Abnormal Uterus Shapes and Miscarriage Risk

An abnormal uterus can sometimes be a risk factor for miscarriage and, in some cases, a cause of recurrent miscarriages. Only some types of uterine malformations increase the risk of miscarriage and require treatment; others may not cause any problems with pregnancy at all. It's thought that 8 percent to 23 percent of women who have recurrent miscarriages have some type of uterine abnormality.

Some uterine malformations are present from birth, while others develop during adulthood. Most often, women with uterine abnormalities do not have any symptoms and are not aware of these malformations before they become pregnant. Diagnosis of congenital uterine malformations usually comes after a hysteroscopy or hysterosalpingogram.

Learn more about the various types of uterine malformations, below. 

Uterine Septum

normal shaped uterus
Miscarriage risks due to abnromal uterine shape and structure. Istockphoto.com/Stock Photo©Spectral-Design

A uterine septum (septate uterus) is the most common congenital uterine malformation, comprising roughly one-third of uterine abnormalities. Since it is a congenital condition, that means that it is present at birth.

A uterine septum is a band of fibrous tissue that partially or completely divides the uterus, usually without a good blood supply. If a fertilized egg implants on the septum, the placenta is unable to grow properly and miscarriage is likely.

For women who have a septate uterus, the risk of miscarriage is between 25 percent and 47 percent (compared with a risk of 15 percent to 20 percent for women without a septate uterus.) For women who do not miscarry, a septate uterus may increase the risk of preterm birth.

Treatment is usually minor surgery that's performed during a hysteroscopy, and it involves the removal of the abnormal tissue. This usually works extremely well to resolve the problem and allow women to successfully carry a full-term pregnancy.

Bicornuate Uterus

A bicornuate uterus is a heart-shaped uterus—essentially a uterus with a dip on top. Bicornuate uteri (as well as unicornuate and didelphic uteri) are considered mullerian duct abnormalities. A mullerian duct abnormality is a type of congenital abnormality of the uterus.

Most women with a bicornuate uterus do not have complications, but in some, a bicornuate uterus can lead to an increased risk of preterm labor. A bicornuate uterus is not believed to increase the risk of a first-trimester miscarriage but it may increase the risk of a second-trimester miscarriage.

Treatment is not usually needed, with the exception of a cervical cerclage in those who are at risk of cervical insufficiency and premature delivery. Most women are unaware that they have a bicornuate uterus until they become pregnant.

Unicornuate Uterus

unicornuate uterus is a horn-shaped uterus that causes the uterus to be smaller than normal. It is a congenital malformation in which one side of the uterus does not develop properly.

A unicornuate uterus increases the risk of ectopic pregnancy, miscarriage, and preterm delivery. One literature review found that the risk of miscarriage was, on average, 37 percent ​and the risk of preterm birth was, on average, 17 percent.

Unlike women with a bicornuate uterus, women with a unicornuate uterus may have symptoms that are suggestive of the anomaly before they become pregnant. Around 65 percent of women with a unicornuate uterus have what is called a rudimentary horn. When this is present, women may have very painful periods because blood gets backed up in the horn during menses.

Along with the uterus abnormalities, women with a unicornuate uterus often have only one functioning fallopian tube (as opposed to two). This, in addition to other factors, may make it difficult to become pregnant in the first place (primary infertility).

The only currently accepted treatment for this condition is the removal of the rudimentary horn and sometimes a cerclage to decrease the risk of preterm delivery.

Didelphic Uterus

didelphic or "double" uterus is a condition in which there are two uteri, and sometimes also two cervices and two vaginas. This condition is quite rare and appears to have a genetic origin (it runs in families). Most women do not have any symptoms prior to becoming pregnant, though some have heavy menstrual periods.

In addition to increasing the risk of miscarriage, a didelphic uterus increases the risk of preterm delivery. It's recommended that women who are living with this condition and want to conceive consult an expert in high-risk pregnancy.

T-Shaped Uterus

A T-shaped uterus is another type of congenital malformation of the uterus that is associated with recurrent miscarriages and an increased risk of preterm labor. Some women who have a T-shaped uterus do not experience problems, while others do.

This specific malformation is sometimes found in women whose mothers took a synthetic estrogen called diethylstilbestrol (DES), which was prescribed to some pregnant women between 1938 and 1971. DES can also cause an increased risk of other pregnancy problems.

Cervical Insufficiency

Cervical insufficiency, or an incompetent cervix, means that a woman's cervix begins to dilate too early in pregnancy—resulting in preterm delivery and sometimes second-trimester pregnancy loss. Cervical insufficiency is not a factor in first-trimester miscarriage. It can be related to congenital malformations or may develop during adulthood.

The condition may occur as a part of a congenital abnormality of the uterus such as a bicornuate or unicornuate uterus, or a short cervix. Acquired causes include procedures such as LEEP, cone biopsy, and repeated D and C's.

Most women do not have any symptoms prior to preterm labor. When it is caught in time, and in subsequent pregnancies, cervical cerclage may be considered.

Fibroids

Roughly 30 to 50 percent of women have uterine fibroids. Some types of fibroids can cause miscarriage or other pregnancy complications. Fibroids usually develop during adulthood.

The chance that a fibroid can lead to miscarriage depends on its location within the uterus. Submucous fibroids (those that project into the uterine cavity and change its shape) and intracavitary fibroids (those within the uterine cavity) are more likely to cause a miscarriage than intramural fibroids (fibroids within the uterine wall) or subserosal fibroids (fibroids outside the uterine wall). Fibroids that lie closer to the middle of the uterus are also more concerning, as well as those that are larger in diameter.

Though medications may be used and a hysterectomy is sometimes needed, the treatment of choice is a myomectomy, a procedure in which the fibroids are removed surgically.

Tipped Uterus

Many women are concerned to hear that they have a tipped or "retroverted" uterus, but there is no evidence that a tipped uterus increases the risk of first-trimester miscarriage.

Of the roughly 20 percent of people who have a tipped uterus, some have pain during intercourse. A tipped uterus can also make it more difficult to find the uterus during a first-trimester ultrasound.

Almost always, a tipped uterus corrects itself during pregnancy. In rare cases, however, a retroverted uterus can result in something called an "incarcerated uterus" during the second trimester of pregnancy, a condition that causes abdominal pain, rectal pain, and urinary obstruction.

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