Fibroids are tumors that grow from muscle tissue in the uterus. They are not cancerous. They can be as small as a pea or larger than a grapefruit. They can grow outside the uterine wall, inside the uterine cavity, or within the uterine wall. Many women have multiple fibroids of different sizes.
An estimated 40% to 60% of women have fibroids by age 35. Up to 80% of women have them by age 50. But detecting them during pregnancy isn’t always easy. That’s because it’s hard for doctors to tell fibroids from the thickening of uterus muscles that occurs during pregnancy. For this reason, doctors think the number of known cases is lower than the real number.
Most women who have been diagnosed with fibroids go on to have normal pregnancies, but sometimes they can cause challenges.
Problems During the First Trimester
Most fibroids don’t grow while you’re pregnant, but if it happens it most likely will be during your first 3 months (first trimester). That’s because fibroids need a hormone called estrogen to grow. Your body produces more of it when you’re pregnant.
The primary problems that could occur are:
- Bleeding and pain. In a study of more than 4,500 women, researchers found that 11% of the women who had fibroids also had bleeding, and 59% had just pain. But 30% of the women had both bleeding and pain during their first trimester.
- Miscarriage. Women with fibroids are much more likely to miscarry during early pregnancy than women without them (14% vs. 7.6%). And if you have multiple or very larg fibroids, your chances go up even more.
Second and Third Trimesters
As your uterus expands to make room for your baby, it can push against your fibroids. This can cause a number of issues during your pregnancy:
- Pain. This is the most common symptom of fibroids, especially if they’re large. Sometimes, fibroids twist, which can cause cramping and discomfort. Other times, the fibroid outgrows its blood supply, turns red and dies. This process, called “red degeneration,” can cause severe stomach pains. In some cases, it can lead to miscarriage. Over-the-counter medicines like acetaminophen (Tylenol) can ease your pain. But avoid ibuprofen (Advil) early in your pregnancy because it may cause problems with gestation and also in your third trimester. It could cause you to miscarry, reduce the amount of amniotic fluid or cause hearts problems in your baby.
- Placental abruption . Ongoing studies seem to show that pregnant women with fibroids have a much greater chance of placental abruption than women without fibroids. That means your placenta tears away from the wall of your uterus before your baby is delivered. It’s very serious because your baby won’t get enough oxygen and you can have heavy bleeding. You could go into shock.
- Preterm delivery. If you have fibroids, you’re more likely to deliver preterm -- meaning your baby is born before 37 weeks of pregnancy -- than women without fibroids.
Many studies show that having uterine fibroids increase your odds of having a cesarean section. That could be because the fibroids can keep the uterus from contracting and they can also block your birth canal, slowing down the progress of your labor. Women who have fibroids are six times more likely than other women to need a C-section.
Breech birth is another potential problem. In a normal birth, the baby comes out the birth canal head first. In a breech birth, the baby’s butt or feet come out first.
Fibroids often shrink after pregnancy. In one study, researchers found that, 3 to 6 months after delivery, 70% of women who had live births saw their fibroids shrink more than 50%.