By Zaira Salvador BSc, MSc (embryologist).
Last Update: 02/15/2018

The uterus or womb is one of the most important reproductive organs of females along with the ovaries. The uterus is a secure home for the fetus during the 9 months pregnancy.

Any anomalies in the Müllerian ducts, also referred to as Congenital Uterine Anomalies (CUA), of the developing embryo are likely to compromise the fertility of the woman, either by causing repeated implantation failure or recurrent miscarriages. Mild uterine malformations usually have no impact on future pregnancies, though.

Uterine anomalies are congenital in most of the cases. However, sometimes they are due to certain pathologies that occur later in life.

The different sections of this article have been assembled into the following table of contents.

Müllerian duct development

The uterus is an inverted, pear-shaped muscular organ that is located under the ovaries, in the middle of the lower abdomen. A normal uterus is about 7 centimeters long, 5 centimeters wide, and 2.5 centimeters thick.

The formation of the uterus starts as early as on the first weeks of embryo development thanks to the growth of the Müllerian ducts, once the female sex of the embryo is determined. On the contrary, the Müllerian ducts disappear if the embryo develops the male sex due to the action of the anti-Müllerian hormone.

The Müllerian ducts are embryonic structures developed by female embryos. They are responsible for the formation of the female reproductive system: womb, Fallopian tubes, cervix, and parts of the vagina.

During embryogenesis, the following steps are involved in the process of female sex differentiation:

  • Formation of the Müllerian ducts
  • Union or fusion of both ducts
  • Resorption of the central septum that was initially separating them

Any kind of abnormality that occurs at any of these stages will cause congenital Müllerian duct anomalies (MDAs), that is, congenital uterine malformations.

Causes & diagnosis

Uterine anomalies are present in about 5.5 percent of the female population. The causes are varied:

  • Genetic abnormalities, whether hereditary or not
  • Radiation exposure during pregnancy
  • Intrauterine infections while pregnant
  • Certain drugs such as dietilestilbestrol and thalidomide

Oftentimes, uterine malformations go unnoticed by patients until they try to conceive and find that pregnancy never occurs. In fact, some women who get pregnant naturally don’t know if their uterus has an abnormal shape or not.

Uterine malformations don’t cause any symptoms like pain or menstrual irregularities. Thus, there are no visible signs that make the woman suspicious that she may have one.

For this reason, these anomalies can only be diagnosed through the following tests:

Transvaginal ultrasound
It is the easiest of all diagnostic tests, and is usually done on routine gynecological exams using an ultrasound scan.
Hysterosalpingography
It is an X-ray examination of the uterus and the Fallopian tubes.
Hysteroscopy
Your health care provider uses a thin viewing tool called hysteroscope to look at the lining of your uterus.

These diagnostic tests are common female fertility tests. They are performed when a woman visits the fertility specialist for the first time, after having realized that pregnancy has not occurred after a year trying to conceive.

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Classification of uterine anomalies

To date, multiple classifications of the different uterine anomalies have been made, mainly due to the wide number of abnormalities, their subtypes, and factors that influence them.

However, the most commonly used classification due to its simplicity and relationship with infertility issues is the one published by the American Society for Reproductive Medicine (ASRM) in 1988:

Müllerian agenesis (MRKH syndrome)

Müllerian agenesis (MA) is a malformation characterized by the absence of uterus and, oftentimes, other organs such as the Fallopian tubes, cervix, and part of the vagina. It is due to a failure of the Müllerian ducts to develop during embryogenesis.

This congenital malformation is also known as Müllerian aplasia, Mayer-Rokitansky-Küster-Hauser syndrome, vaginal agenesis, or MRKH syndrome. Girls affected by MA usually don’t have menstrual periods when they reach adolescence. On the contrary, their sexual development is normal, as they have ovaries.

The MRKH syndrome represents 5-10% of all cases of uterine anomalies. Women affected by it have no alternative but to use a surrogate should they wish to have babies.

Surrogacy is probably the most confusing of all infertility treatments. Transparency is a key value for us when it comes to recommending a clinic or agency for intended parents. You can now use The Calculator to receive a detailed report that will solve any question you may have, and most importantly, to help you avoid potential frauds.

You may also enjoy some further information reading this: What Is the Mayer-Rokitansky-Küster-Hauser (MRKH) Syndrome?

Unicornuate uterus

A unicornuate uterus is smaller than a typical uterus, as it is formed from one only of the paired Müllerian ducts. Its shape resembles a banana, with a unique Fallopian tube.

It has a prevalence of 20% and can appear in any of the following forms:

  • Without rudimentary uterine horn (35% of the cases)
  • With communicating uterine horn containing a functional endometrium (10%)
  • With non-communicating uterine horn containing a functional endometrium (22%): it can cause endometriosis
  • With rudimentary horn that is non-functional (33%)

A unicornuate uterus does not make a woman completely childless. In principle, she can carry a child until birth without complications. However, the risk for preterm birth and C-section may be higher.

Also, it increases the odds of having an ectopic pregnancy, as it is likely that the embryo attaches to the rudimentary uterine horn containing a functional endometrium.

Uterus didelphys

Women with this uterine anomaly, which is sometimes called uterus didelphis or double uterus, have two independent uteruses of a smaller size each.

A uterus didelphis develops when the paired Müllerian ducts do not fuse, but continue developing with normality into a uterus.

This anomaly represents 5% of the cases of Müllerian anomalies of the uterus and can be found in two different forms:

Uterus didelphys bicollis
Each uterus has its own cervix and vagina. It occurs in two-thirds of the cases.
Uterus didelphys unicollis
Both uteruses are communicated between each other through the cervix, and there is only one vagina. It is associated with cervical insufficiency.

The chances of getting pregnant for women with uterus didelphys are almost normal. However, it can lead to the following complications: recurrent miscarriage, premature labor, complicated labor, etc.

Bicornuate uterus

A bicornuate uterus develops when the fusion of the Müllerian ducts is unfinished. This is the reason why it can be confused with uterus didelphys (see the previous section to learn more).

Women with a bicornuate uterus have two symmetric uterine cavities, but the difference is that the horns are not fully developed, which causes them to be smaller than in cases of uterus didelphys.

This Müllerian abnormality is also known as heart-shaped uterus due to the dip that can be seen at the top of the uterus. Depending on its shape, we can differentiate between:

Complete bicornuate uterus
It has a complete transverse vaginal septum, which extends to the external cervical orifice of the uterus (OS).
Partial bicornuate uterus
It has a lesser degree of septation, which extends to the internal cervical OS.

The septum separates both cavities and is composed of a myometrium (middle layer of the uterine wall). Depending on the severity, the remaining space in the cervical canal will be more or less reduced, which has a direct impact on fetal development, and may increase the miscarriage and preterm birth rates.

Septate uterus

It is the most common uterine anomaly, with a prevalence of 55%. Defects in the stage of septal resorption result in a septate uterus.

The uterine cavity is partitioned by a longitudinal septum, which can involve the endometrial canal but not the cervix (partial septate uterus), or extend to the internal or external cervical OS (complete septate uterus).

This type of congenital uterine anomaly is associated with recurrent miscarriage to a large extent, as the risk of miscarriage with a septate uterus reaches 60%.

If an ongoing pregnancy occurred, the rate of preterm birth ranges between 12% and 33%.

In spite of that, a septate uterus can be shaved off through hysteroscopy. Reproductive outcomes are usually very positive after resecting the septum, thereby increasing the pregnancy success rates while reducing the miscarriage risk.

Arcuate uterus

This type of uterine abnormality is considered a variant shape of the normal uterus. There is only a small septum caused by defects in the septal resorption stage during embryo development.

This uterine malformation is actually rather common, but is rarely a cause of female infertility. Eighty five percent of the cases of arcuate uterus result in a normal, ongoing pregnancy, without complications during labor.

FAQs from users

What is uterine arteriovenous malformation?

By Zaira Salvador BSc, MSc (embryologist).

Uterine Arteriovenous Malformation (UAVM) is considered a rare condition that can cause a life-threatening heavy bleeding in young women. It results from the formation of several arteriovenous fistulous communications within the uterus, with a mixture of small capillary-like vessels.

UAVM is associated with multiple births, miscarriage, or previous surgery (D&C, C-section, abortion). It can be treated with a technique called transcatheter arterial embolisation.

Are uterine anomalies hereditary?

By Zaira Salvador BSc, MSc (embryologist).

The truth is that the causes of uterine anomalies are still not known, but there is proof that they are a combination of genetic and environmental causes at the same time. However, these conditions are not thought to run in families.

Can a heart-shaped uterus cause a miscarriage?

By Zaira Salvador BSc, MSc (embryologist).

In principle, women with a bicornuate uterus or heart-shaped womb are not expected to have extra difficulties with conception. Nevertheless, there is a slightly higher risk of miscarriage as well as premature birth.

Can a woman become pregnant without a uterus?

By Zaira Salvador BSc, MSc (embryologist).

Unfortunately, there is no way for a woman without a uterus to become pregnant, either if it’s congenital (MRKH syndrome) or acquired (hysterectomy). The uterus is necessary for the implantation of the embryo. These women may need to turn to surrogacy or adoption to have children.

Learn more: Guide to the Surrogacy Process – The 7 Steps of Surrogacy.

How do congenital uterine anomalies affect reproductive outcomes?

By Zaira Salvador BSc, MSc (embryologist).

The effects of uterine abnormalities on pregnancy outcomes depend on the type of anomaly that is present. Broadly speaking, they are commonly associated with:

  • Miscarriage
  • Delivery before 37 weeks
  • Malpresentation in labor (presentations of the fetus other than vertex)
  • Fetal growth restriction
  • Placental abruption

Why is my uterus tilted backwards?

By Zaira Salvador BSc, MSc (embryologist).

It can occur for several reasons, including: failure of the uterus to move to the forward position as the woman matures, childbirth, intrauterine adhesions and scarring leading to endometriosis or fibroids.

Can uterus didelphys cause infertility?

By Zaira Salvador BSc, MSc (embryologist).

Uterus didelphys rarely causes problems. In fact, some women don’t even know they have it, while others experience very painful menstrual periods. Uterus didelphys can be the cause of recurrent miscarriages. Though rarely, if the woman gets pregnant with fraternal twins, they might be delivered hours or days apart.

Can you have a baby if you have a bicornuate uterus?

By Zaira Salvador BSc, MSc (embryologist).

Yes, there is no problem in principle. However, there’s a higher risk for miscarriage and preterm birth, giving that the septum can prevent the baby from growing properly.

What animals have a bicornuate uterus?

By Zaira Salvador BSc, MSc (embryologist).

A bicornuate uterus has been found in various species, including kangaroos, Tasmanian devils, mice, rats, guinea pigs, rabbits, cats, deers, moose, horses, and hares, among others.

What is the difference between a septate and bicornuate uterus?

By Zaira Salvador BSc, MSc (embryologist).

A bicornuate uterus occurs when the two Müllerian ducts do not fuse completely. On the contrary, it is considered a septate uterus when the two Müllerian ducts fuse but the resorption of the septum is incomplete.

What does it mean if you have two uterus?

By Zaira Salvador BSc, MSc (embryologist).

Having two uteruses is a uterine malformation known as didelphic uterus or uterus didelphys. It means that you literally have two uteruses and two cervices. In some cases, the woman has two vaginas as well. It is a very rare condition, though.

What does it mean when you have a T shaped uterus?

By Zaira Salvador BSc, MSc (embryologist).

A T-shaped uterus is another type of uterine anomaly wherein the shape of the uterus resembles the letter T. It has been associated with pregnancies of women who took diethylstilbestrol while pregnant (DES-exposed women). Daughters affected by this condition are commonly referred to as DES daughters.

Suggested for you

When a couple has been trying to conceive for a long time without luck, the next step is to visit a fertility specialist to be counseled about their potential fertility options. To learn more about the basic tests that are usually required for females, check this out: Female Fertility Tests – How Do You Know if You Can’t Get Pregnant?

Throughout this post, we have made multiple references to repeated or recurrent implantation failure (RIF). Want to learn more? Read: Repeated Implantation Failure – Reasons Why Embryos Don’t Implant.

Women with severe uterine anomalies, that is, abnormalities that prevent them from carrying a child until birth, can turn to surrogacy to become mothers. Get more info by clicking the following link: What Are the Different Types of Surrogacy? – Names & Definition.

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Authors and contributors

 Zaira Salvador
BSc, MSc
Embryologist
Bachelor's Degree in Biotechnology from the Polytechnic University of Valencia (UPV). Embryologist specializing in Assisted Procreation, with a Master's Degree in Biotechnology of Human Assisted Reproduction from the University of Valencia (UV) and the Valencian Infertility Institute (IVI). More information
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