The inaccuracy of total uterine volume measurements: sources of error and a proposed solution.

Abstract

Inaccuracies in total intmuterine volumes calculated using the prolate el~ lipse equation have been reported. No previous study has examined all the sources of error. In this study, a comprehensive approach was undertaken. Measurements were obtained from scans of the pregnant uterus in the prone position using an automated water-path scanner (Octoson) and in the supine position using standard static B-mode scanners. Several conclusions could be drawn : 1) From the Octoson prone scans, uterine volumes obtained using the prolate ellipse formula were markedly different from the true uterine volumes obtained by the summation of stepped areas. This showed that the prolate ellipse formula was inaccurate. 2) From the static supine scans, many observer inconsistencies were found in uterine volumes obtained from the prolate ellipse formula. This made the prolate ellipse formula unreliable. 3) Previously published graphs calculated from the prolate ellipse equation, comparing fetal age with total intrauterine volume, were found to vary accuracy, presumably as a result of 1 and 2. A more accurate approach is proposed. Using the outer uterine wall as the boundary, the stepped areato-volume values of transverse scans taken at 3-cm intervals were found to closely approximate true volumes, with an average error ofonly 3.5 per cent. Since these measurements encompass the intrauterine contents and the myometrium , it is suggested that the term •'total uterine volume'" be used instead of "total intrauterine volume."

Sonographic evaluation of uterine volume and its clinical importance

“…5 When the uterine volume is more than indicated for vaginal hysterectomy but the surgeon feels that a vaginal route may succeed, a trial vaginal hysterectomy (similar to trial forceps delivery in obstetrics to avoid opening the abdomen in a well-counseled obstetric patient) should be planned, with facilities for laparoscopic assistance or cesarean section available. 4,6 When uterine volume exceeds 240-350 cm 3 , trial vaginal hysterectomy is favored or laparoscopic technique considered. One of the authors of this paper is used to performing vaginal hysterectomy on a contraindicated enlarged uterus and comfortably accepts the vaginal route for uterine volumes of 500-800 cm 3 as trial vaginal hysterectomy cases; if these fail vaginally, laparoscopic assistance or opening of the abdomen is utilized.…”

“…The uterine length (L) was defined as the distance between the internal orifice of the uterus (os) to the dome of the fundus and the maximum width (W) and anteroposterior (AP) diameter were taken perpendicular to the axis of the uterine length. Kurtz et al found that the prolate ellipse formula was not ideal for uterine volume calculation …”

“…Kurtz et al found that the prolate ellipse formula was not ideal for uterine volume calculation. 3 The cervix was not included in the uterus volume calculation for two reasons: (i) the formula of volume calculation is based on the presumption that the uterus is ellipsoid in shape and the cervix is cylindrical, and (ii) the cervix does not significantly contribute to the volume (approximately 15 cm 3 ). Therefore, before making a conclusion, it is worth adding or subtracting the cervical length for apt comparison.…”

“…The average uterine weight in grams was strikingly higher than the measured volume in cm 3 in all but nine out of 800 cases ( Table 2). The average uterine weight was 115, 180, 240, 280, 410 and 499 g for average uterine volumes of 90, 130, 180, 240, 355 and 460 cm 3 , respectively, which, in turn, was for uteri sized 6, 8, 10, 12, 14 and 16 weeks, respectively. Uterine weight range varied in a similar manner.…”

Fetal Biometry

“…Therefore, the CV was approximated as a sphere in this study for the following reasons:1) The formula for a prolate ellipse volume has been called inaccurate by Kurtz et a1. 22 2) When these two methods of calculating volumes were compared by using four masses of modeling clay of known volumes (in the shape of a flat-sided prolate ellipsoid), the spherical volumes were about 90% of the true volumes, and the prolate ellipsoid volumes were about 80% of the true volumes. Thus, the spherical volumes more closely approximated the true volumes.…”