Smaller “Occiput-spine angle” during first stage of labor increases risk of operative delivery-a new sonographic parameter to assess fetal head deflexion.

Occiput-spine angle
courtesy http://onlinelibrary.wiley.com/doi/10.1002/uog.14342/full

Clinical Pearls:

• Fetal deflexion in first stage of labor is directly correlated with higher chances of operative  delivery.
• A new sonographic parameter of Occiput-spine angle < 125 predicts high chances of prolonged labor and operative intervention.
• This finding is easily reproducible in subsequent observations.
• This parameter cannot be measured in  frank occiput posterior positions due to  technical limitations.

According to a population based study labor dystocia is the most common indication for primary cesarean section, followed by abnormal fetal CTG, fetal malpresentation, multiple gestation, and suspected fetal macrosomia.[1]

Fetal deflexed cephalic malpresentation is responsible for nearly 30% cases of labor arrest leading to operative intervention. The degree of fetal head extension results in variety of malpresentation including sinciput, brow, and face.[2]

The diagnosis of labor dystocia and fetal cephalic malpresentation is traditionally made by digital examination, but use of ultrasound is increasingly made in labor ward to support the clinical diagnosis especially in face and brow presentations. [3] [4]

A recent study published in July issue of American Journal of obstetrics and Gynecology [5] aimed to determine whether ultrasound measurements quantifying occiput-spine angle can be used as a parameter to determine the course, outcome and operative intervention in labor.

Study was conducted at maternity units of one of the oldest university in Europe at Bologna and Parma from January 2014 to April 2015. In this prospective cross-sectional study 108 pregnant women who presented with uncomplicated singleton pregnancies at term gestation of 37 weeks or beyond were recruited as study participants. The inclusion criteria were regular and active uterine contraction, with cervical dilatation between 3-6 cm, fetal head station at 0. Patients with occipital posterior position and PROM > 24 hours were excluded from the study.

A 2 dimensional sagittal sonogram of fetal head and cervical spine was taken and stored. On the image an offline measurement of the angle formed between the two lines drawn tangential to occipital bone and first vertebra was performed to quantify the  position of fetal head.                           

The results of the ultrasound were not known to the obstetrician who managed the labor. The labor outcome, mode of delivery and the result of ultrasound examination were correlated retrospectively.

After accounting for other confounding factors and excluding cesarean deliveries for non-reassuring FHS, multivariate logistic regression was performed. 

It was seen that out of 108 study subjects, spontaneous vaginal delivery occurred in 79 patients. Of the remaining patients  10 required vacuum assistance and 19 has to undergo cesarean section. The patients needed vacuum or cesarean section because of  labor arrest in 19 patients and nonreassuring fetal heart rate in 10 patients.

Multivariable logistic regression analysis showed that narrow occiput-spine angle values (OR 1.08; 95% CI 1.00−1.16; P = .04) and nulliparity (OR 16.06; 95% CI 1.71−150.65; P = .02) were independent risk factors for operative delivery. A larger occiput-spine angle width (i.e., >125°) showed to be significantly associated with a shorter duration of labor (hazard ratio = 1.62; 95% CI 1.07−2.45; P = .02).

This is a very important first study that correlated  fetal head deflexion objectively in the first stage of labor and  its impact on  labor outcome. The occiput spine angle in the first stage was directly related to the station of the fetal head, the deeper the head the greater the angle.

Cases that required intervention ,all had smaller occiput-spine angle at a similar station indicating diminished flexion of the fetal head. In all those fetuses with occiput-spine angle <125°, the duration of labor was considerably prolonged.

Other similar studies were conducted in the past but none measured the fetal deflexion quantitatively and the findings were not reproducible. About 30-50% of fetuses occupy a frank occiput posterior position in early first stage of labor and the measurements cannot be assessed in such cases. A  recent study have shown that in such patients the chances of operative delivery are quite high.

The study described a new sonographic parameter of occiput-spine angle that can predict with accuracy the chances of operative intervention. Fetuses with deflexed head resulting in smaller occiput-spine angle (<125°) are at increased risk for operative delivery.

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[1] http://www.ajog.org/article/S0002-9378(14)00055-6/fulltext

[2] Jacobson, L.J. and Johnson, C.E. Brow and face presentations. Am J Obstet Gynecol. 1962; 84:1881–1886

[3]Ghi, T., Maroni, E., Youssef, A. et al. Intrapartum three-dimensional ultrasonographic imaging of face presentations: report of two cases. Ultrasound Obstet Gynecol. 2012; 40: 117–118

[4] Lau, W.L., Cho, L.Y., and Leung, W.C. Intrapartum translabial ultrasound demonstration of face presentation during first stage of labor. J Obstet Gynaecol Res. 2011; 37: 1868–1871

[5] Ghi T, Bellussi F, Azzarone C, et al. The “occiput–spine angle”: a new sonographic index of fetal head deflexion during the first stage of labor. Am J Obstet Gynecol 2016;215:84.e1-7.