RDMS OB/GYN key terms

The earliest reliable sonographic sign of an intrauterine pregnancy: a small anechoic fluid collection surrounded by an echogenic trophoblastic rim, implanted within the central endometrium and visible transvaginally from about five weeks of menstrual age.

A thin-walled echogenic ring inside the chorionic cavity, connected to the embryo by the vitelline duct; it is the first structure seen within the gestational sac and confirms that the pregnancy is intrauterine rather than simulated by fluid.

The maximum length of the embryo or fetus from the top of the head to the bottom of the buttocks, measured in a neutral midsagittal view; between roughly 7 and 13 weeks it is the most accurate method of assigning gestational age.

Two concentric echogenic rings around an early gestational sac, formed by the decidua capsularis and decidua parietalis; their presence supports a true intrauterine pregnancy and helps distinguish it from a pseudogestational sac.

The serum hCG level above which a normal intrauterine gestational sac should be visible on transvaginal scanning; failure to see a sac at or above this level raises concern for an ectopic or failing pregnancy.

A fluid collection within the endometrial cavity of a patient with an ectopic pregnancy, produced by hemorrhage and decidual breakdown; it is typically central and irregular and lacks the thick echogenic rim of a true sac.

A pregnancy in which a gestational sac enlarges but no yolk sac or embryo ever develops. Diagnosis requires a mean sac diameter of at least 25 mm with no embryo, or failure of normal growth on follow-up scanning.

Bleeding between the chorion and the uterine wall, seen as an anechoic or hypoechoic crescent beside the gestational sac; it is a common cause of first-trimester bleeding and may be associated with pregnancy loss.

Implantation of the conceptus outside the endometrial cavity, most often within the fallopian tube; sonographic clues include an adnexal mass separate from the ovary, an empty uterus and occasionally free pelvic fluid.

The anechoic space between the fetal skin and the soft tissue over the cervical spine, measured at 11 to 14 weeks; increased thickness is associated with aneuploidy, cardiac defects and other structural anomalies.

A transverse measurement of the fetal skull across its widest portion, obtained at the level of the thalami and cavum septi pellucidi; it is used in the second and third trimesters to estimate age and head growth.

The circumference of the fetal skull measured in the same axial plane as the biparietal diameter; it is less affected by head shape than the biparietal diameter and helps detect microcephaly, hydrocephalus and growth disturbance.

The circumference of the fetal abdomen measured at the level of the stomach and portal sinus; it is the biometric parameter most sensitive to fetal nutrition and is central to diagnosing growth restriction and macrosomia.

The length of the ossified femoral diaphysis, measured from the greater trochanter to the distal condyle; it contributes to dating and estimated weight and is a marker for skeletal dysplasia or aneuploidy when abnormally short.

A calculated fetal weight derived from combined head, abdominal and femur measurements using a published formula; it is plotted on population nomograms to identify growth restriction, appropriate growth or macrosomia.

The cerebrospinal fluid space between the cerebellum and the inner skull table, measured in the transcerebellar plane; normal depth is roughly 2 to 10 mm, and enlargement suggests a Dandy-Walker spectrum malformation.

A measurement of the soft tissue behind the fetal occiput obtained in the second trimester on a transcerebellar plane; a value of 6 mm or more is considered thickened and increases the risk of Down syndrome.

A small bright focus within the fetal heart, usually in the left ventricle, caused by calcification of a papillary muscle; it is a soft marker for aneuploidy whose significance depends on the overall risk assessment.

Enlargement of a fetal lateral ventricle, defined as an atrial width greater than 10 mm measured at the posterior horn; mild cases may be isolated, while more severe ones often accompany structural or chromosomal abnormalities.

A small anechoic cyst within the echogenic choroid plexus of a fetal lateral ventricle, usually resolving before birth; it is a soft marker for trisomy 18, particularly when other abnormalities are present.

A semiquantitative assessment of amniotic fluid obtained by dividing the uterus into four quadrants and summing the deepest vertical pocket in each; normal ranges vary with gestational age and are used to detect oligohydramnios or polyhydramnios.

A reduced volume of amniotic fluid, generally defined as a deepest vertical pocket under 2 cm or an amniotic fluid index below 5 cm; causes include ruptured membranes, placental insufficiency and fetal renal abnormalities.

An excess of amniotic fluid, commonly defined as a deepest vertical pocket greater than 8 cm or an amniotic fluid index above 24 cm; it is associated with diabetes, fetal swallowing disorders and gastrointestinal obstruction.

Placental tissue that covers or lies immediately adjacent to the internal cervical os; it is described as complete, partial or marginal and is a leading cause of painless third-trimester vaginal bleeding.

Premature separation of a normally implanted placenta from the uterine wall; sonography may reveal a retroplacental hematoma but can also be normal, so the diagnosis rests largely on clinical findings.

Insertion of the umbilical cord into the fetal membranes rather than directly into the placental disk, leaving umbilical vessels unprotected as they travel to the placenta; it predisposes to vessel compression and fetal hemorrhage.

The number of chorions, and therefore placentas, in a multiple pregnancy; it determines the risk of complications such as twin-twin transfusion and is best assigned in the first trimester using the lambda or T sign.

A complication of monochorionic twin pregnancy in which unbalanced placental vascular connections make one twin a donor with oligohydramnios and the other a recipient with polyhydramnios and cardiac strain.

A composite fetal assessment scoring five components: fetal breathing, movement, tone, amniotic fluid volume and a nonstress test, each scored 0 or 2; the total correlates with fetal acid-base status.

The maximum anteroposterior measurement of the double-layer endometrium on a sagittal transvaginal view; it changes through the menstrual cycle and is used to evaluate abnormal bleeding, infertility and endometrial pathology.

A benign ovarian germ cell tumor containing elements of at least two germ layers, typically fat, hair and calcium; it appears as a cystic mass with an echogenic mural nodule or diffuse internal echoes.

An ovarian cyst formed by ectopic endometrial tissue and filled with old blood that produces homogeneous low-level internal echoes; it is often called a chocolate cyst and typically shows no internal vascularity.

A dilated, fluid-filled fallopian tube that appears as a folded tubular structure with thin walls and incomplete septations; it usually results from chronic pelvic inflammatory disease and can impair fertility.

Twisting of the ovary, and often the tube, around its vascular pedicle, compromising venous then arterial flow; ultrasound may show an enlarged edematous ovary with peripheral follicles and reduced or absent Doppler signal.

A procedure in which sterile saline is infused through a catheter into the uterine cavity during transvaginal scanning, distending the cavity so that polyps, submucosal fibroids and adhesions become clearly visible.

Reduced echo amplitude deep to a structure that strongly reflects or absorbs sound, appearing as a dark band behind bone, calcification or gas; it can hide anatomy lying deeper in the field.

Increased echo brightness deep to a fluid-filled structure because fluid attenuates sound less than the surrounding tissue; the artifact helps confirm that a mass is cystic rather than solid.

A Doppler artifact that occurs when blood velocity exceeds the Nyquist limit, producing a wraparound of the color scale or a reversed spectral display; raising the velocity scale or adjusting the baseline corrects it.

A displayed value that estimates the potential for mechanical bioeffects such as cavitation from the ultrasound beam; it is kept low, especially in early pregnancy and whenever contrast agents are used.