Abnormal Uterus Shapes and Miscarriage Risks

An abnormal uterus can sometimes be a risk factor for miscarriage and, in some cases, a cause of recurrent miscarriages. However, only some types of uterine malformations increase the risk of miscarriage and require treatment. Others may not cause any problems with pregnancy at all. 

About 18% 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 hysterosalpingogram (HSG), but this test can miss some conditions, such as uterine septum. Any abnormal HSG should be followed by a hysteroscopy.

Uterine Septum

normal shaped uterus
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A uterine septum (septate uterus) is the most common congenital uterine anomaly, comprising roughly 55% of mullerian duct anomalies. Congenital 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 significant. In fact, in one study nearly 67% of the women in the study with a septate uterus experienced pregnancy loss.

For women who do not miscarry, a septate uterus may increase the risk of preterm birth. In fact, the same study found that the likelihood of having a full-term, normal-sized baby was only on 25%.

Treatment is usually minor surgery, performed during a hysteroscopy, to remove 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. Pregnancy with a unicornuate uterus is generally considered to be high risk, which will mean extra monitoring.

One study found that the risk of miscarriage in the first trimester was nearly 25% ​and the risk of preterm birth was, on average, 44%. Meanwhile, the live birth rate was only 29.2%.

Unlike women with a bicornuate uterus, women with a unicornuate uterus may have symptoms that are suggestive of the anomaly before they become pregnant. A significant number 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.

Women with a unicornuate uterus often have only one functioning fallopian tube (as opposed to two). While it is possible to become pregnant with one fallopian tube, women with a unicornuate uterus may have an increased risk of infertility.

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 20 to 60% of reproductive age 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, the treatment of choice is myomectomy, a procedure in which the fibroids are removed surgically. Studies show that myomectomy relives fibroid-related symptoms in 80 to 90% of women. Hysterectomy is only considered when no other options resolve the situation. In the past, this surgery was more common for fibroids, but today there are better and less extreme options for most women. If your doctor suggests hysterectomy, get a second opinion before you proceed.

Tipped Uterus

Of the roughly 20% 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.

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.

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, this condition causes abdominal pain, rectal pain, and urinary obstruction.

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