|ECR 2019 / C-2647|
|Obstetric Ultrasound - A Practical Approach|
Findings and procedure details
The first trimester evaluation
The first trimester of gestation corresponds to the period between the confirmation of a viable gestation up to 13 weeks and 6 days. Ultrasonographic follow-up of this period has a predictable, accurate and reproducible pattern, with a variation of approximately half a week of expected (figure 1). The first trimester evaluation consists on the ultrasound until 10 weeks and 6 days and morphologic ultrasound from 11 weeks to 13 weeks and 6 days .
- First trimester ultrasound (until 10 weeks and 6 days)
Indications: to determine an accurate gestational age, to evaluate the implantation site, vitality, to diagnose multiple gestation and its classification, to evaluate intercurrences. The transvaginal ultrasound complementation on this period its important .
Uterus (figure 2)
Sagittal: demonstrating the fundus, body and cervix, also the longitudinal and anteroposterior diameter. Another picture showing the inferior uterine segment and cervix .
Axial: on the level of the uterine body accomplish the measure of the transverse diameter .
Gestational sac (figure 3)
A round or oval structure with smooth and well defined borders, it has a decidua that is 2 mm or greater, and should be positioned in the upper uterine.
Sagittal: longitudinal and anteroposterior diameter .
Axial: transverse diameter [2,3].
Calculate the average diameter of the gestational sac. It is an important parameter to estimate the gestational age (GA) while the embryo is no characterized [2,3].
Mid-sagittal: evaluate the crown-rump length (CRL) the concept must be in neutral position. It is the most accurate estimation of gestational age between 6-13 weeks. The yolk sac and extremities must be excluded from the picture. Make at least 3 measures and use the arithmetic mean to estimate the gestational age [2,3]. (figure 4)
B+M mode: assess the fetal heart rate (FHR). The cardiac activity should be present in an embryo with a CRL ≥7 mm. You may use the pulsed Doppler for few seconds to demonstrate the concept vitality and for the mother listens the FHR [2,3]. (figure 5)
Yolk sac evaluation (figure 6)
The yolk sac is usually round, annular and anechoic and is located between the amniotic and chorionic membrane [2,3].
Evaluate its features and measure the diameter.
It measures between 3-6mm. After 10 weeks, the yolk sac begins to shrink until it is no longer visualized beyond 14 weeks GA [2,3].
Amniotic membrane (figure 7)
Observe the amniotic membrane integrity [2,3].
Chorionicity and amnionicity  (figure 8)
The visualization of multiple yolk sacs is the earliest sign of a multigestational pregnancy. The number of yolk sacs matches the number of amniotic sacs if the embryos are alive.
The identification of two separate gestational sacs evidences the dichorionic and diamniotic gestation.
The "lambda" signal consists of the union of the two amnions and two chorions of each gestational sac, which determine the dichorionic gestation. When monochorionic and diamniotic gestation occurs, the presence of only two amniotic membranes can be observed in the placental insertion, making this union thinner usually with a thickness below 2 mm, evidencing the so-called "T" sign.
Possible difficulties and how to overcome them: during the CRL evaluation, in multigestational pregnancy, if there are differences between CRL measurements, the smallest fetal measure should be considered.
Sagittal: longitudinal and anteroposterior diameter.
Axial: transverse diameter.
Assess the corpus luteum gravidarum. Use the Doppler to facilitate the identification and make a functional study (figure 9).
Adnexal region and pouch of Douglas
Evaluate the presence of adnexal masses or liquid.
- First trimester ultrasound (11 weeks to 13 weeks and 6 days) – the first trimester morphologic ultrasound.
Indications: investigation of aneuploidy (nuchal translucency, nasal bone and venous duct) and fetal anomalies. Generally, the transabdominal evaluation is enough.
You must evaluate the uterus, gestational sac, amniotic membrane, ovaries and adnexal region as previously reported. The fetal evaluation at this point is more detailed [7,8].
Head: axial section – trans thalamic plane measure the biparietal diameter (BPD), occipitofrontal diameter (OFD) and cephalic circumference (CC); evaluate the central nervous system development (figures 10 – 11).
Nuchal translucency (NT)  (table 1 and figure 12)
One of the factors evaluated in the screening of fetal aneuploidies. For an accurate measurement, all the parameters described in the table 1 must be present.
The greater NT thickness the greater the risk of aneuploidies and fetal anomalies. The normal NT is below the p95 of the normality curve.
Possible difficulties and how to overcome them: In cases where the umbilical cord is around the cervical region of the fetus, it is recommended to measure the NT cranial and caudally of the region and calculate the average between them.
Face – nasal bone (figure 13)
Mid-sagittal imaging to evaluate the absence or hypoplasia of the nasal bone, which rises the aneuploidy risk.
Spine  (figure 14)
Sagittal, axial and coronal imaging to evaluate the integrity and form.
Chest [7,8] (figure 15-16)
Axial imaging on the four cardiac chambers level; measure the anteroposterior and transverse diameter; assess the rhythm and fetal heart rate; Doppler evaluation of the tricuspid valve; observe the diaphragmatic integrity.
Abdomen [7,8] (figure 17 – 18)
Axial imaging on the stomach level and portal sinus; measure the anteroposterior and transverse diameter and the abdominal circumference. Evaluate the stomach, small bowel, kidneys and bladder; assess the abdominal wall integrity.
Humerus and femur (figure 19)
Sagittal imaging showing the diaphysis and measure the length.
Extremities (figure 20)
Evaluate the integrity of superior and inferior limbs, bilaterally.
Placenta (Figure 21)
Locate and visualize the parenchyma.
Amniotic fluid (AF)
Subjective assessment of the AF.
Umbilical cord (Figure 22)
Evaluate the insertion on the anterior abdominal wall and the placenta; observe if there are 1 vein and 2 arteries on color Doppler next to the fetal bladder; observe the umbilical arteries flow and the final diastolic component. Doppler: evaluate the flow velocity pattern (positive final diastolic velocity), pulsatility index. The insonation can be peformed at any point of the umbilical cord 
Second and third trimester ultrasound 
The minimum standard of the ultrasound study from the 14th week until the end of gestation was described. The screening of abnormalities should be done whenever possible. We will highlight now the main changes of this period.
Head, face, spine, chest, heart, abdomen, limbs, extremities and umbilical cord: the basic assessment, with the necessary measurements and images was quoted earlier, the development of structures as well as their shape should always be evaluated. (figure 23 – 28).
Fetal gender: the assess may be by a mid-sagittal image of the lower abdomen below the cord insertion demonstrating penis and scrotum caudally to the cord insertion or the flat pubis caudal to the cord insertion. Transverse image, just below the level of the bladder, best taken with the knees separated, observe the penis presence (male) or absence (female) (Figure 29).
Placenta: it can be recognized from the tenth week and was initially identified as a thickening of the hyperechogenic halo that surrounds the gestational sac, can later be subdivided into chorionic plaque, parenchyma placenta and basal plaque. The placental maturation correlates with the biochemistry of fetal lung maturity, and is usually described during the evaluation (figure 30). The placenta evaluation consists: determine the localization, visualize the parenchyma and measure the thickness (Figure 31).
Amniotic fluid: there are three forms to evaluate the AF: the subjective evaluation, the deepest vertical pocket and amniotic fluid index (Figures 32 – 34)
Cervix: the cervical length is most accurately assessed with the transvaginal probe, the length, distance between the internal and external hole in the longitudinal axis at 24 weeks is about 35 mm. A short cervix is lesser than 25mm. Funneling cervix is defined as protrusion of the amniotic membranes into the cervical canal, is considered by some as an additional risk factor for preterm delivery. (figure 35)
The obstetric Doppler has an important role for obstetrician and it is being apply in many clinical issues. The pulsatility index is the most used relation and in some circumstances other velocities may be necessary. The obstetric Doppler can be performed since the first trimester morphologic ultrasound .
Technical principles 
The pulsed Doppler must be performed during no respiratory and body movements of the fetus, if necessary, you can ask the patient to hold a breath.
If the absolute velocity evaluation is the most important parameter such as fetal anemia, is necessary to correct the insonation angle.
Adjust the sample volume.
Adjust the frequency to optimize the penetration and resolution.
Adjust the wall filter of the vessel to lesser than 50-60 Hz.
Adjust the velocity of the horizontal scan: showing 4-10 waves.
Adjust the pulse repetition frequency. The flow velocity wave must be occupying 75% of the screen.
The Doppler must be reproducible: if the is any discrepancy between the measures, it will be necessary to re-measure.
Adjust the gain to reduce artifacts.
Uterine arteries  (Figure 36)
Isthmic uterine region, next the crossing of the external iliac artery.
Sample volume settled about 1 cm of the crossing with the external iliac arteries.
If the uterine artery bifurcates before this crossing, the sample volume must be settled before the bifurcation.
Do the same with the contralateral artery.
Umbilical arteries  (Figure 37)
Any umbilical cord segment can be analyzed. In multiple pregnancy, the evaluation must be next to the belly of each fetus.
Mid-cerebral artery  (Figure 38)
Axial imaging, caudally to the transtalmic imaging, identify the Willis polygon with color Doppler.
Prefer for the proximal middle cerebral artery; set up the sample volume on the proximal third of the middle cerebral artery and keep the insonation angle next to 0 degrees.
Be careful to not pressure the transducer and compress the cephalic pole.
Venous duct (VD)  (Figure 39)
The venous duct connects the intrahepatic portion of umbilical vein with the inferior vena cava, nexto to the right atrium. Be careful to not confuse venous duct with the inferior vena cava and hepatic veins.
Identify the VD on sagittal imaging of the fetal trunk or oblique axial.
Evaluate the initial portion of VD (next to umbilical vein) in the high speed area or aliasing area during the color Doppler.
First trimester main abnormal US findings
Considering the well-defined landmarks in the first trimester pregnancy, any deviation may be indicative of failure to gestation. The miscarriage is defined as spontaneous ending of pregnancy before 22 weeks’ gestations. The other diagnoses of this period are suspicious or gestational failure and pregnancy of unknown location, which is the term given to the transient state of early pregnancy during which no definite intrauterine pregnancy (IUP), at this stage, the three main possibilities include early IUP, occult ectopic pregnancy, and completed spontaneous abortion. It should be noted that the evaluation of the individual components of gestation, such as gestational sac, yolk sac, amnion, embryo, cardiac activity and decidua, help to determine the prognosis of gestation . (figure 40-41)
Second and third trimester abnormal US findings
The intrauterine fetal death is defined when the fetus death occurs after the 20th week of pregnancy. The ultrasound findings are absent FHR, absent fetal movements, the Spalding sign which is the overlapping of skull bones, gross distortion of fetal anatomy (maceration), soft tissue edema (skin > 5mm) and an echogenic amniotic fluid.
The fetal distress can be chronic or acute, the latter occurs suddenly, usually during delivery due to lack of oxygenation, and chronic fetal distress occurs progressively during pregnancy. The obstetric Doppler has an important role in the diagnosis and to define the management of the fetal distress. The warning signs of obstetric Doppler for fetal distress are: in the umbilical artery - increased resistance, zero or reverse diastole; in the middle cerebral artery - evaluation of fetal centralization with a pulsatility index greater than or equal to 1; in the venous duct - presence of negative A wave, which shows imminent risk of fetal death .
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