Fifteen pregnant women were studied with magnetic resonance (MR) imagi
ng. In 14 women, the myometrium exhibited distinct hypointense bulging
on T2-weighted images that distorted the gestational sac and decidua
but not the outer uterine coutour. In 12 women, an obvious discrepancy
was observed between the inner myometrial configuration on T1-weighte
d images and that on intermediate and T2-weighted images. In three wom
en who underwent serial T2-weighted studies separated by about 30 minu
tes during one MR examination, hypointense myometrial bulging was obse
rved at a different site on each study. In two women who underwent fol
low-up MR study within a week after dilation and curettage, the previo
usly distinct hypointense bulging completely disappeared. These findin
gs confirm the transient and mobile nature of this myometrial bulging.
This phenomenon can be attributed to sustained uterine contractions,
which partially force the blood out of the uterus. The decrease in blo
od volume results in a decrease in water content, which accounts for t
he decrease in myometrial signal intensity on T2-weighted images.