Sonographic detection of physiologic pleural fluid in normal pregnant women.

Purpose: This prospective study was conducted to determine the normal incidence of a small amount of pleural fluid in healthy pregnant women on sonography.

Methods: Chest sonography to detect pleural fluid was performed in 47 pregnant volunteers, first with the subjects leaning on 1 elbow (the "elbow position") and then with subjects in a sitting position. A 9-12-MHz linear-array probe was used. If the result of this first examination was positive (showing a 2-mm or thicker anechoic layer), the subject was rescanned with a 3-6-MHz large-radius curved-array probe.

Results: A thin layer of fluid with a typical wedge-shaped appearance was visible in the pleural cul de sac space of 28/47 (59.5%) pregnant volunteers, on both sides in 18/47 (38.3%) and unilaterally in 10/47 (21.2%). The mean (+/- standard deviation) thickness of fluid layer (mean of measurements obtained in both positions) was 2.9 +/- 1.1 mm (range, 1.8-6.4 mm). The number of pregnant women with visible pleural fluid was the same regardless of the fetus' sex, but the difference in fluid layer thickness between the 2 groups was statistically significant (by the t-test, p <0.05). A fluid layer thicker than 3 mm was easily detected with the 3-6-MHz abdominal curved-array probe in 7/47 (15%) women.

Conclusions: Small amounts of pleural fluid can be visualized by chest sonography in healthy pregnant women. This result, if isolated, should not be taken as a sign of occult thoracic disease.