An Overview of a Unicornuate Uterus

How Pregnancy Is Affected

Pregnant African American woman holding her stomach in hospital
Jose Luis Pelaez Inc / Getty Images

You may have been told that you have a unicornuate uterus, or instead, you may have had a miscarriage and are concerned that a unicornuate uterus may be responsible. What exactly is this condition and what role may it play in pregnancy and miscarriage?

What a Unicornuate Uterus? 

A unicornuate uterus is a type of congenital uterine abnormality (müllerian duct abnormality). It is smaller than a typical uterus (often about half the normal size and usually has only one functioning fallopian tube (rather than two).

The other side of the uterus may have what is called a rudimentary horn.

A unicornuate uterus is congenital, meaning that you are born with it, yet many women are unaware they have the condition until they become pregnant.

What is a Rudimentary Horn?

A rudimentary horn is an underdeveloped “horn” on one side of the unicornuate uterus. About 65 percent of women with a unicornuate uterus have a rudimentary horn. The rudimentary horn may or may not be connected (called a "communicating horn") with the rest of the uterus and vagina.

Symptoms of a Rudimentary Horn

If the rudimentary horn is not connected to the rest of the uterus and vagina, then a woman may experience very painful periods because the menstrual blood becomes backed up (since it is unable to flow out through the vagina.) If the rudimentary horn is connected to the rest of the uterus and vagina, or if a woman doesn't have a rudimentary horn, then a woman may not experience any symptoms throughout her lifetime until she begins trying to conceive.

At that point, she may experience difficulty getting and/or staying pregnant (discussed below). 

Unicornuate Uterus Statistics

In women with normal reproductive histories, about two to four percent are thought to have some sort of congenital uterine anomaly, with a unicornuate uterus present in roughly one in 1000 women.

In women who have a history of recurrent miscarriages, the incidence of the unicornuate uterus is much higher, ranging from five to 30 percent.

Roughly two to eight percent of women evaluated for infertility are found to have a unicornuate uterus.

Diagnosing Unicornuate Uterus

A unicornuate uterus may be suspected based on a history of infertility, recurrent miscarriages, or premature birth. Most of the time, it is not detected on regular pelvic exams.

Imaging studies, such as hysterosalpingogram (HSG) or ultrasound, may indicate that a woman most likely has a unicornuate uterus. In a hysterosalpingogram, a dye is inserted through the cervix into the uterus and then x-rays are taken to visualize the uterus and fallopian tubes.

A hysteroscopy (a test in which a doctor inserts a small telescope through the cervix to visualize the inside of the uterus), three-dimensional ultrasound, or laparoscopy might also be used to confirm the diagnosis.

Treatment Options for a Unicornuate Uterus

Researchers are experimenting with surgical treatment methods for the unicornuate uterus but right now, the only generally accepted treatment is surgical removal of the rudimentary horn when needed (see below) and careful monitoring of pregnancies which occur.

What a Unicornuate Uterus Means for Miscarriage and Pregnancy

Having a unicornuate uterus, unfortunately, brings a significant risk of both pregnancy loss and preterm labor, as well as ectopic pregnancy (that's when a fertilized egg implants outside the uterus—usually in the fallopian tubes—and must be terminated). Estimates vary by study, but the odds of a full-term delivery of a healthy baby are roughly 50 percent. Miscarriage appears to occur in roughly a third of pregnancies. The risk of preterm birth is also significantly increased, ranging from 10 to 20 percent.

Doctors believe that the higher miscarriage risk is due to abnormalities in the blood supply of the unicornuate uterus that might interfere with the functioning of the placenta (or increase the odds of implantation in the fallopian tubes.)

Other pregnancy complications which are increased with uterine anomalies include:

Unicornuate Uterus and Preterm Labor

The higher risk of preterm labor exists because of space restrictions; since a unicornuate uterus is smaller than a typical uterus, the growth of the baby might trigger early labor. Doctors may recommend a surgical procedure called cerclage for women at risk of preterm labor—it's a procedure in which the cervix is sewn closed during pregnancy. 

Pregnancies Which Implant in the Rudimentary Horn

Women who have a rudimentary horn that connects with the rest of the uterus face additional risk. Since the rudimentary horn of the unicornuate uterus has considerable space restrictions, a fertilized egg that's implanted there faces the possibility of rupture (similar to ectopic pregnancies that implant in a fallopian tube). This risk of uterine rupture may be as high as 50 percent when a pregnancy implants in the rudimentary horn. For this reason, doctors sometimes recommend surgery to remove the rudimentary horn.

Bottom Line on Unicornuate Uterus

If you've recently learned you have a unicornuate uterus, you're probably feeling frightened and frustrated. Reading about the statistics can worsen those fears, yet it's important to understand what you are facing.

Keep in mind, however, that there are many different variations to a unicornuate uterus—they are not all the same. A rudimentary horn may or may not be present, may or may not connect, and the size of the uterus can also vary.

Having a unicornuate uterus can increase the risk of infertility miscarriage, and preterm birth, plus lead to ectopic pregnancy or the risk of rupture if the fetus implants in the rudimentary horn. There are also additional pregnancy complications which may occur.

On the other hand, it's important to emphasize that nearly have of the women in a few large studies have gone on to deliver full term healthy babies (and some women have even delivered healthy twins despite a unicornuate uterus.)

If you've been diagnosed with a unicornuate uterus, make sure to ask a lot of questions. Ask about a rudimentary horn. If you have had a miscarriage, ask your doctor what she might expect if you are to become pregnant again, remembering that there are many variations of the condition. Inquire about how your pregnancy would be monitored if you choose to become pregnant. It's often recommended that women with a uterine anomaly be followed by a perinatologist or obstetrician who specializes in high-risk pregnancies.

Most of all, face your future in a way that you are being true to yourself, not honoring the opinion of someone else. Many women choose to become pregnant even when there are increased risks involved, and many complications can be averted or decreased with careful and watchful medical care.

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