Volume 8 Issue 3 - Australasian Society for Ultrasound in Medicine

POLICIES and STATEMENTSGuidelines for the Performance ofFirst Trimester UltrasoundMay 1995, Revised October 1999, Revised October 2002, Revised July 2005Introduction and equipmentStudies should be performed using anabdominal and/or vaginal approach.A high frequency transducer shouldbe used and the equipment should beoperated with the lowest ultrasonicexposure settings capable of providingthe necessary diagnostic information.A vaginal transducer should alwaysbe available and a transvaginal scanshould be offered to the patient whenit is anticipated that this would resultin a more diagnostic study. The patientmay choose to accept or refuse this offerand undue persuasion is inappropriate.Reference should be made to theGuidelines for the Performance of aGynaecological Scan regarding thefacilities and preparation for such anexamination.ASUM policy on disinfectionof vaginal transducers should befollowed.HistoryEstimate gestation based on last menstrualperiod or time of conception.Document symptoms and, if possible,the result and date of any pregnancytest – Human Chorionic Gonado-trophin(HCG).Gestation sacThe gestation sac should usually bevisible from four and one half (4.5) tofive (5) weeks using high frequencytransvaginal ultrasound.When a gestation sac-like structureis seen but no live fetus demonstrated,it is important to attempt to ensurethat it is not a 'pseudo gestational sac'.Look for the echogenic trophoblastrim and the yolk sac, and ensure thatthe fluid in the gestation sac is echofree.If a gestation sac is not visiblein the uterus of a patient believed tobe pregnant, the adnexa should becarefully examined looking for evidencesuggesting the presence of anectopic pregnancy – most ectopicscan be suspected with high frequencytransvaginal ultrasound.In a patient with a positive pregnanttest but either:■ no gestational sac is seen withinthe uterus or elsewhere in the pelvis■ an apparent sac is seen but no fetalstructures (including yolk sac) orheart movements are visibleconsider the following:■ pregnancy not as advanced asthought (e.g. delayed ovulationand conception in that cycle)■ ectopic pregnancy■ failed pregnancy including completemiscarriageInterpretation of the scan may bemore accurate if the result of the quantitativeHCG levels is known.There is more than one acceptedlaboratory standard for reporting HCGlevels. The units for the local serviceshould be taken into account whencorrelating sac development and HCGlevel. In general when the level is >2000 IU/1 a gestation sac should beseen in the uterus on transvaginalscanning. If no sac is visible an ectopicmust be considered.If the level is < 1000 IU/1 thenfurther follow up by serial HCG isappropriate and/or a repeat scan if thediagnosis is uncertain.Gestational ageThis is most accurately assessed in thefirst trimester. The earlier the crownrump length (CRL) is measured, themore accurate is the assessment of gestationalage. The CRL can be measuredfrom six weeks gestation. The compositeCRL chart in the ASUM Policiesand Statements Folder is recommended.From eleven (11) weeks multiparameterassessment can be used. Biparietal diameter(BPD) is the most often used secondmeasurement.Fetal heart movementsWith a high resolution vaginal transducer,fetal heart movements are oftenvisible from five (5) to six (6) weeks(i.e. CRL = 2 mm), but may not beseen until CRL = 3–4 mm (See paragraphon pregnancy failure).Fetal numberThe diagnosis of a multiple pregnancyrequires the visualisation of multiplesacs prior to six (6) weeks and subsequentlyvisualisation of multipleembryos.The first trimester is the optimumtime to determine chorionicity of thefetuses. The chorionicity of the fetusesshould be stated in the report.The presence of separate sacs and thethickness of the intervening membraneand the shape of its junction with theplacenta should be assessed. Be awarethat early in the first trimester anintervening amnion may not be visiblein diamniotic, monochorionic twins.Later in the first trimester the numberof placentas can be evaluated.Pregnancy failureAn experienced operator using highquality transvaginal equipment maydiagnose pregnancy failure undereither or both of the following circumstances:1 When no live fetus is visible ina gestation sac and the mean sacdiameter is 2.0 cm or greater.2 When there is a visible fetus witha CRL of 6 mm or more but nofetal heart movements can be demonstrated.The area of the fetalheart should be observed for aprolonged period of at least thirty(30) seconds to ensure that there isno cardiac activity.In situations where pregnancy failureis suspected by an operator whoeither does not have extensive experiencein making the diagnosis or doesnot have access to high quality equipmentor if there is any doubt about theviability of the fetus, a second opinionor a review scan in one week should berecommended in the report 1 .ASUM Ultrasound Bulletin 2005 August; 8 (3)33