The uterus itself most closely approximates the geometrical shape of a prolate ellipsoid.10 The endometrial cavity is a hollowed-out space which appears to be fairly symmetrical within the uterus itself. Additionally, since we can eliminate the cervix and the cervical canal (which are not part of the endometrial cavity), the endometrial cavity itself can be assumed to also take the shape of a prolate ellipsoid, and in fact removal of the cervix ensures that the cavity more closely approximates a prolate ellipsoid than does the whole uterus. The volume of a prolate ellipsoid is given by the following formula:
where V = volume of the endometrial cavity (mL)
π~3.14
L = length (fundus to internal os of endometrial cavity)
W = width (between cornual ostia of endometrial cavity)
and AP = anteroposterior diameter of the endometrial cavity perpendicular to L.
Endometrial cavity volume has been measured and varies between 5 and 10 mL.11
Transfundal endometrial cavity width varies widely from as little as 7 mm in very narrow cavities to the more usual 22–34 mm depending on parity. There is a wide variation in uterine measurements, but it is possible to calculate boundary values which will accommodate most sizes and volumes.