The basic obstetric ultrasound examination provides an accurate and safe clinical assessment of the gravid uterus throughout a woman’s pregnancy including characterizing pregnancy location, identifying the number of embryos present, and aiding in the prenatal diagnosis of fetal anomalies. In 2013, 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 4 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 exam is described below.
The first trimester basic ultrasound is typically performed to confirm a viable intrauterine pregnancy. The exam may be performed either trans-abdominally or trans-vaginally. 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 exam 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: 1.) Crown-rump length of 7 mm or greater and no heartbeat, 2.) Mean sac diameter of 25 mm or greater and no embryo, 3.) the absence of cardiac activity in an embryo 2 weeks or more after a scan that showed a gestational sac without a yolk sac, and/or 4.) the absence of cardiac activity in an embryo 11 days or more after a scan that showed a gestational sac with a yolk sac. [3]
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 patient’s clinical presentation suggests a miscarriage (i.e heavy vaginal bleeding) but she is stable for expectant management, a follow-up ultrasound 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]
In 2014, 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 her 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 Table 1).
Table 1: American College of Obstetricians and Gynecologists’ Guidelines for Redating a pregnancy based on ultrasonography rather than menstrual dating. [5]
Gestational age in weeks