What Is 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 occurs in about 1 in 4,000 people. A unicornuate uterus usually has only one functioning fallopian tube (rather than two). And, the other side of the uterus may have what is called a rudimentary horn, also called a hemi-uterus.

A rudimentary horn is an underdeveloped “horn” on one side of the unicornuate uterus. About 75% of people with a unicornuate uterus have a rudimentary horn. The rudimentary horn may or may not be connected with the rest of the uterus and vagina. If it is, it is called a "communicating horn."

Signs and Symptoms

A unicornuate uterus is congenital, meaning it is present from birth. Yet many people are unaware they have the condition until they become pregnant and the uterus is seen on ultrasound.

If the rudimentary horn is not connected to the rest of the uterus and vagina, menstrual blood becomes backed up (since it is unable to flow out through the vagina). This can result in very painful periods.

If the rudimentary horn is connected to the rest of the uterus and vagina, or if the person with a unicornuate uterus doesn't have a rudimentary horn, then they may not experience any symptoms until they begin trying to conceive. At that point, they may have difficulty getting and/or staying pregnant. 

Unicornuate Uterus and Pregnancy

Estimates vary by study, but the odds of a full-term delivery of a healthy baby are roughly 50%. Having a unicornuate uterus brings a significant risk of both pregnancy loss and preterm labor, as well as ectopic pregnancy (when a fertilized egg implants outside the uterus and must be terminated).

People 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 an ectopic pregnancy).

This risk of uterine rupture may be as high as 50% when a pregnancy implants in the rudimentary horn. For this reason, doctors sometimes recommend surgery to remove the rudimentary horn.

Miscarriage

Miscarriage appears to occur in roughly a third of pregnancies in cases of unicornuate uteruses. 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).

Preterm Labor

The risk of preterm labor is higher in people with unicornuate uteruses, ranging from 10% to 20% higher than in people with typical uteruses. Since a unicornuate uterus is smaller than a typical uterus, the growth of the baby might trigger early labor.

Other Pregnancy Complications

Other pregnancy complications which are increased with uterine anomalies include:

• Malpresentation (such as breech presentation or transverse lie)
• Premature rupture of membranes (which often leads to preterm labor)
• Intrauterine growth retardation
• C-section delivery
• Placenta previa
• Placental abruption
• Intrauterine fetal demise (stillbirth)

Diagnosis 

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 during regular pelvic exams.

Imaging studies, such as hysterosalpingogram (HSG) or ultrasound, may show 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 tiny camera through the cervix to visualize the inside of the uterus), three-dimensional ultrasound, or laparoscopy might also be used to confirm the diagnosis.

Causes

Scientists aren't sure what causes a unicornuate uterus. It happens when the uterus doesn't form properly while a fetus is developing.

Treatment

Researchers are experimenting with surgical treatment methods for the unicornuate uterus. Currently, the only generally accepted treatment is surgical removal of the rudimentary horn when needed and careful monitoring of pregnancies which occur.

Doctors may recommend a surgical procedure called cerclage for people at risk of preterm labor—it's a procedure in which the cervix is sewn closed during pregnancy. 

Coping

If you've 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 and may or may not connect. The size of the uterus can also vary.

A Word From Verywell

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 they 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 people 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.