The principle of sonography depends on the transmission of a pulsed sound beam through a particular area of the body and on the interpretation of echo pattern produced from interfaces of tissues with different acoustic impedances or densities. Then it is limited to frequencies in the range from 1 to 20 million cycles/sec. in clinical practice. Real time examination provides many sonographic informations. Namely, determination of gestational age without an accurate last menstrual period is difficult, but the correlation between fetal biparietal diameter by ultrasound and gestational age is compromised by normal biologic variation in fetal growth rates. It is possible to estimate fetal weight with reasonable accuracy by meansuring biparietal diameter and abdominal circumference. Ultrasound scanning is also helpful in differential diagnosis of pelvic masses. So, ultrasound as a non-invasivd iagnostic tool has a wide variety of practical applcations which are extremely useful in obstetrical and gynecological practice.