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Anatomical Causes of Recurrent Pregnancy Loss
A blog by Joseph A. Hill, III, MD, Fertility Centers of New England
Sometimes, a woman’s uterus has physical differences that may reduce her chances of a successful pregnancy and increase the chance of miscarriage. These potential anatomical causes of recurrent pregnancy loss can be divided into either:
- congenital anomalies, meaning a woman is born with them, or
- acquired anomalies, meaning a woman was not born with them and has acquired them since birth.
Anatomical anomalies are associated with adverse pregnancy outcomes, which is why they are labeled as potential causes of pregnancy loss. Being associated with pregnancy loss does not necessarily mean they cause pregnancy loss; however, these anomalies should not be summarily dismissed as being unimportant.
The most common congenital malformation of the uterus that has been associated with recurrent pregnancy loss is a septate uterus or intrauterine septum. This anomaly occurs in the early female fetus due to incomplete reabsorption of the septum formed where the two uterine horns fused during development.
Embryologically, the uterus and fallopian tubes — called the mullerian system — start out as two separate, horn-shaped structures near the kidneys and migrate down into the fetal female pelvis where they join. The area where they join is a septum made of fibrous tissue that extends from the top of the uterus into the upper third of the vagina. So, initially after mullerian fusion, there is a large septum in all female fetuses.
But in some women reabsorption of this septum is not complete, and in the majority of such uncompleted cases there is a fibrous band extending a centimeter or more into the intrauterine cavity. This anomaly is called a septate uterus, which is different from a true bicornuate (two-horned) uterus that occurs when there is incomplete mullerian fusion.
A true bicornuate uterus is not traditionally associated with miscarriages, but is associated with preterm labor; while a septate uterus is not associated with preterm labor but is associated with miscarriage and, in some cases, infertility. Implantation difficulty leading to either infertility or miscarriage may occur due to an inadequate blood supply to this septum.
Abnormal-Shaped Uterus Due to DES
Another congenital uterine abnormality associated with recurrent pregnancy loss, and possibly infertility, is an abnormal shaped uterus caused by a female fetus being exposed to Diethylstilbesterol or DES, a synthetic estrogen that women took to prevent miscarriage or preterm delivery between 1938 and 1971. DES may have caused a “T-shape” uterus in women whose mothers took this drug during pregnancy.
Women who were exposed to DES in utero tend to have smaller (hypoplastic) than normal uteruses. DES is also associated with uterine cervical abnormalities that may increase the chance for cervical cancer. Thankfully, this drug was taken off the market in 1973 and is no longer prescribed.
Acquired anatomical anomalies associated with recurrent pregnancy loss are lesions that have occurred since birth. These abnormalities involve lesions that either increase or decrease the configuration of the intrauterine cavity. Such lesions include:
- uterine polyps,
- uterine fibroids,
- intrauterine adhesions (scars), and
- adenomyosis, a condition in which the uterine lining (endometrium) grows into the uterine musculature.