The aim of this study was to evaluate morphological characteristics of the pelvic floor in pregnant women using 2- and 3-dimensional (D)-transperineal ultrasound and compare our findings with findings in non-pregnant women. Methods This case-control study included 40 nulliparous pregnant women at term and 28 nulliparous, non-pregnant women (age-matched). The 2D- and 3D-transperineal ultrasounds were carried out in the semi-supine position, after voiding, at rest and during the Valsalva maneuver. Various biometric parameters related to characteristics of the pelvic fl oor were measured. Results Satisfactory biometric measurements were obtained in all cases. The mean thickness of the levator ani muscle was significantly greater in pregnant women than in non-pregnant women ( P<0.05). The mean levator hiatus angle and transverse diameter of the levator hiatus were significantly lower in pregnant women than in non-pregnant women ( P<0.05). The anteroposterior diameter of the levator hiatus was not significantly different between pregnant women and non-pregnant women. Conclusion Pregnant women had significantly thicker the levator ani muscles but smaller hiatal areas, as measured by the levator hiatus angle and transverse diameter, than did non-pregnant women. Pregnancy itself may cause morphological changes to the pelvic floor to support the birth canal by closing the lower end of the pelvic cavity as a diaphragm. Further studies are needed to evaluate morphologic changes of the pelvic floor following delivery as measured by 2D and 3D-transperineal ultrasound.