Objective To detect a direct transition from a haemorrhagic corpus luteum to an endometriotic cyst by serial transvaginal ultrasonographic scans.
Design Prospective observational study.
Setting An academic tertiary care and referral centre for women with endometriosis.
Population One hundred and nine women younger than 40 years, with regular menstrual cycles, undergoing first-line surgery for endometriomas, and not wanting postoperative oral contraception.
Methods Three-monthly transvaginal ultrasonography during the luteal phase for 2 years after surgery.
Main outcome measure Sonographic identification of progression from a haemorrhagic corpus luteum to a recurrent endometriotic cyst.
Results A haemorrhagic corpus luteum was identified in 13 women. Serial ultrasonographic scans demonstrated transition to an endometriotic cyst in 11 (85%) instances and resorption in two. A unilateral endometriotic cyst without previous detection of a cystic corpus luteum was observed in 14 women.
Conclusions Bleeding from a corpus luteum appears to be a critical event in the development of endometriomas.