Subamniotic Haematoma

A subamniotic haematoma is a collection of blood between the amnion and the chorion on the fetal surface of the placenta, usually from rupture of a fetal chorionic vein near the cord insertion. It is found on ultrasound as a mass that bulges into the amniotic cavity, sometimes mistaken for a tumour. It is uncommon, and matters because most are harmless, while large ones can be linked with growth restriction and fetal loss.

• Risk factors: short cord, cord traction, amniocentesis, abdominal trauma, anticoagulant use.
• Small collections resolve over weeks.
• Large collections on the fetal surface compress cord vessels.

Women usually have no symptoms and the finding comes from a routine scan. Some have light painless bleeding or reduced fetal movements.

• Normal examination and vital signs.
• Uterus may be small for dates if growth is affected.
• Differential diagnosis: chorioangioma, placental cyst, subchorionic haematoma, retroplacental haematoma, mesenchymal dysplasia, normal placental lake.
• Complications: cord compression, fetal growth restriction, fetal anaemia, preterm birth, stillbirth.

The approach is ultrasound with Doppler to distinguish a clot from a tumour.

• Ultrasound: avascular complex or echogenic mass on the fetal surface of the placenta, near the cord insertion, with no internal flow on colour Doppler.
• Measure the collection in three planes; repeat every 2 weeks to detect growth or shrinkage.
• Umbilical artery Doppler, estimated fetal weight, amniotic fluid index; middle cerebral artery peak systolic velocity if anaemia is suspected.
• Haemoglobin, platelets, coagulation screen, blood group; Kleihauer test in RhD negative women with bleeding.

The principle is surveillance for growth and fetal condition, with delivery at term unless complications appear.

• Supportive: reassure; avoid abdominal trauma; daily fetal movement counts; teach to attend for bleeding or reduced movements.
• Pharmacological: paracetamol 1 g oral 6-hourly as needed (maximum 4 g in 24 hours); ferrous sulphate 200 mg oral 8-hourly for anaemia; betamethasone 12 mg IM twice, 24 hours apart, between 24 and 34 weeks and 6 days if preterm delivery is planned; oxytocin 10 IU IM at birth.
• Procedures: growth scans every 2 to 4 weeks with umbilical artery Doppler; delivery at 38 to 39 weeks, earlier for growth restriction, abnormal Doppler or abnormal cardiotocography; send the placenta for histology.
• Special groups: RhD negative women with bleeding give anti-D immunoglobulin 500 IU IM within 72 hours.
• Red flags: bleeding, reduced fetal movements, rapid enlargement of the mass, falling growth centile, abnormal cardiotocography.
• Follow-up: repeat scan every 2 weeks until stable, then every 4 weeks; cardiotocography weekly from 34 weeks; newborn haemoglobin check.