A unicornuate uterus is a uterus that developed from only one side of the body, resulting in a smaller, banana-shaped organ instead of the typical triangular shape. It affects roughly 0.03% to 0.1% of the general population and accounts for 5% to 20% of all congenital uterine anomalies. Many people don’t know they have one until they experience difficulty conceiving or have complications during pregnancy.
How a Unicornuate Uterus Forms
During early fetal development, two tube-like structures (called Müllerian ducts) normally fuse together to form the uterus. In a unicornuate uterus, one of these ducts fails to develop fully, so the uterus forms from only one duct. The result is a uterus that’s about half the normal size, shifted to one side of the pelvis, and typically connected to only one fallopian tube on that side.
Because the kidneys develop around the same time and in close coordination with these structures, kidney abnormalities on the affected side are common. Some people with a unicornuate uterus are missing a kidney entirely or have a kidney that didn’t form in the correct position. This is why imaging of the kidneys is often recommended after diagnosis.
The Four Subtypes
Not every unicornuate uterus looks the same. The American Society for Reproductive Medicine classifies it into four subtypes based on what happened to the undeveloped side:
- Communicating rudimentary horn: A small, partially formed second half of the uterus exists and has an open connection to the main uterine cavity.
- Non-communicating rudimentary horn: A small second horn exists but has no connection to the main cavity. This is the subtype most likely to cause problems, because menstrual blood or even a pregnancy can become trapped inside it.
- Rudimentary horn with no cavity: A small horn is present but contains no internal space, so it’s largely inactive.
- No horn at all: The undeveloped side is completely absent.
Knowing which subtype you have matters because it directly affects your risk profile and whether surgical intervention might be recommended.
How It’s Diagnosed
A standard pelvic ultrasound can raise suspicion, but it often can’t distinguish a unicornuate uterus from other uterine shapes with certainty. A 3D ultrasound provides a clearer picture of the uterine cavity’s outline. MRI is considered the gold standard for confirming the diagnosis and identifying whether a rudimentary horn is present, because it can map the full anatomy of the uterus, cervix, and surrounding structures in detail.
Some people first learn about their uterine shape during a hysterosalpingogram (HSG), the dye test used during fertility workups. On HSG, the uterus appears as a single, narrow, curved cavity rather than the typical inverted triangle. However, HSG alone can’t reliably detect a rudimentary horn, so MRI is usually needed as a follow-up.
Effects on Fertility and IVF Outcomes
A unicornuate uterus is found in about 0.78% to 1.0% of people being evaluated for infertility, roughly ten times the rate in the general population. That said, many people with this anatomy conceive without assistance. The smaller uterine cavity and reduced blood supply are the primary factors that make pregnancy more challenging.
For those who pursue IVF, a 2024 study in Fertility and Sterility compared 74 patients with a unicornuate uterus to nearly 3,900 patients with a typical uterus, all undergoing frozen embryo transfers with chromosomally normal embryos. The sustained implantation rate was 50% for those with a unicornuate uterus compared to about 65% for the control group. After adjusting for age, BMI, and embryo quality, the odds of being discharged pregnant were about 40% lower for the unicornuate group. Overall pregnancy loss rates were also higher: 35% versus 21%.
These numbers don’t mean IVF won’t work. Half of the unicornuate uterus group did achieve a sustained pregnancy from a single transfer. But the data helps set realistic expectations and informs decisions about how many transfer cycles to plan for.
Pregnancy Risks
The most significant concern during pregnancy is preterm birth. The uterus is smaller than usual, so as the baby grows, the uterine wall stretches faster and more dramatically. This can trigger early labor. Babies may also grow more slowly due to reduced space and blood flow, a condition known as fetal growth restriction.
Miscarriage rates are elevated across all trimesters. Some of this is related to the limited uterine capacity, and some may be related to how the placenta implants in a smaller, asymmetric cavity. Breech presentation is also more common, since the baby has less room to turn head-down.
For people with a non-communicating rudimentary horn, there’s a rare but serious risk: a pregnancy can implant inside the horn itself. Because the horn has thin, poorly developed walls, it can rupture as the pregnancy grows. The risk of rupture in a rudimentary horn pregnancy is estimated to be as high as 80%, typically occurring before the third trimester, and can cause life-threatening hemorrhage requiring emergency surgery.
Surgical Considerations
Surgery isn’t needed for a unicornuate uterus itself, since the uterine shape can’t be changed. However, removing a rudimentary horn is sometimes recommended, particularly if it’s non-communicating and contains functional tissue that responds to hormonal cycles. A trapped horn can cause recurring pelvic pain from menstrual blood that has no way to exit. It also eliminates the risk of a horn pregnancy. This procedure is typically done laparoscopically.
For people who experience recurrent second-trimester losses due to cervical insufficiency (where the cervix opens too early under the weight of the pregnancy), cervical cerclage, a stitch placed around the cervix to hold it closed, may help. Case reports have documented successful deliveries after laparoscopic cerclage, with babies carried to 35 weeks or beyond. This approach is considered on a case-by-case basis, particularly for those with a history of mid-pregnancy loss.
What Pregnancy Monitoring Looks Like
Pregnancies in a unicornuate uterus are typically classified as high-risk, which means more frequent ultrasounds to track the baby’s growth, cervical length checks to watch for early shortening, and closer monitoring in the third trimester. Many people with this condition deliver by cesarean section, especially if the baby is breech or if there are concerns about how labor would progress given the uterine shape.
Delivery timing varies. Some pregnancies reach full term without complications, while others require early delivery due to growth restriction or preterm labor. The goal of increased surveillance is to catch problems early enough to intervene, whether that means bed rest, medication to delay contractions, or a planned early delivery when the baby is mature enough.