Ultrasound Clinics: Role of U/S during labor and induction

Key Points

Ultrasound is such a major component of contemporary obstetric practice that we could scarcely imagine managing a patient's pregnancy without this technology. Its role in managing labor at term, however, is less well established. Our purpose here is to explore the usefulness of U/S in term laboring patients or those undergoing induction.

The size and position of the fetus, the shape of the pelvis, and the strength of uterine contractions are integral components of labor and birth. Although clinicians do attempt to predict labor outcomes using abdominal and pelvic exams to assess these components, it's far from an exact science. Given that U/S machines are widely available on many Labor and Delivery units, routinely using U/S in term patients may help you better assess and manage labor and make better decisions regarding mode of delivery. We'll look at the three major ways you can use U/S in full-term laboring patients: in estimating fetal weight, assessing the cervix, and determining fetal position.

Estimating fetal weight

MACROSOMIA IMPLIES GROWTH beyond a specific weight, usually 4,000 g or 4,500 g regardless of gestational age.¹ Weighing a newborn after delivery confirms the diagnosis. Although macrosomia occurs in about 10% of pregnancies, half of shoulderdystocias occur in this group.² It's important to point out, however, that the remainder of shoulder dystocias occur among infants weighing less.¹

Diagnosing macrosomia before birth is difficult, no matter which of the three ways you determine fetal weight: maternal estimation, clinical assessment, or sonographic measurement. The variety of U/S formulas used for estimating fetal weight supports the fact that no single satisfactory formula exists. This begs the question: Is U/S, in fact, clinically useful for guiding clinicians on mode-of-delivery decisions for excessively large fetuses?

Clouding the picture are the higher error rates seen in larger fetuses. For example, Hadlock's formula has an average absolute error of 13% for infants weighing more than 4,500 g and only 8% for nonmacrosomic infants.³

The number of days between U/S and delivery can throw off the accuracy of U/S for the EFW before induction of labor. Seeking to avoid this effect, Ben-Haroush and colleagues performed a prospective study among 800 women who were admitted for labor induction at term. The researchers evaluated the accuracy of sonographically measured EFW 1 to 3 days before delivery, calculating measurements using the abdominal circumference and femur length (FL). They omitted the biparietal diameter (BPD) to avoid inaccurate measurements associated with head engagement and molding. In predicting a fetal weight of 4,000 g or more, U/S had a low sensitivity (47%) and positive predictive value (46%), but a high specificity (95%) and negative predictive value (95%). Given the proximity to delivery, these measurements may be considered equivalent to measurements performed during labor.

THE ABDOMINAL CIRCUMFERENCE (AC) measurement has been reported to be a better predictor of macrosomia than the combination of the BPD, FL, and AC.⁴ One team of investigators evaluated how well a single measurement predicted macrosomia. They concluded that an AC of 35 cm or more was useful in screening for macrosomia, as it identified more than 90% of macrosomic infants.² Induction of labor in this group appeared to raise the risk of shoulder dystocia more than threefold compared to the women who had augmentation or spontaneous labor.

When U/S measurements at term suggest fetal macrosomia, clinicians may feel compelled to induce labor in an attempt to improve maternal and neonatal outcomes and avoid birth injury. However, a meta-analysis found that inducing labor in a woman suspected of having a macrosomic baby leads to a higher cesarean delivery (CD) rate without improving perinatal outcomes and therefore is not a valid indication for induction.⁵