Fibroids are a benign growth of uterine muscle cells that can impair fertility in three ways. Fibroids may inhibit conception, cause pregnancy loss due to poor placental attachment or may cause pregnancy complications such as pre-term birth, breech birth or postpartum hemorrhage. The effect that fibroids have on fertility is dependent on the number, size and location of the fibroids.
Prevention of Pregnancy
The presence of fibroids alone is not always a cause of infertility. The size, number and placement of fibroids determine whether conception will be inhibited. Fibroids are abnormal growths of uterine tissue that grow either superficially in the top uterine layer (submucosal), more deeply in the middle muscle layers (intramural) or located mostly on the outside wall of the uterus (sub-serosal) facing the abdominal cavity. Fibroids projecting into the uterine cavity where embryo implantation takes place are most likely to prevent conception. Uterine fibroids can interfere with normal uterine contractions, which help transport sperm to the egg for fertilization to occur. Fibroids may grow too close to the uterine opening of the Fallopian tube, blocking access to the tube. If Fallopian tubes are blocked, fertilization cannot happen because eggs and sperm come together in the Fallopian tube. Although controversial, some physicians strongly recommend surgical removal of fibroids (myomectomy) before undergoing assisted reproductive technology procedures to increase the chances of conception and full-term pregnancy.
Pregnancy Loss
If embryo implantation occurs over a submucosal fibroid, the fibroid is likely to interfere with formation of a strong placental attachment site, which can lead to uterine bleeding and failure of the placenta. If the placenta's blood supply is interrupted or suboptimal, the pregnancy may be lost. Submucosal fibroids that push out into the uterine cavity are more likely to interfere with strong placental attachment to the uterus. Pregnancy, if it occurs, is more likely to be lost in women who have submucosal fibroids. Deeper fibroids (intramural or sub-serosal) may not prevent pregnancy, but depending on their size and position may cause problems or fetal loss later in the pregnancy.
Pregnancy Complications
Other complications of pregnancy caused by uterine fibroids are pre-term labor, tearing of the placenta from the uterus (placenta abruptia) and slowed fetal growth. Because the fibroid continues to grow inside the uterus alongside the baby, it may be large enough to affect how the baby lies in the uterus, causing a breech presentation, which often requires Cesarean section for the delivery. Having a large fibroid in the uterus may cause the placenta to attach too close to the cervical opening so that the growing placenta may actually grow over the cervical opening (placenta previa), requiring a Cesarean-section delivery. The risk of postpartum hemorrhaging is also greater in pregnancies complicated by fibroids.
