The folly of fully dilated

There are a few myths about the idea that women need to be fully dilated to give birth, and that this means the same thing for every woman.

And these myths need to be addressed.

The assessment of progress in labour is a complex and multi-faceted issue which raises questions in many different areas. In recent years, midwives and researchers have challenged some of the key assumptions that have been made about the cervix. Not least of which are the perception that it opens in a unidirectional manner (e.g. Daviss and Johnson 1998, Gaskin 2003, Bowman 2006). I once summarised a few of these challenges and concluded that, “[a] simple focus on quantification of the cervix is clearly, for so many reasons, neither useful nor appropriate” (Wickham 2009: 41).

Despite these challenges, however, both the assessment of cervical dilation in labour and the documentation or charting of this are deemed key elements of maternity care within the obstetric paradigm. Even in areas where work such as that undertaken by Leah Albers (1999) has led to changes in our expectations of how fast cervixes dilate, the measurement of this still forms an important part of the midwife’s role. This measurement, in turn, is based on a further assumption that some midwives have long understood to be fallacious and yet which has barely been challenged in the literature. The assumption to which I refer is that which equates the concept of being fully dilated with the measurement of ten centimetres.

 

Measuring to Ten

I have often wondered how this assumption originated. Mathematically, the number ten is the base of the decimal numbering system, and it is generally assumed that our preference for this system (and thus our view of the number ten as a nice neat ‘round’ number) arose because humans have ten fingers. Indeed, it was only a couple of decades ago that midwives would measure dilation in fingers rather than centimetres, although it was more common in many areas to perform rectal rather than vaginal examination and so it is difficult to directly compare these different approaches. (Not saying that was OK, by the way, just saying that it happened).

Other numbering systems also exist; the duodecimal (base 12) system was one of those used by the Romans, and its influence can still be seen in some forms of imperial measurement (there are, for instance, twelve inches in a foot) and in the Western calendar where there are twelve months in a year. The roots of this kind of measurement might also have played a part in fuelling our modern assumptions. Roman scholars, including Vitruvius (whose name was immortalised in Leonardo da Vinci’s Vitruvian Man) were fascinated by the relationships between different proportions of the human body, and a number of older forms of measurement including the cubit, foot and fathom derive from body measurements, albeit generally of the male form. Somewhat ironically, da Vinci’s drawing which depicts an ideal rather than acknowledging the inevitable variations which exist between people of different shapes and sizes is often used to symbolise Western medicine.

I cannot claim to know for sure whether the assumption that a woman’s cervix is fully dilated at ten centimetres is related to our cultural use of the decimal system, whether this ideal partly derived from the notion of an “ideal” body or whether the truth lies somewhere else. What I do know is that, as you are reading this, midwives and doctors all over the world are busily comparing women’s current cervical dilation to the magical number ten.

 

Acknowledging Variation

I wonder how many of them also think that this is a crazy thing to do?

We know that both women and babies come in different shapes and sizes, and it seems illogical to me to base a whole area of practice on the notion that every woman’s cervix will need to dilate to exactly ten centimetres before her baby can be born. Babies’ head circumferences, the degree of moulding and the shape and diameters of women’s pelves all vary between individuals. I have looked after women who gave birth to very premature babies who probably only needed their mother’s cervix to be about eight centimetres dilated before they could be born. And I have looked after plenty of women who needed to dilate to eleven, twelve or perhaps more centimetres before their cervix was truly fully dilated. I know I am not the only one. I have been asking other midwives and obstetricians about this over the past few months and many of them have similar experiences.

Even the centile charts that help us assess babies’ head circumferences accept that these fall within a range, so why do we not extend this thinking to accept the fact that “fully” encompasses a range of centimetres rather than assuming that every woman’s cervix is conversant with the history of Western mathematics as well as with the textbooks which dictate ten centimetres as the goal of the labouring cervix?

 

And in practice?

This issue has significant implications for women’s experiences and midwifery practice. One of the key issues is that, while textbooks may tell us that full dilation equals ten centimetres, many midwives understand that real women exhibit variation in this area. For example, Robbie Davis Floyd (2004) shared the experience of Sandi, who realised that she had unintentionally conveyed this erroneous linkage to a student when she observed the student telling a woman who Sandi knew was not yet in second stage that she was ready to push.

“‘It’s not about ten centimetres!’ Sandi exclaimed. ‘When I checked the mother her cervix was ten centimetres dilated, but I could still feel the cervical lip.” (xi)

While Sandi may have understood that full dilation does not always equal ten centimetres, however, this issue is barely discussed in the literature. Moreover, our relative disregard of individual variability may be exacerbated by the way that we use vaginal examination to measure dilation in labouring women.

 

The Os or the Edge…

I have come to realise by talking to midwives over the years that, while we might all have slightly different techniques that we use when undertaking vaginal examination and measurement of dilation, there are some key similarities that many of us share. One of these is that many midwives take different approaches depending on the stage of labour. When we find a cervix that is, say, two or three centimetres dilated, we tend to measure the os, as Nizard et al (2009) describe:

“Cervical dilation … is obtained traditionally by the insertion of 2 fingers through the vagina into the cervix, which is then spread apart until the fingers reach opposite margins of the cervical os. The distance between the tips of the fingers is estimated by the operator and expressed in centimeters.” (402.e1)

This approach works really well in early labour. However, when a woman is nearing the end of labour, midwives tend to focus on feeling what remains. Maybe we can’t stretch our fingers far enough apart, or the presenting part prevents us from doing this. More importantly, we don’t want to cause the woman more discomfort than absolutely necessary by performing digital gymnastics while she is trying to cope with labour. Rather than making our fingers into soft calipers which we spread apart to measure the os, we feel carefully around the edges of whatever is presenting in order to determine how much of the cervix can still be felt. Then we subtract however many centimetres we feel are remaining from the magical number ten.

 

…and the Implications

The point at which we switch from measuring the os to measuring the edge may vary according to the individual. The implications, however, are universal. If we are attending a woman who has a perfectly round cervical os and a textbook baby who needs that cervix to open to exactly ten centimetres, we have no problem.

But what about the woman with a larger baby who is going to need her cervix to dilate to eleven centimetres? When her midwife switches from measuring the os to measuring the edge, she is going to effectively lose a whole centimetre of the work that she has done and appear to be making slower progress than she really is.

It’s sort of the cervical equivalent of that Saturday night in March when, in the UK, we lose an hour of sleep because the clocks go forward into British Summer Time.

The implications of this loss of progress depend a bit on how closely labour is monitored. But in a situation where the partogram and strict time limits are seen as key it is possible that women are having unnecessary intervention as a direct result of our lack of attention to individual variability.

I am not, by the way, arguing that the caliper approach should be used throughout labour. Neither am I planning to conclude that this supports the introduction of those rather un-woman-centred machines that mechanically measure cervical dilation by applying probes to the cervix and making women labour on or near machines that can read them.

Frankly, neither of these things are necessary. We need to find a way to take into account the fact that we are not all the same size and that full dilation encompasses a range of possibilities which do not all equate to exactly ten centimetres. Which I do not believe is a particularly revolutionary idea.

I mean, I can understand why Copernicus had a hard time convincing people about heliocentric cosmology because the idea that the earth revolved around the sun was a radical theory which changed the way that humanity viewed itself, but is the theory that human women come in all different shapes and sizes really that hard to accept?

 

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A version of this article was originally published as Wickham S (2008). The Folly of Fully Part 1. TPM 12(5):50. & Wickham S (2008). The Folly of Fully Part 2. TPM 12(6):62.

 

References

Albers LL (1999). The Duration of Labor in Healthy Women. Journal of Perinatology 19(2): 114 –119.

Bowman L (2006). Cervical reversal/regression. Midwifery Matters, 108: 14.

Daviss BA and Johnson K (1998) Statistics and Research Committee [statistics on cervical ripening] MANA Newsletter, 16(2): 16-17.

Gaskin IM (2003). Going backwards: the concept of `pasmo’. TPM 6(8): 34-37.

Wickham S (2009). Stepping Stones and Cervical Wisdom. Birthspirit Midwifery Journal 1: 39-42.

Davis-Floyd RE (2004). Foreword. In: Downe S (2004) Normal Childbirth: Evidence and Debate. Churchill Livingstone, Edinburgh.

Nizard J, Haberman S, Paltieli Y, et al (2009) How reliable is the determination of cervical dilation? Comparison of vaginal examination with spatial position-tracking ruler. American Journal of Obstetrics and Gynecology 200: 402.e1-402.e4.

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