Fibroids don’t typically originate during pregnancy. Most were already present before conception, often too small to detect. What pregnancy does is create the perfect hormonal environment for existing fibroids to grow, sometimes rapidly. Between 1% and 10.7% of pregnant women have fibroids identified on ultrasound, and as more women delay childbearing into their 30s and 40s, that number is climbing.
How Pregnancy Hormones Fuel Fibroid Growth
The primary driver is the dramatic rise in progesterone and estrogen that sustains a pregnancy. For decades, estrogen got most of the blame, but progesterone plays an equally significant role. Fibroid tissue contains more progesterone receptors than normal uterine muscle, which means fibroids are essentially primed to respond more aggressively to the flood of hormones your body produces during pregnancy.
When progesterone binds to those receptors, it triggers a chain of events inside fibroid cells. It promotes new blood vessel formation around the fibroid, strengthens the fibroid’s structural scaffolding (the dense connective tissue that gives fibroids their firm, rubbery texture), and activates growth-signaling pathways that push cells to multiply while simultaneously blocking the normal process of cell death. The result is a fibroid that grows faster, builds a better blood supply for itself, and resists the body’s usual checks on abnormal tissue growth.
Estrogen contributes by stimulating the production of a compound called insulin-like growth factor-I (IGF-I), which is found at higher levels in fibroid tissue than in surrounding normal uterine muscle. IGF-I accelerates cell division and further suppresses cell death. Insulin itself has a similar, though weaker, growth-promoting effect on fibroid tissue. During pregnancy, when insulin resistance naturally increases and growth-related hormones surge, these factors combine to create an environment where fibroids can enlarge substantially.
Blood Supply and Rapid Growth
Fibroids depend heavily on their blood supply to survive and expand. During pregnancy, blood flow to the uterus increases dramatically to support the growing fetus, and fibroids benefit from that increased circulation. Research consistently shows a positive correlation between blood flow to a fibroid and its size. Fibroids also produce higher levels of vascular endothelial growth factor (VEGF) than normal uterine tissue, essentially signaling the body to build new blood vessels around them. This self-directed blood vessel construction is essential for fibroid growth; without it, fibroid tissue cannot sustain itself in living conditions.
The hormonal surge of pregnancy amplifies this process. Estrogen and progesterone stimulate local production of multiple growth factors with blood-vessel-building properties, acting through signaling loops within the fibroid itself and the surrounding uterine wall. The fibroid develops a vascularized capsule, a shell of blood vessels at its outer edge, that feeds the denser, less vascular core inside.
When Fibroids Outgrow Their Blood Supply
Sometimes a fibroid grows so quickly during pregnancy that its blood supply can’t keep up. When this happens, the interior of the fibroid begins to die, a condition called red degeneration. It occurs in roughly 8% of pregnant women with fibroids, most often during the second or third trimester.
Three things contribute to this problem during pregnancy. First, the rapid growth driven by estrogen and progesterone can simply outpace the fibroid’s ability to build new blood vessels. Second, as the uterus expands, it can compress or twist the vessels feeding the fibroid. Third, blood clots can form in the small veins at the fibroid’s outer edge, cutting off circulation.
Red degeneration typically causes sudden, intense pain localized to where the fibroid sits, sometimes accompanied by a low-grade fever and nausea. It can be alarming, but it is not dangerous to the baby in most cases. Pain management usually involves rest, fluids, and carefully chosen pain relief. Anti-inflammatory medications are used cautiously, particularly after the second trimester, because prolonged use can affect fetal heart and kidney development. In rare cases of severe, unmanageable pain, stronger pain medication or even surgical removal of the fibroid may be considered, though surgery during pregnancy is uncommon.
How Fibroids Affect Pregnancy Outcomes
Most pregnancies with fibroids proceed without serious complications, but fibroids do measurably increase certain risks. A large meta-analysis found the following patterns:
- Preterm birth: About 48% to 72% higher risk compared to pregnancies without fibroids.
- Placental abruption: 85% to 94% higher risk, likely because fibroids can distort the uterine wall where the placenta attaches.
- Breech or abnormal positioning: Roughly double the risk. Fibroids, especially large ones or those in the lower uterus, can physically limit the space available for the baby to turn head-down.
These are relative increases, meaning the absolute risk for any individual pregnancy depends on fibroid size, number, and location. A small fibroid on the outer surface of the uterus poses far less concern than a large one embedded in the uterine wall near the placenta.
Detection During Pregnancy
Most fibroids found during pregnancy are discovered incidentally on routine ultrasound. Ultrasound has a sensitivity of about 99% and specificity of 91% for detecting fibroids, making it highly reliable even with a growing fetus present. Some fibroids that were too small to notice before pregnancy become visible as they enlarge in response to pregnancy hormones. Others were already known but are monitored more closely once pregnancy begins.
Your provider will typically note the fibroid’s size, location, and proximity to the placenta during standard prenatal ultrasounds. Fibroids in the lower uterine segment or near the cervix receive particular attention because they can obstruct delivery.
Management During Pregnancy
Treatment during pregnancy is almost entirely conservative. The goal is to manage symptoms, primarily pain, rather than to treat the fibroid itself. Rest, hydration, and mild analgesics handle most cases of fibroid-related discomfort. Surgical removal during pregnancy is reserved for specific situations: a fibroid causing intractable pain that doesn’t respond to medication, one that’s growing rapidly, or a large fibroid (over 5 cm) positioned in the lower uterine segment where it could complicate delivery.
When surgery is necessary, it can be performed safely in the first or second trimester. Outcomes for mother and baby are comparable to those managed without surgery, though women who undergo fibroid removal during pregnancy are more likely to deliver by cesarean section due to concerns about the uterine scar’s integrity during labor. Procedures like uterine artery embolization, which cuts off blood flow to the fibroid, are not an option during pregnancy.
If you’ve had a fibroid surgically removed before or during pregnancy, your provider will likely recommend a planned cesarean delivery before labor begins, particularly if the surgery cut into the deeper layers of the uterine wall.
What Happens After Delivery
The hormonal environment that fed fibroid growth reverses after delivery. As estrogen and progesterone levels drop, fibroids commonly shrink during the postpartum period. Regression is typical between early pregnancy measurements and three to six months after delivery. In one observed case, the largest fibroid showing maximal shrinkage (an 88.5% reduction in volume) started at roughly 4.3 cm in diameter.
Vaginal deliveries are associated with slightly higher rates of fibroid regression than cesarean deliveries (74% vs. 68%), though the difference is modest. Longer pregnancies also tend to produce more fibroid shrinkage afterward. Some fibroids regress completely. Others shrink but persist, potentially growing again during a subsequent pregnancy or in response to other hormonal changes later in life.