Ultrasound in Third Trimester Doubles Detection Pre-Term Birth Risk

Babies born prematurely, as well as those who are considered small or large for their gestational age, account for up to 30 percent of newborn deaths, and they face a greater likelihood of developing other short-term and long-term problems. But, conducting an ultrasound early in the third trimester could help reduce the dangers, according to new research.

Published in BMC Medicine, a study, led by Romy Gaillard, M.D., Ph.D., in the department of pediatrics at University Medical Center in Rotterdam, The Netherlands, revealed an ultrasound screening around 30 weeks gestation, coupled with an evaluation of maternal characteristics, can double the detection of unborn babies at risk for negative birth outcomes. This is the first study to assess whether ultrasound screening later in pregnancy is effective for pinpointing pre-term birth risk, and the knowledge could give providers a chance the step in and reduce the likelihood of significant problems.

“Improved detection of fetuses at risk of preterm birth, small gestational age, and large gestational age provides the clinician the opportunity to optimize monitoring and interventions,” Gaillard’s team said.

For the study, the team enrolled 7,677 pregnant women during 2001 to 2005. They collected data about the mother’s age at time of enrollment, height and weight, ethnicity, parity, and history of smoking. Overall, the group was deemed a healthy, low-risk population.

The team also conducted ultrasound screenings in the first, second, and third trimesters, collecting fetal head circumference, abdominal circumference, and femur length during the second and third scans. The measurements were used to determine fetal size compared to gestational age.

Knowing whether a fetus is too small or too large at any given time during pregnancy is critical, they said, because those babies are at a higher risk of adverse outcomes, as well as perinatal morbidity and mortality.

During the second and third trimesters, the team also measured uterine artery resistance and umbilical artery pulsatility indices. If these levels of vascular resistance are elevated, it also points to an increased risk for negative birth outcomes.

According to the study results, 345 newborns (approximately 4.5 percent) were born early, 768 (10 percent) were small for gestational age, and 767 (10 percent) were large for gestational age. Based on an analysis, a third-trimester ultrasound did a better job than a second-trimester screening of accurately predicting when newborns would be born early (7.2 percent vs. 5.9 percent), too small (21.8 percent vs. 13.9 percent) or too large (18.1 percent vs. 11.7 percent).

In particular, the team noted, a third-trimester ultrasound coupled with the vascular flow measures nearly doubled the detection of fetuses at risk for preterm birth – sensitivity increased from 19 percent for the second-trimester scan to 38 percent for the third trimester with 90 percent specificity.

In addition, an ultrasound at 30 weeks also bolstered detection of fetuses at risk of being too small at birth by nearly a third, improving sensitivity from 33 percent from the second trimester ultrasound to 50 percent with the third trimester, again with 90 percent specificity. And, a similar improvement was seen in pinpointing fetuses at risk for being too large at birth – 28 percent sensitivity during the second trimester and 43 percent during the third trimester (90 percent specificity).

Interestingly, the team noted, when they removed maternal characteristics, the screening performance of the third-trimester ultrasound in detecting preterm birth was considerably lower.

“We found that in absence of maternal characteristics, the screening models had an inferior screening performance compared to when maternal characteristics were taken into account,” the team wrote. “Thus, our findings underline the importance of considering maternal characteristics within low-risk populations for screening of adverse birth outcomes and the potential value of third trimester ultrasound.”

Ultimately, the authors wrote, conducting an ultrasound early in the third trimester and determining whether the fetus could be at risk for a negative birth outcome should be considered as part of prenatal care.

“Our results suggest that third trimester ultrasound examination, in addition to maternal characteristics, has the best screening performance for detecting fetuses at risk for preterm birth, small gestational age, and large gestational age, compared to second trimester ultrasound or combined second- and third-trimester ultrasound,” the team wrote. “[These findings] provide further evidence for critical evaluation of current obstetric care guidelines."