Pregnancy Ultrasound Evaluation

Anatomy and Physiology

Ultrasound can be used transvaginally or transabdominally to view the female reproductive anatomy, with a transvaginal approach allowing for better visualization of structures. The uterus is located posterior to the bladder and anterior to the colon. The three parts of the uterus can be visualized with the use of transvaginal ultrasound, beginning most caudal: cervix, body, and then the fundus. The uterus contains the gestational sac during pregnancy which is typically visualized in the center of the uterine body at 4.5 to 5 weeks of gestation and is the first structure visualized in pregnancy.[2][3] The ovaries and Fallopian tubes are paired female reproductive organs that can be found lateral to the uterus. The fetal heartbeat becomes detectable at 6 weeks of gestational age and is described initially as a “flickering” structure. The placenta is visible by 10 weeks gestational age with a transabdominal ultrasound. It appears as a uniformly echogenic structure with rounded margins, usually located along the anterior or posterior uterine wall, where it is seen as a thickened echogenic rim of tissue surrounding the gestational sac.[4]

Indications

If there are no concerning symptoms in pregnancy, a routine ultrasound should be performed at 10 to 13 weeks gestational age to establish accurate gestational age, determine viability, and determine the number of fetuses [5]. However, in the presence of concerning symptoms such as vaginal bleeding, abdominal or pelvic pain in a female with a positive pregnancy test, an emergent pelvic ultrasound must be performed to rule out an ectopic pregnancy.

In the setting where a female of reproductive age presents with minor or major trauma, pregnancy is automatically assumed until it is ruled out with serum or urine pregnancy tests. [6] During the FAST (Focused Assessment with Sonography in Trauma) exam, a transabdominal ultrasound is performed to assess for free fluid in the abdomen and to investigate the potential presence of intrauterine pregnancy (IUP). If IUP is identified, further assessment should include fetal heart rate, the amount of amniotic fluid, fetal movement, and the placenta. In cases of traumatic injury in females with confirmed or estimated pregnancy greater than 20 weeks gestation, OBGYN expert consultation should be sought as soon as possible, and a transabdominal ultrasound must be performed to evaluate fetal well-being as soon as the mother is stabilized. 

Contraindications

While the only absolute contraindication to transabdominal and transvaginal ultrasound is patient refusal, the use of transvaginal ultrasound may be relatively contraindicated late into pregnancy term or in the high-risk patient population.[7] Special consideration should be made in the acute setting with a pregnant patient with vaginal bleeding and an unknown past medical history. In this case, a transabdominal ultrasound should be performed first to evaluate the placenta, followed by transvaginal ultrasound and speculum examination if it is deemed safe. However, transvaginal ultrasound is relatively safe in the setting of placenta previa as the angle between the cervix and the vaginal probe is adequate to prevent the probe from slipping into the cervix and disrupting the placenta.[8] 

Equipment

The equipment needed to perform a 2-D prenatal ultrasound includes two ultrasound probes. A High-frequency transvaginal probe (7.5-10 MHz) is used for a transvaginal portion of the examination, and the curvilinear probe (1-6 MHz) is used in the transabdominal approach.[9]

Technique or Treatment

The overall technique for transabdominal and transvaginal ultrasound is the same for all pregnancies, but it differs in the setting of the initial evaluation of pregnancy with trauma. 

The transabdominal view in pregnancy is obtained using the curvilinear probe with the patient in a supine position. The probe is placed with the indicator to the head of the patient above the pubis symphysis in the midline. An adequate view of the uterus includes having the length of the uterus in addition to the cervix and part of the vagina visualized.[10] It is important to distinguish the endometrial stripe in the transverse view as a dot or oval and the vaginal as a stripe going across the screen. For transabdominal assessment, it is best to obtain imagining in both longitudinal and transverse views.

During a transvaginal examination, the tactile indicator that is located on the handle of the probe is pointed up towards the ceiling, and the probe is inserted approximately 4 to 5 cm in the sagittal plane. The first and critical step is the determination of landmarks in which the bladder is identified, and subsequently, the juxtaposition between the bladder and the uterus is recognized, which makes it less likely that the exam is being performed in the adnexa.[10] Next, the ovaries and Fallopian tubes must be investigated when suspicion is high for ectopic pregnancy; the most common location for an ectopic pregnancy being the ampulla of the fallopian tube.[11]

In the setting of trauma, the patient should undergo an assessment with an ultrasound of the abdomen with a FAST exam to evaluate for free fluid in the peritoneal cavity and pericardium. The FAST exam is performed with a curvilinear probe and evaluates the right upper quadrant (Morrison pouch), left upper quadrant, subxiphoid, and suprapubic with additional attention spent evaluating the uterus for intrauterine pregnancy (IUP) while in this position. 

Complications

The overall risk of complications with ultrasound is low; however, a transvaginal ultrasound may cause complications such as anxiety, pain, or vaginal bleeding.

Clinical Significance

Ultrasound is the most commonly used diagnostic procedure in obstetrics. The use of ultrasound is a convenient, painless procedure that yields immediate results and is considered widely safe.[12] Ultrasound has uses in both routine care and in the acute or emergent setting. The goal of ultrasound during routine prenatal care is to establish viability, determine gestational age, and to assess the number of fetuses.[13]

Fetal viability is confirmed by the presence of an embryo that has cardiac activity. Cardiac activity is often present when the embryo itself measures 2 mm or greater during the 6th week of gestation.[5] If cardiac activity is not evident, other sonographic features of early pregnancy can predict viability. Since the first sonographic evidence of pregnancy is the gestational sac (which can be visualized at 4.5 to 5 weeks), its growth and progression are used to assess viability in early pregnancy. The gestational sac should increase by 1.13 mm a day in normal pregnancy.[2][14] To obtain gestational sac diameter with ultrasound, the caliper function is used and then placed from the inner to the outer wall of the gestational sac. The mean sac diameter (MSD) is calculated by added the longitudinal, anteroposterior, and transverse dimensions of the chorionic space and then taking the sum of these 3 measurements and dividing the sum by 3 to obtain the MSD.[15] Tracking the progression of the MSD through early pregnancy helps predict potential pregnancy failure. An MSD of 25 mm with no embryo indicates pregnancy failure, while MSD between 16 and 25 mm without an embryo is suggestive of a possible pregnancy failure.[16][17] Another predictor of non-viability is the absence of cardiac activity in an embryo with a crown-rump length (CRL) greater than 7 mm. CRL is measured by taking the average of 3 measurements of the longest fetal length.[18] CRL is taken by placing the calipers on the measurement setting and measuring the maximum straight-line length of the embryo in the longitudinal axis. It is important to ensure that when measuring CRL that the embryo isn't too flexed or too extended.[15] 

The yolk sac size and shape can be used to predict viability. The yolk sac can be evaluated as early as 5 weeks. Abnormal yolk sac shape (non-round or oval) predicts poor pregnancy outcomes with a sensitivity of 29% and a specificity of 95%.[19] Other characteristics of the yolk sac associated with poor outcomes include the size (smaller than 2 mm or larger than 5 mm in diameter) and the presence of a hyperechoic or hypoechoic rim, hyperechoic yolk sac center, or a number of yolk sacs unequal to the number of embryos. Abnormal yolk sac size and shape are associated with statistically significant higher abortion rates.[20]

Subchorionic hemorrhage is another condition associated with poor pregnancy outcomes that can be assessed with ultrasound. The overall spontaneous abortion rate is doubled in pregnancy cases in which the separation between the uterine wall and the chorionic membrane is considered large compared to small or moderate. A large separation is found to be associated with an approximate 3 fold increase in spontaneous abortion rates.[21] In addition to the size of the hemorrhage, the identification of subchorionic hemorrhage earlier in pregnancy is associated with higher rates of subsequent pregnancy failure.[22]

Fetal bradycardia during the first trimester is another important predictor for potential fetal demise. Fetal bradycardia in the early first trimester below 80 bpm is an ominous sign, and physicians should prepare their patients for likely poor outcomes such as spontaneous abortion and potential intrauterine fetal demise. Follow-up scans are done weekly until the heart rate is no longer present or until it becomes normal.[23] 

It is recommended that all pregnant women undergo a routine ultrasound at 10 to 13 weeks of gestation to determine an accurate gestational age. Getting an accurate gestational age is highly important and pertinent for the optimal assessment of fetal growth later in pregnancy. Ultrasound is the most reliable method for establishing a true gestational age by measurement of crown-rump length, which can be measured either transabdominally or transvaginally. CRL allows for an accurate determination of the day of conception +/- 5 days in 95% of pregnancies. Between 11 and 13.6 weeks gestation, it is recommended that CRL be used to determine gestational age unless it is >84 mm, then head circumference (HC) should be used as it is slightly more precise when compared to biparietal diameter (BPD). Head circumference is measured around the outer table of the fetal skull. The technique of BPD measurement depends on the nomogram used. If the nomogram uses outer-to-outer measurements of the fetal skull, then the same should be done during a fetal ultrasound. However, the outer edge of the skull to the inner edge of the skull nomograms are available as well. In order to obtain reliable information, the largest symmetric axial view of the fetal head must be used to visualize the choroid plexus, the third ventricle, or thalamus.[5]

Towards the end of the first trimester, a routine scan offers the opportunity to detect any visible gross fetal abnormalities. Measuring nuchal translucency in the first trimester has been associated with having both high sensitivity and specificity for identifying pregnancies that are at risk for chromosomal anomalies, the cut-off being 3 mm or greater.[24] It is important to note that not all fetal abnormalities manifest in the first trimester; thus, it is important to undergo standard transabdominal ultrasound during the second trimester.[25] 

Another important goal of the routine obstetric ultrasound at 10 to 13 weeks of gestation is the determination of the number of fetuses as well as amnionicity and chorionicity in twin gestations. Amnionicity is determined by the absence or presence of the intertwin membrane (ITM). If no ITM is identified, pregnancy is described as "mono-amniotic and mono-chorionic," and it implies the highest potential rate of complications (cord entanglement and fetal death).[26] Chorionicity describes different types of placentation and also predicts risks in twin pregnancies. If ITM is identified, monochorionic or dichorionic placentation is investigated next. Monochorionic placentation has a single placenta, while dichorionic placentation describes two separate placentas in twin pregnancies. In order to determine chorionicity, chorionic membrane base and thickness are evaluated. The presence of the lambda sign (a thick, Y-shaped base of the chorion) is used to predict chorionicity with greater than 95% accuracy, but only if it's used before 14 weeks of gestation. If the lambda sign is identified, a dichorionic pregnancy is diagnosed.[27]

In the second trimester, the goal of routine ultrasound examination, typically performed between 18 and 22 weeks, is to assess the fetal anatomy and further evaluate the placenta.[28] The reported advantages of routine ultrasound include early detection of many major anomalies, earlier determination of genetic diagnosis, and easier pregnancy termination if appropriate.[5] Ultrasound evaluation of the placenta can confirm that the placenta has implanted in the correct anatomical position and help recognize variations of placenta previa or placenta accreta. During the second trimester, the placental volume can be measured and calculated using volume organ computer-aided analysis. This measurement of placental volume aids in predicting neonatal birth weights. Women who have lower mean second-trimester placental volume have higher rates of neonates that were born small for gestational age (SGA) compared to women with normal placental volumes in the appropriate gestational age (AGA) group. Placental volumes for the AGA group typically measured with ultrasound to be 213.5 ± 75.8 versus the SGA group measured at 170.6 ± 49.8.[29]

As opposed to asymptomatic pregnant patients undergoing routine obstetric ultrasound imaging, pregnant patients with concerning symptoms must be evaluated on an emergent basis. In cases in which a stable patient presents for an evaluation of pelvic pain, abdominal pain, or vaginal bleeding early in suspected pregnancy, it is important first to establish a diagnosis of an IUP. If pelvic ultrasound reveals an IUP, it can be assumed that the cause of symptoms is unlikely to be secondary to an ectopic pregnancy.[30] However, it is important to consider heterotopic pregnancy, which is defined as the simultaneous presence of intrauterine and ectopic pregnancies; this condition is more common in women undergoing in-vitro fertilization (IVF) or women that have a history of tubal disease. The incidence of ectopic pregnancy is 0.006 to 0.001% in spontaneous cases, while in assisted reproductive techniques, such as IVF, it is 1 to 3%.[31] Once ectopic pregnancy is ruled out, other causes of vaginal bleeding or pelvic pain may be considered, such as spontaneous abortion, threatened abortion, gestational trophoblastic neoplasm, trauma, or most commonly normal pregnancy. Management of a stable patient with minimal bleeding in early pregnancy includes outpatient follow-up within 1 to 2 days and strict return precautions. It must be noted that for pelvic pain or vaginal bleeding complaints that occur later in pregnancy (>23 weeks), a speculum or digital vaginal exam should be held until placenta previa is ruled out by ultrasound.[6] Placenta previa accounts for 13 percent of maternal deaths secondary to hemorrhage in the United States in a recent report. It is associated with other complications, including the need for blood transfusion, organ injury, need for hysterectomy, venous thromboembolism, and increased infection risk, making identification of placenta previa an important part of obstetric ultrasound.[32] 

Trauma in pregnancy is the leading cause of non-obstetric maternal mortality. Non-lethal injuries during pregnancy most commonly occur from motor vehicle accidents and domestic/intimate partner violence. Although most injuries during pregnancy are classified as minor, fetal losses that are associated with minor trauma range from 60 to 70%. Importantly, any female of reproductive age with minor or major injury should be assumed pregnant until proven otherwise. The immediate focus is always on stabilizing the patient. Once successfully stabilized, all female patients of reproductive age should undergo a definitive pregnancy test or an ultrasound scan. In the setting of trauma, the patient may not be conscious or able to report their pregnancy status or gestational age; thus, estimating gestational age with bedside ultrasound can be a useful tool. Identifying pregnancy and estimating gestational age can help anticipate the need for prompt delivery. Ultrasound can assist in the evaluation of fetal cardiac rate and rhythm, amniotic fluid volume, placental localization, exclusion of previa, delineating possible fetal injury, and confirming fetal demise. If a patient is estimated to be >20 weeks gestation, fetal monitoring should be initiated as soon as possible in a stable patient. In the setting of trauma in pregnancy, a critical concern is placental abruption, a condition that occurs most commonly in trauma in which the patient experiences intense and severe uterine pain, in addition to uterine contractions and vaginal bleeding. It is important to note that the use of ultrasound for detecting placental abruption has a poor sensitivity; however, it is very specific if identified; resulting in a positive predictive value of 88% if seen and a negative predictive value of 53% if not identified. Thus fetal prolonged monitoring is advised in pregnant patients with trauma. A follow-up obstetric ultrasound and examination should be done 2 weeks after any hospital admission for either maternal or fetal factors (regardless of the gestational age) to document and assess normalcy after the traumatic event.[6][33]

Enhancing Healthcare Team Outcomes

The care of a pregnant patient requires coordination between an interprofessional team comprised of physicians, nurses, pharmacists, and other healthcare professionals. Not every complaint in pregnancy is an obstetric emergency; however, it is important to recognize and promptly diagnose the conditions that have been discussed above (ectopic pregnancy, placental abruption, etc.). The use of rapid bedside ultrasound by emergency department providers improves patient outcomes and allows for rapid assessment of both mother and baby, which in turn aids in the ability to communicate more rapidly with the obstetrical specialists in the setting of surgical obstetric emergencies.  Ultrasound reduces maternal and neonatal mortality in primary care facilities and should be included in the assessment by all providers routinely caring for pregnant patients.[33]

Nursing, Allied Health, and Interprofessional Team Interventions

When a stable pregnant patient presents for an evaluation with complaints of pelvic pain, vaginal bleeding, or trauma, the initial action is to get the patient on a monitor and obtain a full set of vital signs. Vital signs must be monitored closely in the pregnant patient to ensure shock is promptly recognized. In addition to cardiac monitoring and continuous pulse oximetry, 2 large-bore intravenous (IV) lines must be established, coordinating care between the physician and nursing staff of paramount importance.

In cases when an unstable female patient presents for an evaluation in an acute setting (e.g., the emergency department), staff must work together to ensure that portable bedside ultrasound is available and ready for use by trauma or emergency medicine providers (preferably before the unstable patient arrives). While the primary assessment is being performed, nursing staff should obtain a full set of vital signs and establish at least 2 large-bore IVs. In cases of known pregnancy or if bedside ultrasound reveals pregnancy, emergent consultation with obstetrical specialists must be requested. Nursing should be prepared and plan for a prompt sign-out to obstetrical services in the setting of a pregnant patient.[33]