contraction /kəntrak″shən/ [L, con + trahere, to draw] , 1. a reduction in size, especially of muscle fibers. 2. an abnormal shrinkage. 3. (in labor) a rhythmic tightening of the musculature of the upper uterine segment that begins as mild tightening and becomes very strong late in labor, occurring as frequently as every 2 minutes and lasting over 1 minute. Contractions decrease the size of the uterus and propel the fetus through the birth canal. 4. abnormal smallness of the birth canal or part of it, a cause of dystocia. Inlet contraction exists if the anteroposterior diameter is 10 cm or less or if the transverse diameter is 11.5 cm or less. Midpelvic contraction exists if the sum of the measurements in centimeters of the interspinous diameter (normally 10.5 cm) and the posterior sagittal diameter (normally 5 cm) is 13.5 cm or less. Outlet contraction exists if the intertuberous diameter is 8 cm or less. See also clinical pelvimetry, concentric contraction, dystocia, eccentric contraction, x-ray pelvimetry.
