RDMS OB/GYN flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

MSD discriminatory threshold, transvaginal

An embryo should be visible once the mean sac diameter reaches 25 mm on a transvaginal scan; a sac this large with no embryo supports the diagnosis of anembryonic pregnancy.

Cardiac activity at CRL 7 mm

Cardiac activity should be documented when crown-rump length reaches 7 mm by transvaginal scanning; its absence at this size indicates embryonic demise.

Crown-rump length dating formula

For a crown-rump length up to 84 mm, gestational age in days equals the CRL in millimeters plus 42, and CRL is the most accurate first-trimester dating parameter.

Yolk sac diameter limits

A secondary yolk sac normally measures 2 to 6 mm internally; a diameter of 7 mm or more is associated with an increased risk of pregnancy loss.

Nuchal translucency timing window

Nuchal translucency is measured between 11 weeks 0 days and 13 weeks 6 days, when crown-rump length is 45 to 84 mm, on a mid-sagittal fetal profile.

Double decidual sac sign

Two concentric echogenic rings surrounding an intraendometrial fluid collection indicate a true gestational sac and help distinguish it from a pseudogestational sac of ectopic pregnancy.

Interstitial ectopic myometrial mantle

An interstitial ectopic sac sits eccentrically with less than 5 mm of myometrium between the sac and the serosa; the interstitial line sign helps confirm the location.

Cephalic index formula

Cephalic index equals biparietal diameter divided by occipitofrontal diameter times 100; normal is 70 to 85, with lower values indicating dolichocephaly and higher values brachycephaly.

Cisterna magna normal range

In the transverse cerebellar plane the cisterna magna measures 2 to 10 mm; a measurement over 10 mm warrants evaluation for the Dandy-Walker continuum or aneuploidy.

Fetal lateral ventricle threshold

The atrium of the lateral ventricle is measured on an axial plane; less than 10 mm is normal, and 10 mm or greater defines fetal ventriculomegaly.

Nuchal fold abnormal thickness

In the second trimester, nuchal fold thickness of 6 mm or more, measured at the level of the cisterna magna, is a marker for aneuploidy, particularly trisomy 21.

Abdominal circumference measurement plane

Abdominal circumference is measured in an axial bone-free plane showing the stomach bubble and the junction of the umbilical vein with the left portal sinus.

Renal pelvis dilation thresholds

Anteroposterior renal pelvis diameter under 4 mm before 20 weeks and under 7 mm after 20 weeks is normal; larger measurements suggest fetal hydronephrosis.

Absent cavum septi pellucidi

The cavum septi pellucidi should be visible by 18 weeks; its absence raises concern for holoprosencephaly, agenesis of the corpus callosum, or other midline malformation.

Amniotic fluid index thresholds

The amniotic fluid index sums the deepest vertical pocket in four quadrants; below 5 cm is oligohydramnios, 5 to 24 cm is normal, and 24 cm or more is polyhydramnios.

Placenta previa definition

Placenta previa is placental tissue covering the internal cervical os; a low-lying placenta has its edge within 2 cm of the os without covering it.

Cerebroplacental ratio

The cerebroplacental ratio is middle cerebral artery pulsatility index divided by umbilical artery pulsatility index; a value below 1.0 suggests fetal brain sparing.

MCA peak systolic velocity

Middle cerebral artery peak systolic velocity above 1.5 multiples of the median suggests fetal anemia and guides decisions about intrauterine transfusion in red cell alloimmunization.

Twin peak versus T sign

A lambda or twin peak of placental tissue between membranes indicates dichorionic twins, whereas a thin T sign where membranes meet the placenta indicates monochorionic diamniotic twins.

Short cervix threshold

A transvaginal cervical length of 25 mm or less before 24 weeks predicts preterm birth; measurement is taken with an empty bladder for at least three minutes.

Postmenopausal endometrial thickness cutoff

In postmenopausal bleeding, endometrial thickness of 4 mm or less is reassuring; greater thickness requires tissue sampling to exclude endometrial carcinoma or hyperplasia.

Endometrioma ultrasound appearance

Endometriomas appear as unilocular cysts with homogeneous low-level ground-glass echoes, no internal vascularity, and often reduced compressibility with the transvaginal probe.

Hemorrhagic cyst lacy pattern

Acute hemorrhage within an ovarian cyst produces a lacy reticular fibrin network without internal flow; over time strands retract and a fluid-debris level forms.

Ovarian torsion ultrasound signs

The torsed ovary is enlarged with peripherally displaced follicles and decreased venous flow; arterial flow may persist, so normal Doppler findings do not exclude torsion.

Adenomyosis ultrasound features

Findings include asymmetric myometrial thickening, small myometrial cysts, echogenic islands, and fan-shaped shadowing, with an ill-defined junctional zone.

PCOS follicle count criterion

With modern transvaginal transducers, 20 or more follicles in a single ovary, or an ovarian volume of 10 mL or more, supports polycystic ovarian morphology.

Thermal index in obstetrics

Thermal index estimates potential temperature rise; soft tissue index guides first-trimester scanning and bone index at the focal zone guides later scans, always following ALARA.

Aliasing in color Doppler

Aliasing appears when blood velocity exceeds the Nyquist limit of half the pulse repetition frequency, shown as color wraparound; increasing PRF or lowering frequency corrects it.

Posterior acoustic enhancement

Fluid attenuates sound less than surrounding tissue, so echoes deep to a cyst appear brighter; this artifact supports the diagnosis of a fluid-filled structure.

Reverberation artifact

Sound reflecting repeatedly between two strong reflectors creates equally spaced false echoes at increasing depth, often seen behind the bladder or within the anterior uterus.

Amniocentesis needle guidance

Continuous real-time imaging keeps the needle tip visible throughout, choosing a path that avoids the placenta and cord when possible; a 22-gauge needle is typical.

Sonohysterography timing

Saline infusion sonohysterography is scheduled in the early follicular phase, days 5 to 10, when the endometrium is thin, after pregnancy and infection are excluded.