OBJECTIVE The objective of this study was to investigate the occurrence of in utero defecation as a normal function in the human fetus. STUDY DESIGN The anuses of 240 fetuses were studied sonographically between weeks 15 and 41 of gestation. Fetal defecation was defined as the expulsion of rectal contents through the anus into the amniotic fluid. The diameter and area of the anus were measured sonographically at times of maximum anal aperture. RESULTS One or more defecations were documented in all fetuses. The frequency of defecations was highest between week 28 and 34 of gestation. CONCLUSION This study confirms that defecation in utero is a normal function and supports the view that the evacuation of rectal contents into the amniotic fluid is no departure from normal fetal physiologic behavior.