Ultrasound can provide significant information in the fields of obstetrics and gynecology. Fetal presentation, presence of multiple gestation, and gross fetal abnormalities are easily determined. Measurement of the biparietal diameter correlates more closely with gestational age than does use of the fetogram. In addition, there is no evidence of fetal toxicity by this method, and the placenta is easily located. Ultrasound is the preferred method for diagnosis of placenta previa. Abnormalities of the early gestational period including missed abortion, ectopic pregnancy, and molar pregnancy also can be diagnosed by ultrasound. The cystic or solid nature of pelvic masses can be assessed as well as their relationship to the uterus and adnexa.