16.1 Early Gestation: Normal Development & Structures

Scanning Approaches: Transabdominal vs Transvaginal

Early pregnancy is evaluated with two complementary approaches. Transabdominal (TA) scanning uses a curvilinear transducer (typically 3-5 MHz) placed on the lower abdomen with a full urinary bladder serving as an acoustic window. The filled bladder displaces gas-filled bowel loops out of the pelvis, lifts the uterus into the field of view, and provides a fluid path for the beam. TA scanning gives a global overview of the uterus and adnexa but is limited by distance, patient body habitus, and lower-frequency resolution.

Transvaginal (TV) scanning places a high-frequency endocavitary probe (typically 5-9 MHz) in the vaginal fornix, only centimeters from the uterus. Because the beam travels a short distance with little attenuation, higher frequencies can be used, producing far better spatial resolution. The practical result: TV scanning identifies every landmark of early pregnancy about one week earlier than TA scanning. The bladder should be empty for TV imaging; a full bladder pushes the uterus away from the probe and degrades the image. On the ARRT exam, if a question asks which approach first demonstrates the yolk sac or cardiac activity, the answer is almost always transvaginal.

The Gestational Sac and Decidual Signs

The gestational sac is the first sonographic evidence of an intrauterine pregnancy, visible transvaginally at about 4.5 to 5 weeks (transabdominally closer to 6 weeks). It appears as a small anechoic fluid collection surrounded by an echogenic rim, embedded in the decidua on one side of the endometrial cavity.

Two signs confirm a true sac:

• Intradecidual sign — the tiny sac is buried within the thickened decidua adjacent to, but not deforming, the central endometrial echo line. This is the earliest sign (around 4.5 weeks) but is less specific on its own.
• Double decidual sac sign (DDSS) — two concentric echogenic rings surround the sac: the inner ring of decidua capsularis plus chorionic villi, and the outer ring of the decidua parietalis (the opposed uterine wall lining). The double bleb sign seen slightly later refers to the yolk sac and amniotic sac within the chorionic cavity.

These signs matter because a pseudosac — the fluid collection seen inside the uterus in roughly 10-20% of ectopic pregnancies — lacks a double decidual ring, is centrally located within the cavity, often has pointed or irregular margins, and contains no yolk sac. Differentiating true sac from pseudosac is a classic exam point.

The Decidual Layers

The endometrium of pregnancy (decidua) has three named regions:

The Yolk Sac

The yolk sac is the first structure identified within the gestational sac, visible transvaginally at about 5 to 5.5 weeks. It appears as a small round echogenic ring with an anechoic center. Normal mean diameter is 3 to 6 mm (measured inner-to-inner) during the first trimester. The yolk sac provides early nutrition, hematopoiesis, and contributes to the developing gut before placental circulation takes over.

Abnormal yolk sac morphology is prognostic: a yolk sac larger than 6-7 mm, a very small yolk sac, an echogenic or calcified yolk sac, or a persistent yolk sac beyond its expected regression (normally resorbed by about 12 weeks) is associated with early pregnancy failure. When an exam question describes a normal sac with a large or calcified yolk sac and asks about prognosis, the answer is guarded.

Amnion and Chorion

Two membranes surround the embryo. The chorion is the outer membrane, derived from trophoblast; its villous portion (chorion frondosum) becomes the fetal placenta, and the space between chorion and amnion is the chorionic (extraembryonic) cavity. The amnion is the thin inner membrane enclosing the embryo in the amniotic cavity. Early on, the amniotic cavity is tiny — the thin amnion may be seen as a fine line parallel to the embryo — while the chorionic cavity (containing the yolk sac) dominates. The amniotic cavity grows steadily, and the amnion and chorion fuse at about 14 to 16 weeks, obliterating the chorionic cavity. A membrane persisting well beyond 16 weeks raises the question of amniotic band or hemorrhage.

The Embryonic Pole and Cardiac Activity

The embryonic pole — a small echogenic thickening adjacent to the yolk sac — becomes visible transvaginally at about 5.5 to 6 weeks. Cardiac activity follows almost immediately: flickering motion is expected transvaginally once the crown-rump length (CRL) reaches about 5 to 7 mm, around 6 weeks. By consensus guidelines, an embryo of 7 mm or greater without cardiac activity is diagnostic of demise, so 7 mm is the number to memorize.

The embryonic heart rate follows a predictable curve: about 100-115 bpm at 6 weeks, rising steeply to a peak of roughly 170 bpm at 8-9 weeks, then gradually settling into the normal 120-160 bpm range maintained through the rest of pregnancy. An early rate below 90 bpm carries a poor prognosis.

hCG Discriminatory Levels

Serum human chorionic gonadotropin (hCG) roughly doubles every 48 hours in early viable pregnancy. The discriminatory level is the hCG value above which an intrauterine gestational sac should be visible: historically quoted as 1,500-2,000 mIU/mL transvaginally, but ACOG now advises setting it conservatively high — as much as 3,500 mIU/mL — because a sac is reliably seen only near that level and acting on the lower figure risks interrupting a viable intrauterine pregnancy. The transabdominal figure is about 6,500 mIU/mL. Above a conservatively set discriminatory level with an empty uterus, ectopic pregnancy or completed abortion should be considered, but a single value never establishes the diagnosis on its own. Importantly, a single hCG value below the zone does not exclude a normal early pregnancy — correlation with serial values and sonography is required.

Fetal Number, Chorionicity and Amnionicity

Counting embryos is a required element of the first-trimester examination, not a formality — and the first trimester is the only window in which chorionicity and amnionicity can be assigned reliably. After roughly 14 weeks the dividing membrane thins and the diagnostic signs fade, so a determination missed now may never be recoverable.

Work through it in two steps:

1. Count the gestational sacs — that gives chorionicity. Two separate sacs, each with its own thick echogenic rim, means dichorionic; a single sac means monochorionic.
2. Count the amnions and yolk sacs within each sac — that gives amnionicity. In early gestation the number of yolk sacs generally tracks the number of amnions.

Two membrane signs carry the answer into the second trimester. The twin-peak (lambda) sign — a thick triangular wedge of chorion extending into the base of the dividing membrane — indicates dichorionic. The T sign, where a thin membrane meets the placenta at a right angle with no wedge, indicates monochorionic. Membrane thickness corroborates: a dichorionic membrane carries four layers and usually measures more than 2 mm, a monochorionic diamniotic membrane only two layers and under 2 mm.

Chorionicity is the single most consequential determination in a twin pregnancy. Monochorionic twins share a placental circulation and carry the risk of twin-to-twin transfusion syndrome, twin anemia-polycythemia sequence, and TRAP sequence; monoamniotic twins add cord entanglement. Report the finding explicitly — "monochorionic diamniotic" — rather than simply "twins."