Continuous measurement of normal contractions in 57 primiparae and multiparae showed a minimum in the mornings and a marked peaking of frequency between 8:30 PM and 2:00 AM. This could be due to a periodic fluctuation in the parameters influencing uterine motility. At night, motility-enhancing factors such as estrogens and prostaglandins predominate. These results suggest that in cases at high risk for premature delivery, with high pelvic scores, therapy should be continued at night. In addition, the time when the contractions were recorded should be taken into account when assessing their frequency, and the measurement repeated if necessary.