By measuring the temporal distance in M mode from peak to peak, most modern ultrasound machines will be able to estimate the fetal heart rate. The M mode caliper should be placed over the left ventricle. The fetal heart should be measured using M mode as established by As Low As Reasonably Achievable (ALARA) recommendations. However, lack of fetal movement can have different significances and can be affected by the fetal sleep cycle. įrequently, fetal movement can be identified and should be documented. However, including additional measurements in the formulas has not demonstrated increased accuracy. Formulas that used less than three fetal body part measurements did not perform well compared to formulas that include fetal head, abdomen, and femur measurements. Multiple formulas have been developed to estimate fetal weight, with one of the most widely used being that of Hadlock and colleagues. On the other hand, estimation of fetal size is best performed during the third trimester. Commonly used dating measurements include: Measurements of fetal parts can be used in isolation or combined to estimate gestational age. Gestation dating by ultrasonography may be inaccurate during the third trimester. Several measurements are indicated for appropriate assessment of the status of the fetus and the maternal structures, starting with the estimation of the gestational age. Sagittal images are obtained by maintaining the indicator pointing towards the ceiling, while transverse imaging requires rotating the probe to have the indicator facing the patient's right side. The probe should be inserted with the indicator in the vertical direction. If a transvaginal examination is indicated, the patient should be placed in the lithotomy position. The probe indicator should be directed to the cephalad for the sagittal images and towards the patient's right side for transverse images. The probe can be placed in either sagittal or transverse orientation. The initial evaluation should commence by placing a curvilinear probe with the appropriately selected OB settings over the suprapubic area. The location of the placenta is variable, and it can be identified as a heterogeneous structure connected to the wall of the uterus and attached to the umbilical cord leading to the abdomen of the fetus. The amniotic fluid should be seen as an anechoic space surrounded by the isoechoic walls of the uterus. During the third trimester, the fetal anatomy and cardiac activity should be well defined and evident. In a cephalad to caudal direction, the fundus, body, and cervix of the uterus can be appreciated in that particular order. The uterus is typically found wedged between the more anterior bladder and the more posterior colon. However, due to the higher frequency transducer used, this can result in difficulty visualizing more caudal (maternal) and deeper structures inside and outside the uterus. The transvaginal approach generally offers better visualization of the anatomy due to the improved proximity to the organs and structures of interest. Further detailed studies about abnormal findings are described more extensively under their respective activities.ĭuring the third trimester, ultrasound can be performed transvaginally or trans-abdominally, with preference given to the latter approach due to the theoretical risk of precipitating clinically significant hemorrhage if placenta preview is present and undiagnosed. The purpose of the study serves as a guide for what is evaluated during the examination. Sonographic assessments in the third trimester are not routinely performed unless the patient has had no initial sonographic examination or prior pathological or concerning conditions are identified during the first or second trimesters. There is currently no universal, standardized protocol for assessing the progression and status of the pregnancy during this period. Traditionally, the third trimester is defined as the antenatal period between 28 and 42 weeks of gestation. Thus, the ultrasonographic applications in the third trimester of pregnancy differ from previous trimesters in both scope and objectives. The use of ultrasound in the third trimester of pregnancy serves a multitude of general and specialized purposes that include but are not limited to the determination of fetal number and presentation, assessment of growth disorders, and characterization of the placenta and amniotic fluid.
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