Non-cephalic presentation in late pregnancy
Authors: James M Nicholson
Journal: BMJ (2006)

Caesarean section rates continue to increase around the world. Although non-cephalic presentation is not the most common indication for caesarean section, it may be one of the most preventable.1 Timely diagnosis of this condition, and an attempt at external cephalic version at about 36 weeks' gestation, has been shown to safely reduce the need for caesarean section.1-3 However, timely and efficient diagnosis of fetal malpresentation requires a screening test with a high sensitivity and high specificity.

In this issue of the BMJ Nassar and colleagues report a cross sectional study of the diagnostic accuracy of clinical examination for the detection of non-cephalic presentation in late pregnancy.4 Their findings are worrying: non-cephalic presentation was correctly diagnosed in only 70% (91/130) of cases and in only 38% of obese women (3/8). The authors correctly point out that missing the diagnosis of non-cephalic presentation precludes the ability to offer external cephalic version and increases the likelihood of caesarean section and adverse outcomes associated with the spontaneous onset of labour in women with a malpresentation. Consequently, opportunities to prevent caesarean delivery—and various related adverse birth outcomes—are being missed.

A better way to diagnose fetal malpresentation would be to perform an ultrasound examination routinely at 35-36 weeks' gestation on every pregnant woman. However, as noted by the authors, the cost and resource implications of this approach would need to be considered. Further, it is important to consider two factors that were not a part of this study: the skill of the clinician and the confidence they have in their examination. Common sense suggests that an experienced clinician who is confident in their examination would be more likely to determine fetal lie correctly than a less experienced and confident clinician. However, variable accuracy in this task has been documented even among experienced clinicians.5

US family physicians who attend deliveries care for 20-60 prenatal patients a year, and obstetricians care for 80-150. If persistent breech presentation occurs at a rate of 3-4%,6 then an average family physician's practice will contain a late third trimester malpresentation 0-3 times a year and an average obstetric specialist's practice will contain a malpresentation 2-6 times a year. If the findings of Nassar and colleagues' study are applied, and failure to diagnose non-cephalic presentation correctly at 36 weeks' gestation occurs 33% of the time, then a non-cephalic presentation will be missed by a family physician once every two years and by an obstetrician once or twice a year.

This may not seem to be common enough to prompt a change in clinicians' behaviour. However, other tests are routinely used to guide the diagnosis and treatment of relatively rare but important conditions, such as gestational diabetes, group B Streptococcus colonisation, and HIV infection. Because fetal malpresentation can often be successfully managed with either external cephalic version or elective caesarean section, diagnosis and treatment of non-cephalic presentation just before term should be included in the list of possible important prenatal screening activities.

The findings of Nassar and colleagues may not be strong enough to support a call for routine ultrasound examination of all pregnant women at 35-36 weeks' gestation, but the study should remind all clinicians to assess fetal lie routinely at 36 weeks' gestation. If a clinician is well trained, is confident that a fetus has a vertex presentation, and has a good track record of correctly identifying malpresentation, then ultrasound screening is probably unnecessary. If a clinician is relatively inexperienced, is unsure of their examination, or has a history of missing the presence of a fetal malpresentation, then ultrasonography is probably indicated. The findings also showed that patient factors, such as maternal obesity, might increase reliance on ultrasound examination to determine fetal lie.

Despite the known risks of external cephalic version, birth outcomes clearly could be improved if all women were accurately screened for malpresentation before the onset of labour.7,8 The increased use of ultrasound examination to determine fetal lie, at least in questionable situations, would increase the accuracy of such screening.