Fibroids during pregnancy aren’t caused by pregnancy itself. Most fibroids already exist before conception, but pregnancy creates hormonal conditions that make them grow, sometimes rapidly. The surge in estrogen and progesterone during the first and second trimesters fuels fibroid tissue in ways that can turn a small, unnoticed growth into a clinically significant one.
Why Fibroids Grow During Pregnancy
Fibroids are noncancerous growths made of smooth muscle cells in the uterine wall. They contain receptors for estrogen and progesterone, the same hormones that spike dramatically during pregnancy. When those hormone levels rise, fibroid tissue responds by growing. Tissue samples from pregnant women show that fibroids enlarge through a combination of cell swelling (hypertrophy), fluid retention (edema), and progesterone receptor activity.
Growth follows a predictable pattern across the three trimesters. Fibroids typically enlarge during the first trimester, and 51% to 56% of women see continued increases in total fibroid volume through the second trimester. By the third trimester, the pattern often reverses: 50% to 54% of women experience a decrease in fibroid size. This third-trimester shrinkage likely reflects declining progesterone receptor sensitivity and changes in blood supply as the uterus stretches to accommodate the baby.
Not every fibroid grows. Some stay the same size throughout pregnancy, and a smaller percentage actually shrink from the start. But fibroids larger than 5 cm are more likely to outgrow their own blood supply, which leads to a painful condition called red degeneration. This happens when the center of the fibroid loses oxygen and begins to break down, releasing compounds that cause localized inflammation and sharp pain. It’s one of the most common fibroid complications during pregnancy and is managed with pain relief and fluids.
Who Is More Likely to Have Fibroids
Several factors influence whether you’ll have fibroids by the time you become pregnant. Age is one of the strongest predictors: fibroids become more common as women move through their 30s and into their 40s. African American women develop fibroids at younger ages, tend to have larger growths, and experience more severe symptoms compared to white women. This disparity persists even after menopause, with fibroids less likely to shrink in Black women after periods stop.
Other established risk factors include obesity, high blood pressure, family history of fibroids, and vitamin D deficiency. Interestingly, pregnancy itself is associated with lower long-term fibroid risk, and the more pregnancies you’ve had, the lower your risk becomes. But if fibroids are already present when you conceive, pregnancy hormones will likely affect their behavior.
How Fibroid Location Shapes Risk
Where a fibroid sits in the uterus matters more than its existence alone. Fibroids fall into three main categories based on location: submucosal (growing into the uterine cavity), intramural (embedded within the muscle wall), and subserosal (growing outward from the uterine surface).
Submucosal and intramural fibroids pose the greatest concern during pregnancy. They’re associated with lower birth rates compared to subserosal fibroids, and the combination of both types in the same uterus further increases risk. The position on the uterine wall also matters. Fibroids located on the front or back wall of the uterus are more problematic than those at the top (the fundus), likely because they’re more likely to interfere with implantation, placental development, or the baby’s ability to move into a head-down position.
Subserosal fibroids, which grow on the outer surface, carry a different profile. While they’re less disruptive to the pregnancy itself, research has linked them to a higher rate of early pregnancy loss compared to submucosal or intramural types.
Complications Fibroids Can Cause
Most women with fibroids have uncomplicated pregnancies, but the presence of fibroids does shift the odds on several outcomes. Across large studies, preterm birth occurs in about 12.9% of women with fibroids, compared to 9.4% of those without. Placental abruption, where the placenta separates from the uterine wall before delivery, occurs at a rate of 6.3% in women with fibroids versus 5.5% without, and the relative risk is nearly double. Breech presentation is also more than twice as likely: 8.3% of women with fibroids versus 3.7% without. Large fibroids can physically prevent the baby from turning head-down, which often leads to a planned cesarean delivery.
The likelihood of cesarean delivery rises with fibroid size. Women with a single fibroid measuring 3 cm or larger have a roughly 22% higher adjusted risk of C-section. For women with the largest total fibroid volumes, the risk increases by about 59%. This reflects both the direct obstruction some fibroids cause in the lower uterus and the higher rates of malpresentation and labor complications.
Pain and Degeneration During Pregnancy
The most noticeable symptom fibroids cause during pregnancy is pain, and the most common reason for that pain is degeneration. As a fibroid grows faster than its blood supply can support, the tissue at its center becomes oxygen-starved and begins to die. This process releases inflammatory compounds that cause acute, localized pain, sometimes severe enough to mimic appendicitis or placental abruption.
Red degeneration is most common in fibroids larger than 5 cm and typically strikes during the second trimester, when fibroid growth peaks. The pain is usually treated conservatively with rest, hydration, and pain medication. Surgery during pregnancy is reserved for rare, extreme cases. For most women, the pain resolves on its own, though it can recur if the fibroid continues to grow or if multiple fibroids are present.
Outside of degeneration, fibroids can also cause a persistent feeling of pelvic pressure, more frequent urination (from the combined effect of fibroid and uterine growth pressing on the bladder), and in some cases, contractions that require monitoring to distinguish from preterm labor.