Fetal sex can be accurately and rapidly assessed in fetuses of 24 days gestation to term (31 days) by examination of the gonads. At 24 days both testis and ovary are a few millimeters below the kidney. With advancing gestation the testis descends markedly so that by 28 days it lies in the lowest region of the retroperitoneum. The ovary descends only slightly, never reaching a position more than halfway between the kidney and the bottom of the retroperitoneal space. The testis is larger, thicker, kidney-bean shaped and has an epididymis. The ovary is longer, thinner, rice grain-shaped and has a Fallopian tube. The character of the blood supply to the gonads also provides useful supporting information in determining fetal sex. Histological examination was used in 127 fetuses to evaluate the accuracy of anatomical identification of fetal sex and detected 2 errors (1.6%).