Basic Obstetric Ultrasound

The basic obstetric ultrasound examination provides an accurate and safe clinical assessment of the gravid uterus throughout a pregnancy, including characterizing pregnancy location, identifying the number of embryos present, and aiding in the prenatal diagnosis of fetal anomalies.

The American Institute of Ultrasound in Medicine (AIUM), in conjunction with the American College of Radiology (ACR) and the American College of Obstetricians and Gynecologists (ACOG), released updated practice guidelines for performance of obstetric ultrasound examinations. These guidelines describe the indications and key elements of four major types of obstetric ultrasounds, specifically the first-trimester ultrasound, standard second- or third-trimester ultrasound, and limited and specialized ultrasound examinations. ^[1] Further details regarding specific information gathered in each type of ultrasound examination is described below.

First-trimester ultrasound examination

The first-trimester basic ultrasound examination is typically performed to confirm a viable intrauterine pregnancy. The examination may be performed either transabdominally or transvaginally. It is ideally performed before 13 weeks and 6 days of gestation. Ultrasound examination at this time aids in the clinical assessment of pelvic pain and/or vaginal bleeding in the setting of an early pregnancy because it can diagnose an extrauterine pregnancy or an abnormal pregnancy, such as a hydatidiform molar pregnancy, an anembryonic gestation, or an incomplete versus complete abortion.

A definitive diagnosis of an intrauterine pregnancy can be made when a gestational sac containing a yolk sac is visualized within the uterine cavity. Without visualization of a yolk sac (or signs of a further developed pregnancy such as an embryo), the location of the pregnancy cannot be certain and further evaluation is warranted. In some cases where a pregnancy test is positive, but there is no clear intrauterine pregnancy or extrauterine findings concerning for an ectopic pregnancy such as an adnexal mass on ultrasound, a patient may have a “pregnancy of unknown location.” It is important to consider the clinical context of a patient without a documented intrauterine pregnancy to guide further management. This includes the patient’s symptoms (pelvic pain, vaginal bleeding), serial serum beta human chorionic gonadotropin levels (bHCG), and pelvic examination findings.

Additionally, a first-trimester ultrasound examination is useful to diagnose an “early pregnancy loss” which is defined by American College of Obstetricians and Gynecologists as a nonviable, intrauterine pregnancy with either an empty gestational sac, or a gestational sac containing an embryo or fetus without cardiac activity within the first 12 6/7 weeks of gestation. ^[2] The Society of Radiologists in Ultrasound Multispecialty Panel on Early First Trimester Diagnosis of Miscarriage and Exclusion of a Viable Intrauterine Pregnancy have published conservative guidelines to aid with clinical judgment in the diagnosis of an abnormal intrauterine pregnancy. Diagnostic findings of an early pregnancy loss include the following ^[3] :

• Crown-rump length of 7 mm or greater and no heartbeat

• Mean sac diameter of 25 mm or greater and no embryo

• The absence of cardiac activity in an embryo 2 weeks or more after a scan that showed a gestational sac without a yolk sac

• The absence of cardiac activity in an embryo 11 days or more after a scan that showed a gestational sac with a yolk sac

Cardiac activity of an embryo is documented using 2-dimensional video clip or M-mode imaging. ^[1] If no cardiac motion is seen on transvaginal ultrasound in an embryo less than 7 mm, a subsequent ultrasound in 1-2 weeks should be performed to assess for cardiac activity. ^[4] Additionally, if the clinical presentation suggests a miscarriage (ie, heavy vaginal bleeding), but the patient is stable for expectant management, a follow-up ultrasound evaluation performed 7-14 days after initial presentation to assess for interval changes and viability is also appropriate management. ^[2]

In the setting of a confirmed viable intrauterine pregnancy, the first-­trimester ultrasound is utilized to provide an accurate gestational age assessment. When only a gestational sac and yolk sac are visualized, the mean gestational sac diameter may be used to estimate gestational age (Mean sac diameter (mm) + 30 = gestational age in days). However, if an embryo is visualized, then a crown-rump length (CRL) of the fetus should be used to determine an estimated due date because it is the most accurate measurement for establishing gestational age. ^[4]

An embryo should be visible by transvaginal ultrasonography with a mean gestational sac diameter of 25 mm or greater. The crown rump length is the maximum length of the infant from cranium to caudal rump in a longitudinal plane. ^{[4, 5]}  Measurements of the CRL are more accurate the earlier the first-trimester ultrasound is performed. If the CRL measurement is greater than or equal to 84 mm (which corresponds to a gestational age of 14 and 0/7 weeks), second-trimester biometric parameters should be used for calculating the gestational age. ^[5] A reliable formula to calculate gestational age based on CRL is as follows: CRL (mm) + 42 days (+/- 3 days) = gestational age (days). ^[6]  

ACOG published a standardized approach for calculating a patient’s anticipated due date using both ultrasound estimates and menstrual history, specifically the patient’s first day of the last menstrual period. ^[5] If the patient is unsure of the last menstrual period (LMP) or has a history of irregular menstrual cycles, dating should be calculated based on ultrasound measurements. In general, ultrasound dating is used when the discrepancy between menstrual dating and ultrasound dating is greater than the precision of ultrasonography. ^[4] First trimester calculations are more precise compared to later gestational ages. Before 14 0/7 weeks gestation, the mean crown-rump length calculated has a precision of 5-7 days. ^[4] Therefore, before 9 0/7 weeks gestation, the estimated due date should correspond to ultrasound measurements when there is more than a 5 day discrepancy between menstrual dating and ultrasound dating. ^{[4, 5]} Similarly, if the ultrasound dating between 9 0/7 weeks of gestation and 13 6/7 weeks gestation has more than a 7 day discrepancy from the menstrual dating, ultrasound measurements should be used to assign estimated due date. ^{[4, 5]} In the second and third trimester, larger discrepancies reflect less precise measurements based on biometric parameters (see the Table below).

Table. American College of Obstetricians and Gynecologists Guidelines for Redating a Pregnancy Based on Ultrasonography Rather Than Menstrual Dating ^[5] (Open Table in a new window)

In the setting of multifetal gestations, amnionicity and chorionicity should be documented. In the setting of desired genetic testing, nuchal translucency measurement aids in the screening assessment for fetal aneuploidy in conjunction with biomarkers. A first-trimester ultrasound examination is also useful for the evaluation of maternal anatomy including assessment of the uterus, cervix, and adnexal structures. The presence of adnexal masses, ovarian cysts, and/or leiomyomas should be documented and followed throughout pregnancy.

Second- and third-trimester ultrasound examinations

Second- or third-trimester ultrasound examinations use fetal biometry to assess fetal growth and also can provide detailed information on fetal anatomy. A standard obstetric ultrasound examination also may include an evaluation of fetal presentation(s), amniotic fluid volume, cardiac activity, and placentation.

After the first trimester, fetal biometry specifically measures the fetus’ biparietal diameter, head circumferences, abdominal circumference or average abdominal diameter and femoral diaphysis length. Fetal biometry may be utilized to establish an estimated due date for a pregnancy if no prior ultrasound measurement of the embryo was done. However the most accurate gestational age assessment is based on crown rump length measurement and the variability of gestational age estimations increases throughout pregnancy. For this reason, the earliest available ultrasound should always be used to assign an estimated due date and any significant discrepancies between gestational age and fetal measurements on subsequent ultrasounds should raise suspicion for growth abnormalities. The approximate error in fetal weight prediction methods is approximately 15% and is influenced by patient body habitus, weight range of fetus, technical factors such as machine quality and experience of ultrasonographer. ^[4]   In the third trimester, the femur length is the best single biometric measurement of gestational age. ^[4]

Assessment of fetal anomalies, also known as a fetal anatomic survey, should be performed after 18 weeks' gestational age and ideally performed between 18-20 weeks' gestational age. ^[4] Although it may be possible to document anatomic structures before this time, the size, position, and movement of a fetus may limit a comprehensive examination of structures and therefore require repeat ultrasound examinations. ^[4] Additionally if a fetal anomaly is detected in this preferred gestational age window (18-20 weeks), termination of pregnancy may still be an option for the patient.

The basic fetal anatomic examination includes assessment of the following structures: lateral cerebral ventricles, choroid plexus, midline falx, cavum septi pellucidi, cerebellum, cistern magna, upper lip, four-chamber view of heart as well as left and right ventricular outflow tracts, size and location of stomach, urinary bladder and ureters, spinal anatomy, extremities, and gender. A more detailed anatomic survey may be indicated depending on the risk and concern for aneuploidy. The placenta should also be further characterized at this time, specifically noting its location and proximity to the internal cervical os as well as the number of vessels and insertion site of the umbilical cord. ^{[1, 4]}  

Additionally, the second and third trimester basic ultrasound may also be used to diagnose or to monitor maternal anatomical problems, most notably cervical length in the setting of risk factors for preterm birth or cervical insufficiency. Ultrasound monitoring of fibroids and/or ovarian cysts is also important, but this is limited at later gestational ages due to the size of the gravid uterus.

Limited and specialized ultrasound examination

A limited or specialized ultrasound examination may be performed at any gestational age and is typically used to evaluate a specific clinical concern during prenatal care. Some examples include assessment of cardiac activity when fetal heart tones are undetectable with external fetal monitoring devices to rule out fetal demise, notation of fetal presentation in setting of anticipated external cephalic version or to determine mode of delivery, and calculation of estimated fetal weight and amniotic fluid in setting of comorbidities that may predispose infants to growth abnormalities (eg chronic hypertension, preeclampsia, diabetes, multifetal gestation).  

Fetal growth evaluations are typically performed at 3-4 week intervals and usually include an assessment of amniotic fluid as well. ^[4] There are two main techniques to measure amniotic fluid in the second or third trimester, specifically single deepest pocket (SDP) or the amniotic fluid index (AFI). The SDP technique records the single largest vertical pocket of amniotic fluid without evidence of umbilical cord or fetal parts visualized in utero. The AFI technique is the summative measurement of the single deepest vertical pocket of fluid without evidence of cord or fetal parts noted in all four quadrants of the uterus. Oligohydramnios, or significantly low amniotic fluid, is defined as an AFI less than 5 cm or a maximum vertical pocket less than 2 cm. Alternatively, polyhydramnios which is a term to describe an abnormally large amount of fluid, is defined by an AFI greater than 24 cm or a single deepest vertical pocket (SDP) greater than 8 cm. ^[4] In multifetal gestations, fluid evaluation should be performed using the single deepest pocket technique. Comparison of measurement techniques show that measurement of the single deepest pocket leads to fewer interventions (such as induction of labor for oligohydramnios) with no increase in poor perinatal outcomes. ^[4]

Similar to a limited ultrasound examination, a detailed ultrasound examination is a supplemental tool to aid in the management of prenatal care in the setting of concern for fetal well-being due patient history, genetic screening abnormalities, or results of prior ultrasound exams. For example, specialized ultrasound examinations include fetal surveillance with a biophysical profile, fetal Doppler ultrasonography for assessment of placental insufficiency, and a more detailed anatomy scan or fetal echocardiography in setting of concern for fetal anomaly.

Safety

The prenatal ultrasound examination has been proven safe for both mothers and fetuses. As with any clinical test or medical intervention, a risk ­to ­benefit analysis of the test should be considered and should only be performed when there is a medical/obstetric indication or clinical concern. Additionally, if a diagnostic ultrasound examination is required for patient care, it should be done under the “as low as reasonably achievable principle” (ALARA principle) due to possible risks associated with the physical effects from the examination. These effects include mechanical vibrations or increase in temperature under exam conditions as well as unknown risks of ultrasound energy to fetus not yet documented in the literature. ^{[1, 4]} Some ultrasonographic modalities, such as Doppler, deliver more energy to the area of interest, and the use of those modalities should be reserved for specific clinical questions and an attempt made to limit their duration of use.

Obstetric ultrasound examination requires real-time two-dimensional imaging via a transabdominal or transvaginal approach in order to adequately assess pregnancy viability through cardiac activity and fetal movement. There is currently no clinical evidence suggesting a clear advantage of three-dimensional imaging in prenatal diagnosis. ^[4]

The ultrasound transducer frequency must be selected to balance optimal beam penetration versus resolution, and therefore may vary based on a patient’s body habitus. For example, a lower-frequency transducer is beneficial in obese patients to allow for increased penetration and better imaging. Modern equipment typically includes a transabdominal transducer with 3 to 5 MHz frequency and a transvaginal transducer with 5-10 MHz frequency. ^[4]

Modern ultrasound equipment has boundaries set by the manufacturer, limiting the fetal exposure to energy generated 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 personnel and the ultrasound unit. The AIUM has published specific training guidelines for physicians who perform and interpret obstetric ultrasound which includes the following minimum requirements to demonstrate a strong knowledge base, technical skill and competency. ^[1]  Specifically for graduates of residency or fellowship programs to gain proficiency, the physician must perform at least 300 diagnostic ultrasounds. Additionally, a minimum of 170 annual diagnostic obstetric ultrasound examinations is recommended to maintain the technical skills required for competency.

Procedure planning

Minimal preparation is required for a transabdominal or transvaginal ultrasound examination. 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.

Ultrasound transducers require proper cleaning before and after each patient’s use to avoid risk of microbial transmission leading to infection. Transabdominal ultrasound transducers are typically cleansed with disposable antiseptic wipes, and the clean transducer may be applied directly to the patient’s skin. In contrast, transvaginal ultrasound transducers require more extensive cleaning and sterilization. Specifically, transvaginal transducers should be covered with a single-use disposal cover during the patient exam. After the exam is completed, cleaning steps include removal of the disposal cover, cleansing with running water or a damp cloth to remove residual gel or debris from probe, followed by high-level disinfection chemical agents in accordance with recommended guidelines from the US Food and Drug Administration. ^[4]