant head is represented by the black circle and the size of the maternal birth canal (pelvic opening) by the white circle. Other non-human primates also have CPD similar to that of humans. A CPD can lead to an obstructed birth. This means that the progress of the birth is hindered or prevented. If CPD is indeed present, a cesarean section is the only way to get mother and baby safely through the birth process.

Figure 2: Relationship of infant head to maternal pelvis in different primate species. Rosenberg & Trevathan (2002) after Schultz (1969).

According to the WHO, about 2.8% of maternal deaths worldwide are due to CPD. However, data from the U.S. show that approximately 47.1% of secondary cesarean sections were performed due to CPD or obstructed labor. Conversely, do the U.S. figures mean that 47.1% of women in the U.S. would not survive childbirth if life-saving C-sections were not available? Hardly! Looking at data from areas (sub-Saharan Africa and southern Asia) where performing a cesarean section is often not possible, the maternal mortality rate due to CPD or obstructed labor ranges from 2.1 to 2.7% (Say et al., 2014). Why then is the indication for secondary cesarean section CPD or obstetric labor in approximately 47.1% of women in the United States? This may be because CPD is most often diagnosed when there is an obstructed labor in the active phase of labor or when the opening of the cervix is progressing too slowly. This is where Emanuel A. Friedman's so-called partogram comes into play, as many clinical guidelines use this partogram as a foundation for labor time management. The partogram in brief, as there will be a separate blog post on this topic: According to the study published in 1955, which was based on a sample of 500 women, the cervix should open at an average rate of 1cm per hour. In the clinical setting, Friedman's partogram is still used despite studies (e.g. Zhang et al., 2010; Ragusa et al., 2016) showing that the opening of the cervix up to 6 cm takes on average much longer than 6 hours or is not continuous.

What are the implications of work like that of Emanuel A. Friedman or us anthropologists for the individual birthing woman?

In the case of the obstetric dilemma, criticism is emerging within the discipline. Some anthropologists (Walrath, 2003; Dunsworth et al, 2012; Betti et al, 2013; Betti, 2021; Warrener, 2023) argue that birth is a multifactorial process and the female pelvis is also subject to multifactorial evolutionary adaptation (adaptation to upright gait, genetic drift, phylogenesis, epigentics, developmental constraints, etc.). In a New York Times interview, Dr. Warrener spoke critically of the assumption that any human anatomical structure is fine-tuned for a particular task, in the case of the female pelvis for childbirth, by natural selection, since adaptations are often flukes. It is possible that the shape of the human pelvis is simply an evolutionary by-product. What does this mean? Some genes responsible for the formation of the pelvis are also responsible for the formation of other parts of our skeleton (Agusto et al., 2022). Therefore, if one of these other skeletal parts changes due to an evolutionary adaptation, it may be that the pelvic shape also changes in terms of an evolutionary by-product and not as hypothesized by the obstetric dilemma hypothesis, due to the upright gait as well as birth. Those anthropologists who criticize the "obstetric dilemma" hypothesis fear that it leads to a widespread notion that women‘s bodies are prone to failure and childbirth is not possible without medical help.

As Dr. Rachel Reed always says “Women‘s bodies are not machines that function according to certain diagrams or averages“. Every woman's body is unique in its physiology and therefore also during birth. As researchers, we should convey this message to women and thus strengthen their basic trust in the ability to give birth! Because language creates reality!