Obstetric ultrasonography, or prenatal ultrasound, is the use of medical ultrasonography in pregnancy, in which sound waves are used to create real-time visual images of the developing embryo or fetus in the uterus (womb). The procedure is a standard part of prenatal care in many countries, as it can provide a variety of information about the health of the mother, the timing and progress of the pregnancy, and the health and development of the embryo or fetus.

The International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) recommends that pregnant women have routine obstetric ultrasounds between 18 weeks' and 22 weeks' gestational age (the anatomy scan) in order to confirm pregnancy dating, to measure the fetus so that growth abnormalities can be recognized quickly later in pregnancy, and to assess for congenital malformations and multiple pregnancies (twins, etc). Additionally, the ISUOG recommends that pregnant patients who desire genetic testing have obstetric ultrasounds between 11 weeks' and 13 weeks 6 days' gestational age in countries with resources to perform them (the nuchal scan). Performing an ultrasound at this early stage of pregnancy can more accurately confirm the timing of the pregnancy, and can also assess for multiple fetuses and major congenital abnormalities at an earlier stage. Research shows that routine obstetric ultrasound before 24 weeks' gestational age can significantly reduce the risk of failing to recognize multiple gestations and can improve pregnancy dating to reduce the risk of labor induction for post-dates pregnancy. There is no difference, however, in perinatal death or poor outcomes for infants.

Below are useful terms on ultrasound:

In normal state, each body tissue type, such as liver, spleen or kidney, has a unique echogenicity. Fortunately, gestational sac, yolk sac and embryo are surrounded by hyperechoic (brighter) body tissues.

Traditional obstetric sonograms are done by placing a transducer on the abdomen of the pregnant woman. One variant, transvaginal sonography, is done with a probe placed in the woman's vagina. Transvaginal scans usually provide clearer pictures during early pregnancy and in obese women. Also used is Doppler sonography which detects the heartbeat of the fetus. Doppler sonography can be used to evaluate the pulsations in the fetal heart and bloods vessels for signs of abnormalities.

Modern 3D ultrasound images provide greater detail for prenatal diagnosis than the older 2D ultrasound technology. While 3D is popular with parents desiring a prenatal photograph as a keepsake, both 2D and 3D are discouraged by the FDA for non-medical use, but there are no definitive studies linking ultrasound to any adverse medical effects. The following 3D ultrasound images were taken at different stages of pregnancy:

A gestational sac can be reliably seen on transvaginal ultrasound by 5 weeks' gestational age (approximately 3 weeks after ovulation). The embryo should be seen by the time the gestational sac measures 25 mm, about five and a half weeks. The heartbeat is usually seen on transvaginal ultrasound by the time the embryo measures 5 mm, but may not be visible until the embryo reaches 19 mm, around 7 weeks' gestational age. Coincidentally, most miscarriages also happen by 7 weeks' gestation. The rate of miscarriage, especially threatened miscarriage, drops significantly after normal heartbeat is detected, and after 13 weeks.

In the first trimester, a standard ultrasound examination typically includes: