A 28-year-old woman at 8 weeks gestation presents with intermittent bright red vaginal bleeding. Transvaginal ultrasound shows a live intrauterine gestational sac with normal fetal pole and cardiac activity. The structure marked **A** in the diagram is identified as a crescentic hypoechoic collection measuring 15 mm in diameter, occupying approximately 20% of the gestational sac perimeter. Which of the following best describes the most likely diagnosis and its clinical significance?

Explanation

## Why option 1 is correct The structure marked **A** is a subchorionic hematoma (SCH), the most common sonographic finding associated with first-trimester vaginal bleeding, occurring in ~20% of pregnancies presenting with first-trimester bleeding. The key clinical anchor is that a SMALL hematoma (<25% of gestational sac perimeter) detected before 9 weeks carries minimal increased risk of pregnancy loss. This patient's hematoma measures 15 mm and occupies ~20% of the sac perimeter, placing it in the small category. The presence of confirmed fetal cardiac activity and appropriate gestational age indicate a viable intrauterine pregnancy. According to Rumack Diagnostic Ultrasound and Williams Obstetrics, the prognosis for small SCH detected early is favorable, with most resolving spontaneously by 20 weeks. Management is supportive: pelvic rest, serial ultrasound surveillance, and Rh-D prophylaxis if Rh-negative. ## Why each distractor is wrong - **Option 2 (Placental abruption)**: Placental abruption typically occurs in the second or third trimester, is retroplacental (not subchorionic), presents with severe pain and rigid uterus, and is a clinical emergency. This patient is at 8 weeks with a marginal collection and no pain—inconsistent with abruption. - **Option 3 (Vanishing twin syndrome)**: Vanishing twin presents as a partially resorbed twin with either its own gestational sac or yolk sac fragments. This patient has a single viable gestational sac with normal fetal pole and cardiac activity; the hypoechoic collection is between the chorion and decidua, not a separate gestational sac. - **Option 4 (Molar pregnancy)**: Molar pregnancy presents with a heterogeneous "snowstorm" appearance, absence of a fetus (or abnormal fetal tissue in partial molar), and markedly elevated beta-hCG. This patient has a normal-appearing fetus with cardiac activity at appropriate gestational age—molar pregnancy is excluded. **High-Yield:** Subchorionic hematoma is the most common cause of first-trimester bleeding with a live IUP; small SCH (<25%) detected before 9 weeks has favorable prognosis and requires only supportive management and serial ultrasound follow-up. [cite: Rumack Diagnostic Ultrasound 5e Ch 34 (First Trimester); Williams Obstetrics 26e]