The uterine cavity is usually smooth and as the pregnancy grows, the uterus grows. Some women have fibroids inside the cavity of the uterus, known as submucosal fibroids. For most women these cause no problems at all, but in some cases they are linked to miscarriages. A fibroid is a hard ball of smooth muscle cells and a submucosal fibroid can thin out the lining of the uterus and decrease the blood supply to the developing embryo. The pregnancy cannot develop normally and this may lead to a miscarriage.
Repetitive miscarriages can also occur if the uterus has an abnormal shape.
A normal uterus is shaped like a small pear turned upside down. And a normal uterine cavity has a triangular shape. During development of a female embryo, separate parts fuse together to produce a normal uterus and Fallopian tubes. Abnormal structures result if one of the parts fails to develop, or if the fusion is incomplete. If only one side develops, the uterus may be shaped like a comma and there may be only one Fallopian tube.