Adenomyosis is a common gynecologic disease. Pregnancy with adenomyosis is on the increase due to a tendency of delay with first pregnancies and various infertility treatments involved in the process. We encountered decidualized adenomyosis in three patients during pregnancy, who were suspected by magnetic resonance (MR) imaging and were followed monitored post delivery. The MR imaging findings of adenomyosis during pregnancy showed low signal intensity areas with embedded bright foci that expanded to a few mm in diameter on half Fourier single-shot turbo spin-echo images. This finding may reflect decidual change of the stroma within the ectopic endometrium caused during pregnancy. The MR imaging findings of adenomyosis after childbirth showed hemorrhage inside the lesion, which were assumed to be led by rapid decrease in a blood flow to adenomyosis post childbirth.