Chorionic hematoma (also chorionic hemorrhage) is the pooling of blood (hematoma) between the chorion, a membrane surrounding the embryo, and the uterine wall. With an incidence of 3.1% of all pregnancies, it is the most common sonographic abnormality and the most common cause of first trimester bleeding.
Cause and diagnosis 
- Subchorionic hematomas, the most common type, are between the chorion and endometrium.
- Retroplacental hematomas are entirely behind the placenta and not touching the gestational sac.
- Subamniotic or preplacental hematomas are contained within amnion and chorion. Rare.
Ultrasonography is the preferred method of diagnosis. A chorionic hematoma appears on ultrasound as a hypoechoic crescent adjacent to the gestational sac. The hematoma is considered small if it is under 20% of the size of the sac and large if it is over 50%.
Prognosis and treatment 
Chorionic hematomas are associated with an increased risk of miscarriage, 9.3% for patients with a live fetus and hematoma identified in the first trimester according to a study by Bennett et al., but most regress naturally and do not worsen the patient's prognosis. There is no known therapeutic intervention.
- Nagy, Sándor MD; Bush, Melissa MD; Stone, Joanne MD; Lapinski, Robert H. PhD; Gardó, Sándor MD, DSci. Clinical Significance of Subchorionic and Retroplacental Hematomas Detected in the First Trimester of Pregnancy . Obstetrics & Gynecology: July 2003 - Volume 102 - Issue 1 - p 94-100
- Avneesh Chhabra, MD et al. "Subchorionic Hemorrhage" , Medscape.
- Trop, Isabelle and Levine, Deborah. Hemorrhage During Pregnancy: Sonography and MR Imaging . Amer J Roentgenology 2001; 176:607-615.
- Trop I, Levine D. Hemorrhage during pregnancy: sonography and MR imaging. AJR Am J Roentgenol. Mar 2001;176(3):607-15.
- Hodgson DT, Lotfipour S, Fox JC. Vaginal bleeding before 20 weeks gestation due to placental abruption leading to disseminated intravascular coagulation and fetal loss after appearing to satisfy criteria for routine threatened abortion: a case report and brief review of the literature. J Emerg Med. May 2007;32(4):387-92
- Abu-Yousef MM, Bleicher JJ, Williamson RA, Weiner CP. Subchorionic hemorrhage: sonographic diagnosis and clinical significance. AJR Am J Roentgenol. Oct 1987;149(4):737-40.
- Bennett GL, Bromley B, Lieberman E, Benacerraf BR. Subchorionic hemorrhage in first-trimester pregnancies: prediction of pregnancy outcome with sonography. Radiology. Sep 1996;200(3):803-6.