How do fibroids affect pregnancy?

Several ways. Fibroids or myomas are balls of muscle within the wall of the uterus. They can sometimes make it difficult for the fetus to turn to head first before labor.
Be proactive. If you are given a diagnosis of fibroids in your 20s and 30s before you are pregnant, do not accept the common instruction of "if they aren't bothering you, no need to bother them". Decrease your dietary estrogen intake by eating organically or at least avoiding the "dirty dozen". Decrease sugar, trans fats, and refined carbs. Get your body fat % down to a healthy range. This is a good start.
Fibroids. Fibroids may or may not affect the pregnancy. If it increases in size it may cause pelvic pain which can lead to premature contractions. It also may prevent the uterus from contracting effectively, especially postparum causing hemorrhage . Occasionally, it may cause the baby to present in an abnormal position, increasing your risk of a c section. In some cases it may cause infertility & abortions.
Uterine fibroids. Fibroids or leiomyoma are benign muscle tumors of the uterus present in many woman. The smaller fibroids <3cm pose no risk. Larger fibroids may be problematic in that they may grow to significant size (cantelope size) and may obstruct the outlet for the baby. Sometimes a cesarean may be indicated. These are not removed during delivery or cesarean but wait til 6 months after. They can shrink.
Manifold effects. Fibroids can obstruct the birth canal and require cesarean section, even the classical type with excessive blood loss. They can also cause subfertility, increase the fetal wastage risk in the first trimester and lead to painful degeneration at any time during pregnancy or postpartum. Fibroids can also lead to postpartum hemorrhage and may require hysterectomy for their management.
Usually they don't. Many women have fibroids and sometimes they are just discovered during a sonogram. They do grow with pregnancy and sometimes can cause pain. But most of the time they don't have any impact on the pregnancy. They can be a problem if they are so large that it is difficult to estimate the size of the baby or if their location blocks the cervix or causes the baby to stay breech or sideways.
It depends. Fibroids may be submucosal (in the uterine lining), intramural (in the uterine wall), or subserosal (just beneath the outer uterine surface). Sometimes they grow during pregnancy with the increased uterine blood supply and can cause preterm labor or pain. Sometimes they remain small and asymptomatic. If you have known fibroids your doctor will follow them closely during your pregnancy.
It varies. Most fibroids will grow during pregnancy, but usually cause no harm. There are, however, many potential complications including infertility, miscarriages, preterm labor, inability of the baby to grow properly, potential for the baby to be under stress, abnormal position in labor, need for cesarean section, and postpartum bleeding. Work with your doctor to identify any complications early.
Sometimes a problem. Fibroids are estrogen sensitive. Pregnancy is a high estrogen state, therefore, fibroids can grow during this time. Fibroids can prevent pregnancy, increase miscarriage, lead to breech presentation, cause pain as they grow, cause preterm labor, lead to postpartum hemorrhage, and increase the need for cesarean section. You may also need a cesarean if you have had fibroids removed before.
Usually no problem. Fibroids can grow duing pregnancy and cause increased discomfort. Rarely they can cause preterm labor or bleeding . They can make a c-section more difficult.
Multiple ways. Every fibroid is different in size and location. Some women have 1 fibroid and others have 10-20 at a time. Fibroids may grow and cause pain in pregnancy. Some can make delivery difficult. And some do not cause any problems at all.
Mostly no effect. Fibroids generally do not cause too many problems in pregnancy. Sometimes they can cause pain. Sometimes they can prevent the baby from dropping into the vagina.