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Basic Obstetric Ultrasound

  • Author: Serdar H Ural, MD; Chief Editor: Carl V Smith, MD  more...
Updated: Jul 27, 2016


The basic obstetric ultrasound examination may be used to determine the location of a pregnancy and number of fetuses present and to assist in the assignment of gestational age, prenatal diagnosis of fetal anomalies, and early diagnosis of placental insufficiency.

In 2007, the American Institute of Ultrasound in Medicine (AIUM), in conjunction with the American College of Radiology and the American College of Obstetricians and Gynecologists, released Practice Guidelines for Performance of Obstetric Ultrasound Examinations. The guidelines detail key elements of standard first- and second-trimester fetal evaluation. They classify fetal ultrasound examination into 4 categories: first trimester, standard second or third trimester, limited examinations, and specialized examinations.[1]

Also see the article Targeted Obstetric Ultrasound.



The basic obstetric ultrasound is categorized by the gestational age at which it is performed.

During the first trimester of the pregnancy, the first-trimester basic ultrasound is performed to confirm an intrauterine pregnancy after the patient has a positive pregnancy test. In patients with a positive pregnancy test, the first-trimester ultrasound is especially helpful in ruling out ectopic pregnancy in the setting of pelvic pain or to evaluate for miscarriage or molar pregnancy in the setting of vaginal bleeding. In addition, the first-trimester ultrasound is used to determine the number of fetuses and to evaluate maternal masses. A more-advanced first-trimester ultrasound may be used to evaluate the fetus for aneuploidy (eg, measurement of the Nuchal translucency area and determination of anencephaly).

During the second trimester of the pregnancy, a second-trimester basic ultrasound may be performed. This test is commonly known as an anatomy ultrasound and is routinely used to detect fetal anatomical anomalies and markers for aneuploidy. It can also be used to diagnose or to monitor maternal anatomical problems such as fibroid uterus or ovarian cysts.

A limited ultrasound may be performed at any gestational age and is typically used to evaluate a specific clinical concern. Some examples include evaluating for fetal heart tones that are undetectable with conventional methods, evaluating preterm labor, evaluating vaginal bleeding, assisting in the diagnosing rupture of membranes, determining fetal presentation, and assisting with fetal procedures such as amniocentesis. This study is usually reserved to help in managing certain obstetric procedures or emergencies and is not usually used to replace the second-trimester anatomy ultrasound.

The CDC and ACOG recommend that pregnant women who live in or have traveled to areas with ongoing Zika virus exposure should undergo Zika virus serologic testing and fetal ultrasonography to screen for microcephaly or intracranial calcifications as early as 3-4 weeks after symptoms or exposure.[2, 3]  However, the CDC warned that fetal ultrasounds might not detect abnormalities until late second or early third trimester of pregnancy.[2, 4, 5]



Ultrasound has been proven safe to both mothers and fetuses. As with any clinical test, a risk-to-benefit analysis of the test should be considered. Some ultrasonographic modalities, such as Doppler, deliver more energy, and the use of those modalities should be reserved for specific clinical questions and an attempt made to limit their duration of use.


Technical Considerations

Modern ultrasound equipment has boundaries set by the industry, limiting the fetal exposure to energy derived by the equipment.

As with any medical equipment, adequate care and maintenance should be performed as per manufacturer recommendations.

Additionally, personnel involved with the use of ultrasound equipment should have appropriate training. Some ultrasound suites have quality-assurance programs to evaluate performance of the personnel and the ultrasound unit.

Procedure Planning

Minimal preparation is required for a transabdominal ultrasound.

A fasting state is not required, in contrast to other ultrasound studies (eg, gallbladder ultrasonography).

Some practitioners advise their patients to arrive to the ultrasound suite with a full bladder, but there is no consensus regarding this recommendation, especially for an obstetric ultrasound performed after 18 weeks’ gestation.

If a transvaginal approach is to be used, the patient is asked to void just before the study to empty her bladder. This minimizes discomfort and collapses the bladder for better visualization of pelvic organs.

Contributor Information and Disclosures

Serdar H Ural, MD Associate Professor of Obstetrics and Gynecology and Radiology, Director, Division of Maternal-Fetal Medicine, Medical Director, Labor and Delivery Suite, Pennsylvania State University College of Medicine

Serdar H Ural, MD is a member of the following medical societies: American College of Obstetricians and Gynecologists, American Institute of Ultrasound in Medicine, American Medical Association, Association of Professors of Gynecology and Obstetrics, AAGL, Society for Maternal-Fetal Medicine

Disclosure: Received honoraria from GSK for speaking and teaching; Received honoraria from J&J for speaking and teaching.


Pedro Roca, MD, MPH, FACOG Fellow in Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, Hershey Medical Center, Pennsylvania State University College of Medicine

Pedro Roca, MD, MPH, FACOG is a member of the following medical societies: American College of Obstetricians and Gynecologists, American Institute of Ultrasound in Medicine, American Medical Association, American Public Health Association

Disclosure: Nothing to disclose.

Specialty Editor Board

John G Pierce, Jr, MD Associate Professor, Departments of Obstetrics/Gynecology and Internal Medicine, Medical College of Virginia at Virginia Commonwealth University

John G Pierce, Jr, MD is a member of the following medical societies: American College of Obstetricians and Gynecologists, Association of Professors of Gynecology and Obstetrics, Christian Medical and Dental Associations, Medical Society of Virginia, Society of Laparoendoscopic Surgeons

Disclosure: Nothing to disclose.

Chief Editor

Carl V Smith, MD The Distinguished Chris J and Marie A Olson Chair of Obstetrics and Gynecology, Professor, Department of Obstetrics and Gynecology, Senior Associate Dean for Clinical Affairs, University of Nebraska Medical Center

Carl V Smith, MD is a member of the following medical societies: American College of Obstetricians and Gynecologists, American Institute of Ultrasound in Medicine, Association of Professors of Gynecology and Obstetrics, Central Association of Obstetricians and Gynecologists, Society for Maternal-Fetal Medicine, Council of University Chairs of Obstetrics and Gynecology, Nebraska Medical Association

Disclosure: Nothing to disclose.

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Embryo at 12 weeks' gestation.
Fetal heartbeat at 20 weeks' gestation.
Table 1. Mean Gestational Sac Sizes at Which a Yolk Sac and an Embryo Should Be Visible
Visible Feature Mean Transvaginal Gestational Sac Diameter (mm) Mean Transabdominal Gestational Sac Diameter (mm) Serum b-hCG level (mIU/mL) Gestational Age (wk)
Yolk sac visible 8 20 7,200 5-6
Embryo visible 16 25 10,000 >6
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