Why do fibroids shrink during pregnancy? How likely are they to reappear later?

--Robin

Fibroids don't necessarily shrink during pregnancy -- they often grow! Fibroids tend to grow under the influence of estrogen, which is greatly increased during pregnancy. Progesterone tends to keep them in check, and progesterone is also greatly increased during pregnancy. The relative balance between estrogen and progesterone is probably important in determining what happens in any given pregnant woman with fibroids.

In pregnancy, fibroids may cause absolutely no problems -- or they may be associated with miscarriage, preterm labor and delivery, or poor \growth of the baby. The size and location of the fibroids within the uterus are factors. Small or solitary fibroids don't usually cause any problems. A large fibroid located right under the placenta may interfere with blood supply to the placenta and consequently deprive the baby of nutrition. A fibroid that is near the cervix may interfere with labor. Ultrasound is very helpful in determining where the fibroids are, how large they are and whether they are growing during the course of a pregnancy.

If a fibroid causes you -- and the baby -- no problems during a pregnancy, it can be re-evaluated after delivery. Usually, a pelvic exam alone is sufficient to check the size of your uterus. In general, if fibroids are not bothering you (no pain, pressure or heavy bleeding), we don't bother them.