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Diagnostic Medical Sonography ProgramPRACTICUM 2OBSTETRICS SKILLS LAB DIDACTIC COMPONENTNAGELE’s RULETo determine the due date (EDC) from the last menstrual period (LMP) ECD= LMP – 3 months + 7 daysFirst-Trimester measurements 0 to 13 weeksMEAN SAC DIAMETER (MSD)Established gestational age prior to visualization of an embryonic disc.Measure the length, height, and width of the inner to outer borders of the gestational sacMSD (mm) = Length (mm) + Height (mm) + Width (mm) 3Mean sac size can be used to date an early pregnancy. A MEAN DIAMETER is calculated form three planar sectionsMSD (mm) + 30 = gestational age (days)A (MSD – CRL) < 5mm is associated with a high risk of SAB.Crown-rum length (CRL)measured until the 12th gestational weekmost accurate method of dating a pregnancySagittal measurement of the embryo or fetus from the top of the head to the bottom of the rump.Lower extremities are not included in the measurement.The embryo should always be seen sonographically when the gestational sac measures 16mm TV or 25 mm TA.The CRL is the most accurate method of dating a pregnancy sonographically, and is accurate within 3-5 days if measured properly.The correct measurement is from the top of the head to the bottom of the rump, excluding legs. The embryo grows at a rate of 1mm per day.ROT: CRL (mm) + 42 = GA (days)Nuchal translucencyThe gestation should be 11 weeks 0 days to 13 weeks 6 days and the crown-rump length (CRL) should be a minimum of 45mm and a maximum of 84mm.Mid-Sagittal section of the fetus should be in a neutral position.Only the fetal head and upper thorax should be included in the image. Maximum thickness of the subcutaneous translucency between the skin and soft tissue Overlying the cervical spine is measured.Calipers are placed on the hyperechoic lines, NOT in the nuchal fluid.More than one measurement must be taken and the maximum one is to be recorded.Nuchal translucency exceeding 3mm is abnormal.A nuchal translucency scan measures the fluid filled area in the back of the neck between 11.5 to 13.5 weeks (CRL 45-84mm).The measurement can be used to calculate the risk that the fetus has Trisomy 21 or Trisomy 18It is determined to be normal or abnormal based on the CRL of the fetus. Some centers consider the NT of > 3mm to be abnormal.Second-Trimester measurements 14 to 27 weeksBiparietal Diameter (BPD)Two-dimensional measurementAccurate predictor of gestational age before 20 weeksMeasured in a plane that passes through the third ventricle and thalamiTransverse axial plane is most common and includes the following landmarksFalx CerebriThird VentricleThalamic NucleiCavum septi PellucidiAtrium of each lateral ventricleMeasure perpendicular to the falx placing calipers from the outer margin of the upper Cranium to the inner margin of the lower cranium.Measurement of the Biparietal diameter can be obtained from the head circumference plane.Axial measurement performed at the level of he thalami and Cavum septum PellucidumCalipers are placed “outer-to-outer” (leading edge to leading edge)First measurable between 10-12 weeksFor the exact BPD, the angle of insonation should be 90 degrees.Head Circumference Three-dimensional measurementReliable measurement independent of cranial shapeMeasure in plane that must include the Cavum septum Pellucidi and the tentorial hiatus.Measured parallel to the base of the skull, placing he calipers on the outer margins of the cranium.Measurement of the head circumference cannot always be obtained from the BPD plane.Measured at the same level as BPDCircumference should NOT include scalp echoes When Occipital-frontal (OFD) measurements are used in conjunction with BPD, OFD should be measured outer edge to outer edge.More accurate than BPD when the feat head is dolichocephalic or brachycephalicCephalic IndexThree-dimensional measurementDevised to determine the normality of the fetal head shapeNormal cephalic index is approximately 78%Abnormal when less than 74% or greater than 83%Defines had shape, and is determined by dividing BPD by OFD (occipital-frontal Diameter)Normal range is 0.70 – 0.86 (70% to 86%)In cases of Dolichocephaly (CI < 70% or Brachycephaly (CI > 86%) the BPD is eliminated from estimation of gestational age.Abdominal CircumferenceThree-dimensional measurementPredictor of fetal growth, not gestational ageMost difficult measurement to obtainCross-Sectional measurement slightly superior to the cord insertion at the junction of the left and right portal veins (hockey stick sign) or demonstrates a short length of the umbilical vein and the left portal veinPlace calipers on the outer margins of the skin edge.Measured at a level to include the fetal liverPreformed at the level of the stomach and portal sinus of umbilical veinReflects fetal growth through fetal liver sizeMeasurement is taken along skin line to include soft tissue and subcutaneous fatLeast reliable measurement in establishing gestational age due to significant genetic and physiologic variations in size after 25 weeks.Femur lengthOne-dimensional measurementLong bone of choice due to ease of measurementNormal femur demonstrates a straight border and a curved medial borderMeasure parallel to the femoral shaft placing calipers at the level of the femoral head cartilage and the distal femoral condyle.Best obtained with linear array transducerMeasure anterior (upside) femur to reduce artifactual bowingIncludes only ossified diaphysis, excludes epiphyseal cartilage.If femur length falls more than 2 SD below the mean, skeletal dysplasia may be present, and other long bones should be measured.Third-Trimester measurements 28-42 weeksBiparietal Diameter BPDHead Circumference HCAbdominal Circumference ACFemur Length FLAmniotic Fluid Volume (Index)Uterus is divided into four quadrantsVertical (AP) measurement of fluid in each quadrant is obtained.Quadrants are added to obtain the AFIA progressive increase in AFI is noted until approximately 28 weeks, after which time the AFI slowly decreasesNORMAL AFI rage is between 5 – 22 cmTranscerebellar distanceMeasured from lateral aspects of the cerebellum in the axial plane. May be useful in assessing Gestational age in IUGR, or when other head measurements are impossible to obtain.Head-Circumference to abdominal-Circumference ratio (HC/AC)During the early third trimester, the head circumference is slightly larger than the circumference of the abdomenDuring the late third trimester, with the increase of fetal body fat, the abdominal circumferenceIs typically equal to or slightly larger than the head circumference.OTHER PARAMETERSLong bone length such as radius, ulna, tibia, fibulaHumeral lengthFoot lengthBinocular distance: Measured from lateral orbital rim to lateral orbital rim. May be useful in assessing gestational age in IUGR or when obtaining other measurements is impossible doe to fetal position or malformations.Estimated fetal weightMost commonly calculated using the Biparietal diameter, femur length, and abdominal Circumference.Overall accuracy falls within 18% of the fetal actual weight in 95% of the cases.Is an inaccurate predictor of absolute weight, as is less accurate the greater the GA as well as in term and post-term fetuses.Changes in weight from previous sonograms may be used as an indicator of fetal growth.Significant variation in normal fetal weight exists, especially in the 3rd trimester.SUGGESTED HOMEWORK:Show an Ultrasound image of the MSDShow an Ultrasound image of the CRLShow an image of the Nuchal TranslucencyShow an image of the BPD Marking key landmarksShow an image of the HCShow an image of the cephalic index: one with CI normal, one with CI too low (<74% and one with CI too high (>83%Show an ultrasound image of the AC marking key landmarksShow an ultrasound image of a FL measurementShow an ultrasound image of the Amniotic Fluid IndexShow an ultrasound image of the Transcerebellar distanceShow an ultrasound image of the HC/AC ratioShow an ultrasound image of the estimation of fetal weight.Show images and define dolichocephalic and brachycephalic heads.Show an ultrasound image of binocular distance.ImagesGestational sac (MSD)Crown rump lengthNuchal TranslucencyBPDHCCephalic IndexACFLAFITranscerebellar distanceHC/ACHumeral lengthFoot lengthBinocular distanceIMAGES TO INCULEDRequires a definition**Falx Cerebri **Third ventricle **Thalamic Nuclei **Cavum septi pellucidi **Atrium of lateral ventricle **ACIThe hockey stick sign **Fetal stomachFetal liverFemur ossified diaphysisFemoral epiphyseal cartilageCephalic IndexIn certain circumstances (ruptured membranes, breech presentations, multiple gestations) shape changes in the fetal head may lead to even greater errors than those encountered when using “normal fetal anatomy measurements. If one suspects that the shape of the calvarium is other than typical, one should measure the cephalic index of the head to assess head shape. The cephalic index is calculated from the BPD and the fronto-occipital diameter (FOD) measured from the outer edge of the calvarium to the outer edge of the calvariaCephalic index = BPD x 100 FOuDTechnically, manufacturers usually compute a CI from the HC ellipse long and short axes. ................
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