Head circumference better predicts labor dystocia than weight

1. Head circumference was more strongly associated with risk of unplanned cesarean than birth weight.

2. Compared to birth weight, newborn head circumference was also more predictive of operative delivery.

Evidence Rating Level: 2 (Good)

Study Rundown: Labor dystocia, the most common indication for cesarean delivery in the United States, has classically been attributed to problems with the “3 P’s”: passenger, passageway, and power of the uterus. When the passenger (baby) is too large to fit through the passageway (maternal pelvis), a phenomenon referred to as cephalopelvic disproportion results. Inadequate uterine contractions can also lead to labor dystocia. However, clinical pelvimetry poorly predicts pelvic adequacy in labor (passageway) and uterine power cannot be reliably predicted a priori. While many purport that fetal macrosomia is associated with a decreased likelihood of achieving successful vaginal delivery, randomized clinical trials have not shown clinical benefit to scheduled cesarean delivery and cost-effectiveness studies do not recommend scheduled cesarean delivery for fetal macrosomia unless estimated fetal weight is 5000g (11 pounds). Some experts argue that head diameter is a more relevant fetal parameter to predict cephalopelvic disproportion whereas overall fetal weight might serve as a better predictor of shoulder (soft tissue) dystocia. To date, little research has assessed the impact of head circumference on obstetric outcomes. In this study, Israeli researchers assessed the relationship between neonatal head circumference or birth weight and delivery outcomes.

Findings demonstrate that large head circumference was more strongly associated with risk of unplanned cesarean and operative delivery than high birth weight. The major strengths included large sample size. Limitations include retrospective design and the associated risks of bias. Furthermore, the association with newborn head circumference would only prove clinically useful if unplanned cesarean is also associated with sonographic measurement of fetal head circumference (HC) and biparietal diameter (BPD). Prospective investigation of third trimester fetal HC and BPD is warranted to evaluate head circumference as a predictor of labor dystocia.

Click to read the study in AJOG

Relevant Reading: Fetal head circumference and length of second stage of labor are risk factors for levator ani muscle injury

In-Depth [retrospective cohort study]: All singleton term births at 2 Israeli tertiary care centers from 2010-2012 were included in the analysis (n = 24 780). Delivery outcomes for newborns with a HC or birth weight (BW) ≥95%ile (n = 1415 and 1354 respectively) were compared to those for normal sized infants. Unplanned cesarean, operative delivery, maternal and fetal complications were assessed.

Newborn HC ≥95th percentile was associated with a greater likelihood of unplanned cesarean (OR = 2.58, 95%CI 2.22-3.01) and operative delivery (OR = 2.13, 95%CI = 1.78-2.54) compared with normal HC. BW ≥95th percentile correlated with a small increase in unplanned cesarean risk (OR = 1.2, 95%CI = 1.01-1.44) compared with normal BW infants. In subgroup analysis, having a large HC but normal BW was associated with unplanned cesarean (3.08, 95%CI = 2.52-3.75), while normal HC and high BW was not.

Image: CC/Wikimedia Commons/Oyvind Holmstad

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