This is a case report of a primigravida who was noticed to have a large pelvic mass at the time of her "booking" scan. A magnetic resonance imaging (MRI) scan was performed in order to elucidate the exact nature of the mass, which enlarged between successive scans. The appearances were in keeping with a leiomyoma undergoing degenerative change. The patient underwent a laparotomy in pregnancy to remove the leiomyoma, which was found to contain fluid-filled cavities. The uterine cavity was not breached at the time of surgery. The pregnancy continued uneventfully until term, and the baby was delivered by caesarean section. The postnatal period was uneventful.