16.3 First-Trimester Abnormalities

Diagnosing Pregnancy Failure

To avoid ever mislabeling a normal early pregnancy as failed, strict consensus criteria (Doubilet/Benson, endorsed by the Society of Radiologists in Ultrasound) define diagnostic findings of pregnancy failure. A terminology note before the criteria: the 2024 SRU first-trimester lexicon retired several of the legacy words still used on older question banks — pregnancy failure is now early pregnancy loss, blighted ovum is anembryonic pregnancy, missed abortion is embryonic or fetal demise, pseudosac is intracavitary fluid, and heartbeat is cardiac activity. The measurement thresholds themselves are unchanged:

Findings that are suspicious but not diagnostic include CRL < 7 mm without cardiac activity, MSD 16-24 mm without an embryo, an empty amnion, an enlarged yolk sac (> 7 mm), and a small sac relative to the embryo — these call for a follow-up scan, not a final diagnosis.

Anembryonic Pregnancy and Embryonic Demise

An anembryonic pregnancy (historically called a blighted ovum) is a gestational sac that develops without an embryo — the sac and often a yolk sac grow, but no embryonic pole forms. It meets failure criteria when the MSD reaches 25 mm with no embryo. Embryonic demise refers to an embryo that formed and then died: the hallmark is an embryo at or above the 7 mm CRL threshold without cardiac activity, confirmed with M-mode and high magnification before pronouncing absence of motion. Embryonic bradycardia is a warning sign: a heart rate below 90 bpm in the early embryo is associated with a high rate of subsequent demise, while rates of 90-110 bpm warrant short-interval follow-up.

Spontaneous Abortion Spectrum

Spontaneous abortion (miscarriage) is pregnancy loss before 20 weeks. The clinical types form a sequence the exam tests by definition:

• Threatened abortion — vaginal bleeding with a closed cervix and a live intrauterine embryo; the pregnancy may continue.
• Inevitable abortion — bleeding with a dilated/open cervix; loss cannot be stopped. A sac low in the uterus or within the cervical canal supports this.
• Incomplete abortion — only part of the products of conception have passed; the cervix is open and tissue remains in the uterus.
• Complete abortion — all products expelled; the uterus is empty with a thin endometrium and the cervix closes.
• Missed abortion — embryonic demise with retention of the nonviable pregnancy; no bleeding or passage of tissue, cervix closed.

Retained products of conception (RPOC) appear as an echogenic endometrial mass with demonstrable color Doppler flow; an endometrial thickness under about 8-10 mm with no focal mass makes significant RPOC unlikely. RPOC predisposes to hemorrhage and infection and may require evacuation.

Ectopic Pregnancy

An ectopic pregnancy implants outside the endometrial cavity and remains a leading cause of first-trimester maternal mortality from tubal rupture and hemorrhage. About 95% occur in the fallopian tube, and of tubal implantations the ampullary portion is the most common site (roughly 70%), followed by the isthmic portion. Less common sites include interstitial (cornual), cervical, cesarean scar, ovarian, and abdominal implantations. A heterotopic pregnancy — simultaneous intrauterine and ectopic gestations — is rare naturally (about 1 in 4,000-30,000) but rises toward 1 in 100 with assisted reproduction.

Risk factors: prior ectopic pregnancy (strongest), pelvic inflammatory disease (especially chlamydia) and tubal scarring, prior tubal surgery or sterilization, intrauterine device use, in-vitro fertilization, and smoking.

Sonographic signs with an empty uterus and positive hCG:

• Adnexal mass or tubal ring (bagel/donut) sign — an echogenic ring separate from the ovary; the most specific finding is a live embryo with cardiac activity in the adnexa, though an extrauterine sac with yolk sac is also diagnostic.
• Free fluid — echogenic free fluid in the posterior cul-de-sac (pouch of Douglas) or in the hepatorenal recess (Morison's pouch) suggests hemoperitoneum from rupture; scanning Morison's pouch is part of the ectopic workup.
• Pseudosac — a central intrauterine fluid collection in 10-20% of ectopics, lacking the double decidual sign (see Section 16.1).

Gestational Trophoblastic Disease

Gestational trophoblastic disease (GTD) is abnormal proliferation of trophoblastic tissue. The complete hydatidiform mole is classically diploid 46,XX of entirely paternal (androgenetic) origin with no fetal tissue, markedly elevated hCG (often above 100,000 mIU/mL), hyperemesis, and a uterus large for dates. Sonography shows a heterogeneous intrauterine mass of countless small vesicles — the snowstorm or bunch-of-grapes appearance — with no embryo. Ovarian stimulation by the very high hCG produces bilateral multiloculated theca lutein cysts. A partial mole is triploid (typically 69,XXY) and may show a coexisting abnormal fetus with a focally cystic placenta. Complete moles carry a risk of persistent trophoblastic disease and choriocarcinoma, so hCG follow-up after evacuation is mandatory.

Subchorionic Hemorrhage

A subchorionic (subchorionic-placental) hemorrhage is bleeding between the chorionic membrane and the uterine wall, appearing as a crescent-shaped fluid collection lifting the chorion away from the myometrium. Its echogenicity varies with age — hyperechoic or isoechoic when acute, becoming hypoechoic/anechoic as it resolves — and it shows no internal blood flow on Doppler, distinguishing it from a vascular mass. Small collections in a pregnancy with cardiac activity usually resolve with good outcomes; large hemorrhages (involving a substantial portion of the sac circumference or elevating a large placental area) increase the risk of miscarriage, especially with advancing maternal age.