While there is no exact definition of a small uterus, one that is about 25% smaller than average should still be able to support a pregnancy. The average uterus is 3 inches (7.5 cm) long, 2 inches (5 cm) wide, and 1 inch (2.5 cm) deep. Symptoms of a small uterus include irregular or no periods and abdominal pain. However, many women are unaware of uterine abnormalities as there may be no symptoms at all.
A healthy uterus is pear-shaped, hollow, and lined with thick muscular walls. There are two fallopian tubes, each connected to an ovary filled with eggs entering the uterus on either side at the top. A uterus that differs in structure from the norm is considered an abnormality which may or may not lead to difficulties in conception and pregnancy. The uterus, normal-sized or smaller than average, stretches and grows when stimulated by the hormones released during pregnancy. It is only when the size is an indication of other conditions that a small uterus may lead to problems.
What Medical Conditions Are Associated With a Small Uterus?
A small uterus, also known as uterine hypoplasia or hypoplastic uterus, is a congenital condition. Congenital disorders are those which are present in the child from birth. The causes of this abnormality are still unknown.
The process of development of the female reproductive system begins in the embryo with the primordial Mullerian ducts. These are paired ducts that differentiate to form the fallopian tubes, uterus, and vagina in the female child.
A Mullerian duct anomaly is a very rare condition affecting one in every 4,500 newborn girls, called uterine agenesis or Mayer-Rokitansky-Küster-Hauser (MRKH) syndrome. Uterine agenesis results in a very small uterus or none at all and is usually present with vaginal agenesis, which is the lack of or incomplete vagina.
Girls born with a small uterus or MRKH syndrome have a typical chromosome pattern, with 46 chromosomes and an XX pairing. Therefore, the abnormality is only physical. However, their ovaries will function normally, and their genitalia will appear normal externally. Also, their breast and pubic hair will develop as usual.
There are two types of MRKH syndrome. In type 1, only the reproductive organs are impacted by the abovementioned effects. In type 2, other organs (aside from the reproductive ones) can also develop abnormally. For example, a person with MRKH syndrome type 2 might have abnormally formed, positioned, or undeveloped kidneys, skeleton abnormalities, or hearing and heart defects.
Women who are born with this condition might not begin menstruating during puberty. This abnormality can also prevent conception or pregnancy, as mentioned previously. In this case, surrogacy is nearly always the only solution for women to have a child.
Another Mullerian duct anomaly leads to a unicornuate uterus. This is generally only half the size of a normal uterus, and there is only one fallopian tube instead of two. As long as this is healthy and developed, then it should be possible for the woman to conceive, but she will need careful monitoring during the pregnancy and birth as having a unicornuate uterus significantly increases the risk of pregnancy loss and preterm labor.
There are many ways of diagnosing a small uterus. The most common test used first is that of the 2D ultrasound scan, which is the one used by most gynecologists or obstetricians. This is enough to indicate that there is a problem, but the images are not usually sharp enough for a diagnosis. Further tests include 3D ultrasound scan, magnetic resonance imaging (MRI), or a minor operation called a laparoscopy.
How Does a Typical Uterus Work During Pregnancy?
The primary function of the uterus during pregnancy is to house and nurture the growing fetus. This organ, lined with muscular walls, has three parts. It is comprised of the fundus (the top of the uterus), the corpus (the main body), and the cervix (the lower region of the uterus). There is also a part connecting the corpus to the cervix: the isthmus. At the isthmus, the uterus starts to narrow between the corpus and cervix.
The fundus functions as the opening of the uterus. It leads into the corpus, where the fetus grows during pregnancy. The corpus expands during pregnancy to hold the baby. The cervix is a cylindrical tissue at the very bottom of the uterus, connecting the uterus and vagina. The baby is pushed through here during labor.
A uterus that has never been pregnant before is typically around 2.5 - 3 inches (about 7 cm) long. On the other hand, a uterus that is not currently pregnant but has been pregnant previously is typically around 3.5 inches (about 9 cm). The uterus can stretch dramatically throughout pregnancy to accommodate the growing baby's size.
The uterus also plays an important role when a woman goes into labor. Contractions arise when the uterus' muscles contract to push the baby into the birth canal. These muscles in the uterus deliver the baby out of the womb and into the world.
After the labor is over and the baby is delivered, the uterus contracts once more to push the placenta out. The muscles stay contracted to prevent hemorrhage (severe bleeding).
How Does the Uterus Work During Menstruation?
The lining of the uterus is called the endometrium, and this lining sheds during menstruation. The endometrial lining changes in size each month as the body prepares to ovulate, ovulate, and then menstruate.
As the body gets closer to ovulation (the process where the ovaries release an egg), the endometrial lining gets thicker to prepare for the possibility of housing a fetus. When this egg is not fertilized, the uterus sheds its endometrial lining, causing menstruation.
Can a Small Uterus Be Treated?
In some cases, surgery can correct abnormal uteruses. However, surgery is a more feasible and likely solution for other uterus conditions, such as the septate uterus. This condition is when the uterus is divided into two regions by a membrane called the septum.
In those cases where the uterus is unicornuate or only 25% smaller than average, any possible pregnancy must be carefully monitored. However, in many cases, women with a small or underdeveloped uterus must find alternative plans for pregnancy.
Surrogacy is one alternative option. In this process, one woman agrees to carry a baby for another woman or couple, often for a sum of money. Surrogate mothers are impregnated through In Vitro Fertilization (IVF). IVF is the procedure in which sperm is used to fertilize an egg in a laboratory dish before being inserted into the uterus.
As we mentioned earlier, a small uterus is a rare condition. For this reason, it is essential that patients who suffer from it have access to adequate information and support to overcome the difficulties this disorder may present.