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 which are 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.
The process of development of the female reproductive system begins in the embryo with the primordial Mullerian ducts. These are paired ducts which differentiate to form the fallopian tubes, uterus and vagina in the female child. A very rare condition affecting one woman in many thousands, called uterine agenesis, is a Mullerian duct anomaly. 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. 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 unicornate 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.