Evidence that cervical softening in the pregnant rat is independent of increasing uterine contractility.

The cervices of rats were surgically separated from their uterine horns on Day 11 of pregnancy. The increase in the creep of the cervix (a measure of cervical tensile properties) that occurred between Day 11 and Day 19 was unaffected. In other rats the effects of hormonal manipulations on uterine contractility and cervical creep were determined. Uterine contractility, measured using intrauterine balloons in conscious unrestrained rats, was increased on Days 18, 19 and 20 of pregnancy after bilateral ovariectomy on Day 16, compared to that of rats with intact ovaries. Creep of the cervix of these ovariectomized rats on Day 20 was decreased. Treatment with oestradiol-17 beta benzoate (0.5 micrograms/kg) plus progesterone (10 mg/kg) twice daily after ovariectomy decreased uterine contractility and increased cervical creep. It is concluded that cervical softening in pregnancy is not a consequence of an increase in uterine contractility.