Second‐Trimester Sonographic Thymus Measurements Are Not Associated With Preterm Birth and Other Adverse Obstetric Outcomes

Abstract

reterm birth, which is defined as delivery before 37 weeks' gestation, is responsible for 35% of deaths in the first year of life and more than $26 billion in annual health care costs. 1 Current methods for identifying women at increased risk for preterm birth are inadequate. Although universal cervical length screening during the second-trimester anatomic survey may identify patients at risk for spontaneous preterm birth, many patients with normal second-trimester cervical length measurements will still have preterm birth. [2][3][4] Justin S. Brandt, MD, Jamie A. Bastek, MD, MSCE, Eileen Wang, MD, Stephanie Purisch, MD, Nadav Schwartz, MD Received July 2, 2015, ORIGINAL RESEARCHObjectives-Previous studies have demonstrated an association between adverse obstetric outcomes, such as preterm birth, and in utero inflammation. The fetal thymus, which can be visualized in the anterior mediastinum on obstetric sonography, may involute in response to such inflammation and thus may identify pregnancies at increased risk for these outcomes. We therefore sought to determine whether secondtrimester fetal thymus measurements are associated with preterm birth.Methods-Transabdominal fetal thymus measurements were prospectively obtained in singleton pregnancies at gestational ages of 18 weeks to 23 weeks 6 days during a 5-month period. The transverse and anterorposterior thymus diameters and the thymicthoracic ratio were measured. Delivery outcomes were collected from our clinical database. The primary outcome was preterm birth, which we defined as delivery between 24 weeks and 36 weeks 6 days. Small for gestational age (SGA) and pregnancyrelated hypertension, which are adverse obstetric outcomes that may also be associated with in utero inflammation, were included as secondary outcomes.Results-We included 520 patients with thymus measurements and obstetric outcome data. The prevalence of preterm birth was 12.3% (n = 64). None of the thymus measurements were associated with preterm birth. Similarly, there was no association between thymus measurements and SGA or pregnancy-related hypertension.Conclusions-Sonographic assessment of the second-trimester fetal thymus did not identify patients at increased risk for preterm birth, SGA, and pregnancy-related hypertension. Routine thymus measurements during the second-trimester anatomic scan are not clinically useful for prediction of preterm birth and other adverse outcomes.

Small fetal thymus and adverse obstetrical outcome: a systematic review and a meta‐analysis

“…In all, 258 articles were identified, 25 were assessed with respect to their eligibility for inclusion (see Supplementary material, Table ) and 12 studies were included in the systematic review (Table , Figure ) . These 12 studies included 1744 fetuses who had ultrasound assessment of thymus size.…”

“…These 12 studies included 1744 fetuses who had ultrasound assessment of thymus size. Eleven studies were prospective , only one was a retrospective study . Inclusion criteria differed among the included studies; six studies included women at risk of infectious‐related complications, such as those presenting with preterm premature rupture of the membranes (PPROM) or signs of preterm labor , whereas the remaining studies included women with no apparent risk factors (Table ) .…”

“…In nine studies, pregnancies with fetuses with anomalies were excluded . Gestational age at first ultrasound examination ranged between 18th and 40th weeks of gestation.…”

“…Two studies including 551 fetuses explored the strength of association between small thymus and the occurrence of spontaneous PTB <37 weeks of gestation, reporting no difference between the two groups (OR 2.2, 95% CI 0.2–23.6) . However, these two studies differed as regards their inclusion criteria.…”

“…The study by Brandt et al. , included women undergoing second‐trimester scans and did not report any difference in the occurrence of PTB between fetuses with a small and a normal thymus (OR 0.7, 95% CI 0.3–1.7), whereas the study by Di Naro et al. included women with symptoms and signs of PTB and reported an increased risk for PTB in those with a small fetal thymus (OR 8.3, 95% CI 1.7–41.2), with a sensitivity of 75.0% (95% CI 47.6–92.7), a specificity of 73.3% (95% CI 44.9–92.2), an LR+ of 2.8 (95% CI 1.29–7.11), an LR− of 0.34 (95% CI 0.13–0.76) and a DOR of 73.3% (95% CI 44.9–92.2) (Figure , Table ).…”