A 27-year-old primigravida at 9 weeks gestation presents with painless vaginal spotting for 2 days. Speculum examination shows mild blood at the external os; the internal os is closed. Serum β-hCG is appropriate for dates. Transvaginal ultrasound reveals a live singleton intrauterine pregnancy with fetal heart rate 165/min, and a crescent-shaped hypoechoic collection between the chorion and uterine wall measuring 1.5 × 4 cm, as marked **A** in the diagram. Which of the following best describes the pathophysiology of the structure marked **A**?
A. Partial detachment of the placenta from the decidua with bleeding into the subchorionic space
B. Trophoblastic invasion of the myometrium with abnormal placentation
C. Complete separation of the placenta with fetal demise and retained products of conception
D. Rupture of a chorionic villus with direct fetal-maternal hemorrhage
Explanation
Why "Partial detachment of the placenta from the decidua with bleeding into the subchorionic space" is right
The structure marked A is a subchorionic hematoma (SCH), which results from partial detachment of the placenta from the decidua, allowing blood to accumulate in the space between the chorion and the uterine wall. This is the defining pathophysiologic mechanism of SCH in threatened abortion. The clinical presentation—painless spotting, closed cervical os, viable intrauterine pregnancy, and the characteristic crescent-shaped hypoechoic collection on ultrasound—is pathognomonic for this diagnosis. Per ACOG Practice Bulletin on Early Pregnancy Loss and the PRISM trial, SCH is a common finding in early pregnancy (~10–20% of first-trimester ultrasounds with bleeding) and results specifically from this partial placental detachment.
Why each distractor is wrong
Complete separation of the placenta with fetal demise and retained products of conception: This describes missed abortion or incomplete abortion, not threatened abortion with a viable pregnancy. The patient has a live fetus with normal heart rate; there is no fetal demise.
Trophoblastic invasion of the myometrium with abnormal placentation: This describes placenta accreta or morbidly adherent placenta, which is a disorder of placental implantation, not a hemorrhagic collection from partial detachment. SCH is not related to abnormal trophoblastic invasion.
Rupture of a chorionic villus with direct fetal-maternal hemorrhage: While SCH may result in some fetal-maternal hemorrhage, the primary pathophysiology is not villous rupture but rather partial placental detachment from the decidua. This distractor conflates SCH with a different mechanism of bleeding.
High-YieldNEET PG
Subchorionic hematoma = partial placental detachment from decidua; threatened abortion = bleeding before 20 weeks with closed os and viable IUP; larger hematomas (>25% of sac circumference) and maternal age >35 carry higher miscarriage risk.
ACOG Practice Bulletin Early Pregnancy Loss; Coomarasamy et al PRISM trial
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