Fetal membrane distention: Determination of the intrauterine surface area and distention of the fetal membranes preterm and at term

OBJECTIVE: This study was undertaken to obtain an accurate measurement of the intrauterine surface area and the degree of distention of the apposed fetal membranes preterm and at term. STUDY DESIGN: Serial longitudinal images of the uterus in 23 women between 25 and 41 weeks' gestation were obtained by ultrasonography. A Mathematica (Wolfram Research, Inc, Champaign, III) program assembled a 3-dimensional image and calculated the intrauterine surface area for each patient. The surface areas of the placental amnion and membranes were measured in vitro after delivery. From these measurements the degree of distention of each fetal membrane in vivo was calculated. RESULTS: The mean calculated intrauterine surface areas were as follows: 1037 +/- 70 cm(2) (25-29 weeks' gestation, n = 4), 1376 +/- 121 cm(2) (30-34 weeks' gestation, n = 4), and 1876 +/- 307 cm(2) (37-41 weeks' gestation, n = 15, P = .0021 by Wilcoxon rank sum test). The surface areas of the expelled membranes at 25 to 29, 30 to 34, and 37 to 41 weeks' gestation were 737 +/- 61 cm(2), 855 +/- 77 cm(2), and 1115 +/- 149 cm(2), respectively. The ratios of intrauterine surface area to the area of the expelled membrane and hence a measure of the degree of distention in vivo were 1.4 +/- 0.05 at 25 to 29 weeks' gestation (n = 4), 1.6 +/- 0.2 at 30 to 34 weeks' gestation (n = 4), and 1.7 +/- 0.3 at term (n = 15). CONCLUSION: The intrauterine surface area in vivo increases during gestation. The surface area of the fetal membranes as measured in vitro increases to a lesser extent. The fetal membranes are therefore distended in vivo.