labidometer

from The Century Dictionary.

• noun In obstetrics, a scale adapted to the handles of the forceps, which indicates the distance of the blades from each other when applied to the head of the child.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Med.) A forceps with a measuring attachment for ascertaining the size of the fetal head.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun medicine A specialized forceps or attachment to a regular forceps for measuring the size and curvature of the head of a fetus in the pelvis.