RDMS OB/GYN common mistakes
RDMS OB/GYN · Understand the traps and how to avoid them.
Dating a first-trimester pregnancy by mean sac diameter when a measurable crown-rump length is present, or continuing to use CRL for dating beyond the first-trimester threshold.
Candidates memorize a single dating rule and do not check which parameter is valid at the current gestational age; they also may measure MSD on an irregular sac.
What to do instead
Use CRL as the preferred dating parameter when it is measurable within its validated first-trimester range; use MSD only when no embryo is visible, and switch to BPD/HC when CRL exceeds the first-trimester dating range.
Diagnosing early pregnancy failure on a single scan by applying a size threshold without confirming absence of yolk sac/embryo or without a follow-up scan when findings are borderline.
Pressure to give a definitive answer and familiarity with a few numeric cutoffs lead candidates to skip the required confirmatory steps and strict criteria.
What to do instead
Apply strict sonographic criteria for early pregnancy failure, document yolk sac/embryo presence, and recommend follow-up imaging at the appropriate interval when criteria are not definitively met.
Measuring biparietal diameter and head circumference at a plane that is too high, too low, or angled through the orbits or skull base.
Candidates rely on a rough 'oval skull' appearance instead of verifying internal landmarks, especially when fetal position is difficult.
What to do instead
Confirm the correct axial plane with midline falx, cavum septi pellucidi, thalami, and continuous skull table; avoid angling through orbits or cerebellum and use outer-to-inner calipers for BPD.
Measuring femur length along a foreshortened axis or including the distal femoral epiphysis.
Candidates accept the first bright linear structure seen and do not rotate to the true longest axis, particularly with fetal movement or shadowing.
What to do instead
Measure the diaphysis end-to-end at its longest true axis, exclude the epiphysis, and confirm the measurement is reproducible in an orthogonal plane or with real-time adjustment.
Assigning chorionicity in a multiple gestation from membrane thickness alone, without using first-trimester membrane insertion or sac/yolk-sac counts.
Candidates overvalue the 'thick versus thin' membrane sign and do not integrate gestational age, membrane insertion, and number of sacs.
What to do instead
Determine chorionicity in the first trimester using the lambda/T sign, number of gestational sacs and yolk sacs, and membrane insertion; label each fetus and document the basis for the assignment.
Performing amniotic fluid assessment with nonstandard pockets or including umbilical cord or fetal parts in the measurement.
Candidates rush the measurement or use a single pocket that is convenient rather than representative, especially in obese patients or oligohydramnios.
What to do instead
Use a standardized technique: measure the deepest vertical pocket free of cord and fetal parts, obtain AFI by summing four standardized quadrants, and correlate with fetal bladder and renal findings.
Labeling an adnexal cyst as complex or suspicious without first applying simple-cyst criteria and confirming with orthogonal views and color Doppler.
Candidates overinterpret internal echoes, septations, or artifact and do not distinguish true morphologic features from gain, slice-thickness, or bowel artifact.
What to do instead
Confirm a simple cyst using thin-walled, anechoic, unilocular criteria; use orthogonal planes, appropriate gain, and color Doppler to exclude vascularity or solid components before assigning complexity.
Measuring endometrial thickness in the wrong cycle phase or including the hypoechoic halo in the measurement.
Candidates do not correlate with menstrual history and may place calipers at the outer myometrial border rather than the endometrial-myometrial junction.
What to do instead
Correlate with cycle phase and clinical context, measure the double-layer endometrium from anterior to posterior endometrial-myometrial junction, and avoid including the hypoechoic halo.
Interpreting acoustic shadowing, enhancement, or reverberation artifact as a real fetal or pelvic structure.
Candidates do not vary the transducer angle or focal zone and accept an artifactual echo as pathology, especially behind bone or gas.
What to do instead
Change the scanning angle and orthogonal plane, adjust focal zone and gain, and confirm that a suspected structure persists with consistent anatomy rather than artifact.
Omitting pre-procedure safety steps before an interventional OB/GYN sonogram, such as confirming coagulation status, consent, aseptic technique, and needle-guide alignment.
Candidates focus on the imaging target and treat the procedure as a scanning exercise rather than a supervised intervention with checklist requirements.
What to do instead
Use a structured pre-procedure checklist: verify indication and consent, review coagulation and medications, confirm aseptic preparation, and check needle-guide alignment and depth before needle insertion.