Hormonal Control of the Cervix in Pregnant Gilts. I. Changes in the Physical Properties of the Cervix Correlate Temporally with Elevated Serum Levels of Estrogen and Relaxin*

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Abstract
The time course of changes in the physical properties of both the uterine and vaginal portions of the cervix were studied during the last two thirds (days 40 to approximately 115) of pregnancy in the gilt. To obtain insight concerning the hormonal control of these physical changes, serum levels of relaxin, estrone, 17 .beta.-estradiol, and progesterone were also determined. Throughout midpregnancy, the extensibility (softness), lumen diameter, and wet weight of the uterine portion of the cervix were less than those of the vaginal portion of the cervix. After day 80, marked and sustained increases in these three physical parameters occurred in the uterine portion of the cervix whereas slight (extensibility) or moderate (lumen diameter and wet weight) increases occurred in the vaginal portion of the cervix. By day 110, extensibility, lumen diameter, and wet weight of the uterine portion of the cervix were similar to those of the vaginal portion of the cervix. Because the uterine portion of the cervix has a relatively firm consistency and small lumen throughout most of pregnancy, we conclude that it probably plays a more important role than the vaginal portion of the cervix in protecting the uterus and its contents during pregnancy. The marked changes in the physical properties of the uterine portion of the cervix are temporally correlated and elevated serum estrogen levels (days 80-100) or estrogen and relaxin levels (day 100 to term). We conclude that estrogen or, more likely, estrogen and relaxin in combination may promote the modifications in the physical properties of the cervix that occur during the last 35 days of gestation.