First trimester obstetric ultrasound

Clinicians can use a combination of history and physical exam, measurement of human chorionic gonadotrophin levels, and/or pelvic ultrasonography to confirm the diagnosis of pregnancy. First-trimester ultrasound is a commonly ordered diagnostic test in the United States to confirm pregnancy, establish dating, and perform fetal nuchal translucency screening. Beyond these uses, a first-trimester ultrasound is routinely performed to evaluate lower abdominal pain, vaginal bleeding, and assess pregnancy location. The diagnoses of early pregnancy loss and ectopic pregnancy, particularly cesarean section ectopic pregnancy, rely on accurate performance and interpretation of first-trimester obstetric ultrasounds. We focus this article on the specific considerations every clinician should know when performing and interpreting first-trimester obstetric ultrasounds.

Ultrasound probes

The most commonly utilized ultrasound probes to evaluate a suspected first-trimester pregnancy are the curvilinear and endocavitary, used for transabdominal and transvaginal scans, respectively. Which probe is used will depend on multiple patient factors, such as suspected gestational duration, total body weight, distribution of adiposity, and uterine surgical history. The transvaginal approach may result in improved images for patients under 8 weeks of gestational duration, with higher BMIs, increased suprapubic adiposity depth, or prior cesarean section uterine scars. Clinicians can obtain improved lower uterine segment images by asking patients not to void before the transvaginal ultrasound exam.

Early pregnancy development

Estimating a patient’s gestational duration based on the first day of the last menstrual period (LMP) is essential before performing a pelvic ultrasound. Should the LMP date indicate a pregnant person is over 8 weeks of gestation, the use of a transabdominal curvilinear probe will likely be sufficient to image the pregnancy. That said, clinicians will obtain better visualization of pregnancy landmarks with a transvaginal approach. Figure 1 outlines the gestational duration at which pregnancy landmarks appear. A mismatch of the gestational duration based on LMP and visualized pregnancy landmarks on transvaginal ultrasound should prompt considerations for inaccurate gestational duration estimation based on LMP or the presence of an abnormal intrauterine or extrauterine pregnancy.

Figure 1. Gestational Duration at Which Pregnancy Landmarks Are Typically First Seen on Transvaginal Ultrasound

Pregnancy dating

An accurately dated pregnancy is critical for providing evidence-based obstetrical care, specifically the timing of clinic visits, interpretation of screening tests, fetal growth, lab values, and delivery timing.¹ Pregnancies can be dated based on physical exam, menstrual history, ultrasound, or a combination of the above methods.

By convention, an estimated due date (EDD) can be calculated as 280 days from the LMP. This calculation assumes regular 28-day menstrual cycles with ovulation on the 14th day. Therefore, it is essential to obtain a thorough menstrual history, including cycle regularity and cycle length, as an LMP from a person with irregular menstrual cycles will result in inaccurate pregnancy dating. Even in the setting of regular menstrual cycling, establishing a sure LMP may not be possible. In a 2005 prospective cohort study, only 56% of menstruating people were able to recall their exact LMP accurately.² In current United States clinical practice, LMP estimates of gestational duration serve as an initial dating method, a method recommended to be confirmed by first-trimester ultrasound.

Less than or equal to 6 weeks (42 days) gestation: Mean Sac Diameter (MSD) Measurements

Under or equal to 6 weeks or 42 days of gestation, clinicians should not expect to visualize an embryo on transvaginal ultrasound. Instead, ultrasound may be used to confirm the location of a pregnancy, especially helpful for those with prior cesarean section uterine scars, a history of ectopic pregnancy, or to aid in the evaluation of vaginal bleeding or pelvic pain in early pregnancy. In this gestational range, measurement of the mean gestational sac diameter (MSD) can aid in obtaining gestational duration estimates. Gestational sacs are identified on ultrasound as round or oval hypoechoic fluid collection with a surrounding hyperechoic rim.³ Clinicians must know how to differentiate intracavitary fluid, historically called a “pseudosac,” from a gestational sac. The measurements used for calculating a MSD are the gestational sac's length, width, and height. Care must be taken to ensure measurements are of the gestational sac itself, not including the echogenic rim (Figure 2). The average (mean) is taken by adding these measurements (in mm) and dividing by 3. This average diameter (MSD) value can be used to estimate gestational duration with the formula: Days of pregnancy = MSD (in mm) + 30. Some ultrasounds can approximate gestational duration using a different calculation from gestational sac measurements. MSD pregnancy dating should not be used when an embryo is seen. It must be noted that MSD estimates should not be utilized to establish a due date. Instead, clinicians should plan for repeat interval imaging to establish gestational duration.⁴

Between 6 to 10 weeks (42 to 70 days) gestation: Embryonic measurements

Between 6 to 10 weeks (42 to 70 days) of gestation, clinicians should expect to visualize a yolk sac, amnion, and an embryo with cardiac activity on transvaginal ultrasound. Visualization of the yolk sac inside a gestational sac is sufficient to confirm that a pregnancy is intrauterine. Yolk sacs are identified on ultrasound as a thin-rimmed round eccentrically located structure inside of a gestational sac. Yolk sac measurements are not utilized for pregnancy dating. A brief note on terminology: we use the correct term of embryo to refer to a pregnancy that measures less than or equal to 10 weeks 6 days; the term fetus applies to a pregnancy that measures greater than or equal to 11 weeks gestation.³ In the 6-to-10-week gestational duration range, the measurements of the embryo, specifically the crown-rump length (CRL), will provide gestational duration estimates. CRL (Figure 3) is the longest straight-line measurement (in mm) of the embryo in the mid-sagittal line, including the fetal spine longitudinally, from the outer margin of the cephalad region (crown) to the caudal region (rump).⁵ The average of 3 measured CRLs is calculated. Depending on the ultrasound machine's quality and the ultrasonographer's skill, CRLs as small as 2mm can be seen on transvaginal ultrasound. The mean CRL can then estimate gestational duration with the formula: Days of pregnancy = mean CRL (in mm) + 42. It is important to note that this formula cannot be applied to CRL measurements taken transabdominal or greater than or equal to 25mm.⁶ Clinicians can calculate estimates of gestational duration in these cases (transabdominal or transvaginal measurements with a CRL above 25mm) from population estimates programmed into modern ultrasound machines that use a complex equation typically referred to as the Hadlock formula.⁷ The Hadlock formula is also acceptable for gestational dating when CRL is less than 25 mm. Both methods are validated measures of gestational duration and are utilized in practice.

LMP and ultrasound discrepancies

Generally, a sure LMP confirmed by a first trimester pregnancy ultrasound is widely considered the best pregnancy dating. When differences arise between these 2 dating methods, redating is recommended if the discrepancy in the number of days is above a certain threshold. Figure 4 represents the first trimester re-dating recommendations from the 2017 ACOG Committee Opinion.¹ It is essential to recognize that significant discrepancies between LMP and ultrasound estimates are often attributed to incorrect LMP estimates but may indicate pregnancy pathology such as aneuploidy or impending early pregnancy loss.

Figure 4. Ultrasound Guidelines for Redating a Pregnancy

Early pregnancy loss

Early pregnancy loss (EPL) is also known as miscarriage or spontaneous abortion. EPL is common; rates in clinically recognized pregnancies vary from 10% to as high as 53% and are influenced by patient characteristics such as age and number of previous miscarriages.⁸ Patients often seek care in early pregnancy, due to concern for EPL, in the event of vaginal bleeding, lower abdominal cramping, or a history of EPL. We focus this discussion on the ultrasound definition of EPL. We focus our discussion on desired pregnancies, as the desiredness of the pregnancy alters the recommended diagnostic approach and treatment options.⁹ The nuances of evaluating and managing vaginal bleeding, pelvic cramping, and complete EPL are beyond the scope of this article.

EPL ultrasound diagnosis can be made using one-time imaging or serial imaging studies. Before performing a transvaginal ultrasound, assessing a person’s estimated gestational duration through LMP or prior ultrasounds is critical. For example, a previously diagnosed intrauterine pregnancy on transvaginal ultrasound that is no longer seen on repeat imaging is diagnostic of EPL. The same applies to situations where cardiac activity was seen and subsequently not seen. The criteria to diagnose an EPL based on a single imaging study have intentionally been created to reduce the probability of a false positive result, in other words to reduce the chance of diagnosing an EPL in a desired pregnancy that is an appropriately developing pregnancy. The imaging findings that have a 100% positive predictive value for EPL are as follows:¹⁰

• CRL greater than or equal to 7 mm without cardiac activity
• MSD greater than or equal to 25 mm and no visualized embryo
• No visualized embryo with cardiac activity greater than or equal to 14 days after an ultrasound that showed a GS without YS
• No visualized embryo with cardiac activity greater than or equal to 11 days after an ultrasound that showed a GS with a YS

Multiple other ultrasound features can be concerning, but not diagnostic, for EPL, for example, a CRL of 5mm without cardiac activity or a MSD of 20 mm without an embryo. Knowing what is diagnostic versus what is concerning for EPL allows clinicians to provide nuanced counseling, treatment plans, and expectation setting.

Ectopic pregnancy

Any clinician performing early obstetric ultrasound must be well-versed in ultrasound features that indicate ectopic pregnancy. A note on terminology: ectopic pregnancies can be located both intrauterine (cervical, cesarean section scar, intramural, interstitial) and extrauterine (tubal, ovarian, abdominal) and, therefore, should be thought of as pregnancies located in abnormal locations. The presence of no intrauterine pregnancy and a heterogenous extra ovarian adnexal mass, regardless of the presence or absence of a GS, YS, or CRL in this mass, is highly suggestive of tubal ectopic pregnancy. Differentiating a suspected ectopic pregnancy from a corpus luteum ovarian cyst is notoriously challenging. The Society of Radiologists in Ultrasound recommends assessing color flow (active color flow can be present in both structures), location with respect to the ovary (within the ovarian parenchyma more suggestive of corpus luteum), and mobility (immobile from the ovary with ultrasound probe pressure more suggestive of corpus luteum) to differentiate these structures.³ If clinical doubt remains, obtaining additional images from another sonographer and interpreting clinician is always appropriate.

Cesarean scar ectopic pregnancy

In 2022, 32% of United States birthing people delivered via a cesarean section.¹¹ This steady and high rate of cesarean delivery has many health implications. We focus this article on the development of future cesarean scar ectopic pregnancies, which, when unrecognized, can result in severe adverse fetal and maternal outcomes. Clinicians performing early obstetric ultrasound, inevitably occurring in patients with prior uterine incisions, must be able to recognize signs of a cesarean scar ectopic pregnancy. Again, we highlight the utility of having a patient maintain a full bladder to improve transvaginal lower uterine segment imaging. A gestational sac that is located in the lower half of the uterine cavity, particularly the anterior portion of the lower half, should always raise suspicion for cesarean scar ectopic pregnancy and prompt a more detailed ultrasound examination. Additional sonographic details of this diagnosis are outside the scope of this review. That said, a clinician's ability to identify a possible cesarean section scar ectopic pregnancy and make appropriate referrals is crucial.

Terms to avoid

To provide clarity amongst the diverse providers performing and interpreting first trimester obstetric ultrasounds, we focus this final section on current recommendations from the Society of Radiologists in Ultrasound on common terms to avoid.³ Figure 5 is a shortened list of ‘terms of avoid’ and recommended alternative terms. Interested readers can find full explanations for the rationale of these recommendations in the complete article. Generally, many of the changes reflect a desire to replace terms that carry assumptions about the future development of a pregnancy and instead focus on terms that describe current pregnancy details. For example, “heart” and heartbeat” imply a fully formed organ when cardiac development is incomplete in the first trimester.

Figure 5. Society of Radiologists in Ultrasound 2024 Consensus: Terms to Avoid

References

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3. Rodgers SK, Horrow MM, Doubilet PM, et al. A lexicon for first-trimester US: Society of Radiologists in Ultrasound Consensus Conference recommendations. Radiology. 2024;312(2):e240122.
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9. Flynn AN, Schreiber CA, Roe A, et al. Prioritizing Desiredness in Pregnancy of Unknown Location: An Algorithm for Patient-Centered Care. Obstet Gynecol. 2020;136(5):1001-1005.
10. Doubilet PM, Benson CB, Bourne T, Blaivas M. Diagnostic criteria for nonviable pregnancy early in the first trimester. N Engl J Med. 2013;369(15):1443-1451.
11. Osterman MJ, Hamilton BE, Martin JA, Driscoll AK, Valenzuela CP. Births: final data for 2022. Natl Vital Stat Rep. 2024.