Is cervicometry the future for labor assessment?

All of the fetal monitors we currently use are designed to detect the small number (<2%) of fetuses with acidemia and hypoxia in labor. But most complicated labors involve poor progress and many births associated with hypoxia and academia are also prolonged.¹ Dystocia is the most common indication for cesarean delivery (C/S) and accounts for 670,000 of the approximately 1,120,000 labors that end in this form of delivery in the United States every year.² About half of laboring women require oxytocin induction or augmentation, and 80% of patients with labor disorders do respond to oxytocin.³

To diagnose and manage an abnormal labor pattern, the most critical information the ob/gyn needs is cervical dilatation and head station as assessed by digital examination. But as this article reveals, the digital approach has limitations, as have previous attempts at developing instruments to measure cervical dilatation, also known as cervicometry. The one cervicometer approved for use in Europe-the Barnev Cervicometer-appears to have potential and we'll review how it is used, current results with the technology, and possible future applications.

The limits of digital examination

Cervical dilatation. Several investigators have assessed the ability of digital examination to accurately assess cervical dilatation.^4-6 Lacking a "gold standard" for cervical dilatation, they compared digital examination by one examiner to that of another examiner, or assessed the performance of multiple examiners on a cervical model. The two-examiner studies showed an average variability of 1 to 2 cm in cervical examination, but it can be as much as 6 cm.⁴ The studies with cervical models showed that only about half the clinicians were accurate to within 1 cm in assessing cervical dilatation.^5,6 Tufnell and colleagues showed that digital examination by a single observer was consistent (an examiner consistently estimating, overestimating, or underestimating cervical dilatation) only 33% of the time, suggesting that repeat examinations by a single clinician have limited value.⁶

Position. Very few studies have examined how accurate digital examination is for assessing fetal head position in labor. In Sherer and colleagues' study, clinical assessment-done digitally during labor and checked later sonographically-was correct (that is, ± 45 degrees) in only 40% of cases.⁷ Akmal and colleagues used the same methodology to look at fetal head position before forceps application.⁸ The clinician's diagnosis of occiput/transverse position was correct in 54% of cases, but in 25% of cases, the clinical diagnosis would have led to misapplication of forceps.⁸

Station. Fetal head station during labor should describe the distance of the leading edge of the fetal head from the ischial spines in centimeters. A recent study, however, found that 243 caregivers in four university obstetric units used four different definitions of head station in labor.⁹ Some clinicians used the 1-to 5-cm system recommended by the American College of Obstetricians and Gynecologists (ACOG), while others used the old system of dividing the pelvis into thirds and defining station on a 1-to-3 scale. The clinicians also disagreed about when the head enters the pelvis (station 0). Some definitions had the presenting part reaching the spines, while others considered entry to occur when the biparietal diameter reached that level. The result: four different definitions of station. This diagnostic confusion was further exacerbated by the caregivers' lack of awareness of their differing definitions. Asked to diagnose station on a model, 36% to 80% of experienced clinicians failed and station was incorrectly labeled as high, mid, or low in 34% of the assessments.¹⁰

Determining critical times for labor management. Because obstetricians are also very poor at prospectively defining key events in labor, they usually assign the definitions retrospectively. Often what seems to be term or preterm labor is found to be false labor, implying an incorrect diagnosis of onset of labor. Just as it is difficult to pinpoint the onset of labor, so, too, the active phase of labor is usually retrospectively determined. A finding of full dilatation on digital examination implies that the event occurred before the examination.