The septate uterus is the most common müllerian anomaly, accounting for 3% of detected anomalies. It consists of a single uterus divided by a largely fibrous midline septum. If the septum extends to the internal opening (os) of the cervix or even further downward, it is considered a complete septum. If it does not, the uterus has a partial septum and is "subseptate." In addition, a septum may be broken up longitudinally, or segmented. The outside contour of the uterine dome (fundus) may or may not be indented, but the groove does not exceed 1.5 centimeters in depth.
In the female embryo, uterine development is usually complete by 22 weeks' gestation, with the two müllerian ducts fusing together to form a single uterus. In one of the final events of uterine formation, the wall where the ducts fused dissolves, forming a single endometrial cavity. It is the failure of this last process that produces a septate uterus with two endometrial canals, reflecting either a partial or complete failure of the duct walls to dissolve, depending on the extent of the septum. Although there is some evidence of a weak genetic factor at work, researchers still do not know the exact cause of the failure of a septum to resorb.
While a septum is not thought to decrease fertility, it does seem to affect the course of a pregnancy, either through miscarriage or pregnancy complications. Early miscarriage is common within the septate uterus, because the blood-starved median septum is covered by a poorer grade of endometrium than that of the blood-rich sidewalls. An embryo implanting in the septum frequently fails to thrive because of lack of nourishment, and an early miscarriage is the result. Late miscarriage is also common, and its likelihood increases along with the extent of the septum. Some studies suggest that in a completely septate uterus, the prognosis of a live birth is as low as 10%. In a late miscarriage, the pregnancy outgrows available space and the cervix may give way, typically midway in the pregnancy, before the fetus is mature enough to survive. Other complications include premature labor, premature birth, intrauterine growth retardation, and fetal malpresentation at birth. One caveat to keep in mind is that women diagnosed with uterine septa tend to be the ones with the most problems conceiving or carrying to term; cases of septa that do not affect reproduction are probably under-reported.
Unless the woman has a vaginal septum or double cervix, the septate uterus is usually not diagnosed until a woman has had some pregnancy failure. Typically, after failure to conceive or repeated miscarriage, hysterosalpingogram (HSG), in which dye is injected into the uterus and x-rays taken, reveals some degree of septation. Other tests which may reveal a septum are: magnetic resonance imaging (MRI), ultrasound, and hysteroscopy.
Each test has its own advantages and disadvantages. For instance HSG, which shows the inner contours of theuterine cavity and fallopian tubes, can not tell the difference between a septate uterus and a bicornuate uterus because it does not reveal the outer contour of the uterus. A spetate uterus looks normal or nearly normal on the outside, while a bicornuate uterus has some degree of outer division. Although some medical texts state that the angle between the horns may reveal the type of uterus, with an angle less than 75 degrees indicating a septate uterus, this is not reliable; some septa are very broad. MRI, on the other hand is increasingly thought to be more reliable, although sometimes it is difficult to see the uterus in the proper plane. While ultrasound may sometimes reveal the outer shape of the uterus, it is best used as an adjunct to a more definitive test. Hysteroscopy can frequently determine whether the septum is fibrous or muscular, and when used simultaneously with laparoscopy, can provide a very accurate diagnosis. Although a simultaneous hysteroscopy and laparoscopy is invasive, it can be done at the same time as a hysteroscopic metroplasty, if the latter is indicated to reduce a uterine septum.
One of the heartening aspects of having a septate uterus is that it can be repaired through a relatively simple surgery, giving the woman near-normal odds of carrying a subsequent pregnancy to term. Whether or not she needs a metroplasty depends on the extent of the septum, or whether or not she has lost previous pregnancies. Until the mid-1980s, surgery to reduce a septum, called a metroplasty, was done through an abdominal incision. Recent development of the hysteroscopic metroplasty (also known as "septoplasty") has rendered the former technique almost obsolete.
Information from: http://www.wegrokit.com/uterineanomalies/index.htm