RDMS OB/GYN cheat sheet

Transvaginal milestones: gestational sac visible at mean sac diameter 2–3 mm (about 5.0 weeks), secondary yolk sac at 5.5 weeks, embryo with cardiac activity at 6.0 weeks (CRL 1–2 mm).

Mean sac diameter = (length + width + height) / 3; the sac grows approximately 1 mm per day in early pregnancy.

Discriminatory limits: an embryo should be seen when MSD is ≥25 mm transvaginally (≥35 mm transabdominally), and cardiac activity should be seen when CRL is ≥7 mm.

Dating: crown-rump length is the most accurate first-trimester parameter; for CRL ≤84 mm, gestational age in days ≈ CRL in mm + 42.

Yolk sac diameter of 2–6 mm is normal; a diameter ≥7 mm is associated with increased risk of pregnancy loss.

Nuchal translucency is measured from 11 weeks 0 days to 13 weeks 6 days with CRL 45–84 mm; ≥3.0 mm or >95th percentile is abnormal and prompts aneuploidy assessment.

Anembryonic pregnancy = MSD ≥25 mm with no embryo; embryonic demise = CRL ≥7 mm with no cardiac activity.

Ectopic clues: empty uterus with an adnexal ring or tubal mass and free fluid; a pseudogestational sac is centrally located, irregular, and lacks the double decidual sign.

Interstitial ectopic: sac lies eccentrically with <5 mm of myometrium between sac and serosa; the interstitial line sign helps separate it from an angular pregnancy.

Biparietal diameter is measured outer-to-inner at the thalamic level; head circumference is traced around the skull in the same axial plane.

Cephalic index = BPD / OFD × 100; normal is 70–85, values <70 indicate dolichocephaly and >85 indicate brachycephaly.

Abdominal circumference is obtained in a bone-free axial plane at the stomach bubble and the junction of the umbilical vein with the left portal sinus.

Lateral ventricle atria measure <10 mm; cisterna magna measures 2–10 mm; the cavum septi pellucidi is present by 18 weeks.

Nuchal fold thickness ≥6 mm at 15–22 weeks is associated with trisomy 21 and other aneuploidies.

Four-chamber view: the heart occupies about one-third of thoracic area with the apex directed 45° ± 20° to the left.

Renal pelvis anteroposterior diameter <4 mm before 20 weeks and <7 mm after 20 weeks is normal; larger values indicate hydronephrosis.

The anatomy survey is optimally performed at 18–22 weeks; long bones are measured along the diaphysis, excluding the epiphyses.

Amniotic fluid index (sum of deepest vertical pocket in four quadrants): <5 cm oligohydramnios, 5–24 cm normal, ≥24 cm polyhydramnios.

Single deepest pocket: ≤2 cm defines oligohydramnios and ≥8 cm defines polyhydramnios; it is the preferred method in multiple gestations.

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

Umbilical artery impedance rises with placental insufficiency; absent or reversed end-diastolic flow indicates severe fetal compromise.

Middle cerebral artery peak systolic velocity >1.5 multiples of the median suggests fetal anemia from alloimmunization or parvovirus.

Cerebroplacental ratio = MCA pulsatility index ÷ umbilical artery pulsatility index; <1.0 suggests brain sparing in growth restriction.

Monochorionic diamniotic twins show a thin dividing membrane with a T sign at the placenta; dichorionic twins show a lambda or twin peak sign.

Transvaginal cervical length ≤25 mm before 24 weeks predicts preterm birth; measure with an empty bladder for at least three minutes.

Endometrial thickness: premenopausal trilaminar endometrium up to about 12–15 mm; postmenopausal ≤4–5 mm, and >4 mm with bleeding requires tissue sampling.

Cyclic endometrial appearance: menstrual thin and echogenic, proliferative trilaminar, secretory thick and homogeneous.

Leiomyoma: well-circumscribed hypoechoic myometrial mass with edge shadowing, internal calcification, or cystic degeneration.

Adenomyosis: asymmetric myometrial thickening, small myometrial cysts, echogenic islands, and fan-shaped or venetian-blind shadowing.

Simple ovarian cyst: anechoic, thin smooth wall, posterior acoustic enhancement, and no internal vascularity.

Endometrioma shows homogeneous ground-glass low-level echoes; a hemorrhagic cyst shows a lacy reticular fibrin network without internal flow.

Mature teratoma: echogenic mass with acoustic shadowing, Rokitansky nodule, dermoid mesh, or floating echogenic dots.

Ovarian torsion: enlarged edematous ovary with peripherally displaced follicles and reduced venous flow; preserved arterial flow does not exclude torsion.

Hydrosalpinx: tubular fluid-filled adnexal structure with a folded wall, incomplete septa, and the cogwheel sign.

ALARA principle: lowest output for the shortest time; for obstetric scanning keep mechanical index ≤1.9 and ISPTA.3 ≤720 mW/cm².

Thermal index soft tissue (TIS) guides first-trimester exposure; thermal index bone (TIb) at the focal zone applies in later pregnancy when bone lies in the beam path.

Higher transducer frequency improves axial resolution but reduces penetration; lower frequency is required for obese patients and deep structures.

Aliasing occurs above the Nyquist limit (PRF/2); correct it by increasing PRF/scale, lowering frequency, or adjusting the Doppler angle.

Shadowing from high-attenuation structures and posterior enhancement behind fluid are common and often diagnostically useful artifacts.

Reverberation creates equally spaced false echoes at increasing depth; comet-tail and ring-down artifacts arise from strong reflectors.

Amniocentesis: continuous real-time needle-tip visualization, avoid placenta and cord when possible, 22-gauge needle, procedure-related loss about 0.5–1% above background.

Saline infusion sonohysterography is performed on cycle days 5–10 with a thin endometrium, after excluding pregnancy and pelvic infection.

Chorionic villus sampling is performed at 10–13 weeks by transabdominal or transcervical route, giving earlier results with a slightly higher loss rate than amniocentesis.