[Purpose] This study investigated changes in the thickness of the transversus abdominis muscle at various stages of the menstrual cycle. [Subjects] The subjects were 15 young healthy females with regular menstrual cycles. [Methods] A regular menstrual cycle was defined as a 28-day cycle comprising 3 phases: the menstrual phase, the follicular phase, and the luteal phase. For the purpose of the study, measurements were taken at day 3 (menstrual phase), day 12 (follicular phase), and day 21 (luteal phase) of the cycle. An ultrasonic imaging diagnostic device (MyLab 25) and a linear expression probe were used for measurement of the transversus abdominis muscle. [Results] There were no significant differences in the variation rate of the thickness of the muscle at any phase of the menstrual cycle. [Conclusion] The results suggested that the sex hormones associated with the menstrual cycle do not affect the contractility or changes in the thickness of the transversus abdominis muscle. For the reasons stated above, there is little need to consider the menstrual cycle when measuring muscle thickness in physical therapy scenarios because the transversus abdominis muscle does not depend on the menstrual cycle.