Uterine Abnormalities: Uterine Septum, Structural Causes of Infertility
Uterine abnormalities are a significant cause of both infertility and recurrent pregnancy loss. These abnormalities are typically congenital (present from birth) and are referred to as Mullerian anomalies, as the uterus develops from a specialized type of tissue called Mullerian tissue. During embryonic development, a female fetus actually starts out with two small uteri (plural of uterus) – one near each kidney. As the fetus develops, each uterus migrates down toward the tissue that ultimately becomes the vagina, and toward the middle of the patient’s body where it fuses with the uterus from the other side. Under normal circumstances, the wall where the two uteri join reabsorbs completely – from the bottom of the uterus to the top – resulting in a triangular shaped uterine cavity.
Any alteration of this development can lead to a Mullerian anomaly. The most common anomalies include a septate uterus, a bicornuate uterus, a unicornuate uterus, and a uterine didelphys. A septum occurs when the two uteri fuse but the wall between them only partially reabsorbs. The remaining (unreabsorbed) tissue at the top of the uterus behaves like scar tissue – it has decreased blood flow and may lack the receptors found in normal uterine lining that are necessary for implantation to occur. As a result, a successful pregnancy is much less likely to result. Even if implantation does occur, but it occurs at the top of the uterus, miscarriage is much more common.
If the two uteri only fuse at the very bottom leaving a large unreabsorbed vertical wall between the two cavities, a bicornuate (Latin for “2 horns”) uterus results. In this condition, the patient has two small uterine cavities and a single cervix. These cavities each have an attached Fallopian tube, so pregnancy is possible, although the chance of pregnancy in these patients is significantly reduced. In addition, when pregnancy does result, the risk of premature labor is very great as the uterus cannot grow as large as it normally would in a term pregnancy.
Sometimes only one of the two uteri forms during embryologic development. This uterus has only one fallopian tube and it is called a unicornuate (Latin for “1 horn”) uterus. Both ovaries will still be present, as they come from different embryologic tissue. The chance for pregnancy in these patients is about 60% of that in a patient with a normal uterus. In addition, pregnancy can only occur during months in which the woman ovulates from the ovary on the same side as the fallopian tube (remember there is only one tube present in this condition).
On very rare occasions, both uteri are present but they fail to fuse at all. This condition, called a uterine didelphys, results in two separate uteri – each with its own fallopian tube and cervix. This condition is easily recognized by the general gynecologist; as such patients have two cervices (plural of cervix) on pelvic examination.
With the exception of the uterine septum, these abnormalities are not repaired because the chance of improving pregnancy outcome is typically worse following surgical repair than it is without repair. The situation is vastly different for the septate uterus, however, as multiple large studies have shown that surgical repair dramatically improves pregnancy outcome.
Therefore, whenever we diagnose a uterine septum, we will advocate for hysteroscopic repair – usually under laparoscopic guidance. This outpatient procedure is designed to resect the septum so that the entire top of the uterus enlarges, creating a normal area for implantation to occur. Once this procedure has been performed, the uterus is much more capable of carrying a pregnancy to term. In addition, as the repair does not involve making an incision in the muscular uterine wall, patients who conceive after a septum resection are not required to have a Cesarean section.