Fig. 1

Ultrasound image of fetal membranes at 21 weeks 4 days, obtained transvaginally. a An outline was manually drawn around the membranes to determine the area and length, which were used to approximate the average thickness using the equation: average thickness = area/length. b Six thickness measurements were performed along a 1 cm range near the center of the image of the membranes (1 cm scale bar along the top) to identify the maximum and minimum thicknesses. Values are provided in Table 3