Types of Fibroids Explained

Fibroid classification can be done in different ways, but the most widely adopted classification system is the one set by the International Federation of Gynecology and Obstetrics (FIGO). FIGO fibroid classification is based on the fibroids location in the uterus. According to FIGO fibroid classification, there are 9 types of fibroids, which are assigned numbers ranging from 0 to 8 [1]. These numbers describe the location of the fibroid relative to the three main layers of the uterus: the inner lining of the uterine cavity called the endometrium, the smooth muscle of the uterine wall called the myometrium, and the outer surface of the uterus known as the serous membrane. Size and number of fibroids are not currently included in existing staging systems, which assess fibroids primarily on their location [2]. Under the FIGO fibroid classification system, you’ll see that uterine fibroids are described as either pedunculated, submucosal, intramural, or subserosal.

​Pedunculated fibroids grow on small stalks on the inside of the uterus (intracavitary) or outside of the uterus (subserosal). Imagine one of the stalks twisting–pedunculated fibroids can cause intense episodes of acute pain! Type 0 fibroids and type 7 fibroids are considered predunculated fibroids.

Intramural fibroids are located within the uterine wall. Some intramural fibroids can enlarge to span from the outer serosal surface, through the uterine wall, and all the way to the endometrium (the mucosal surface). These are known as transmural fibroids. While intramural fibroids don’t distort the uterine cavity, they are associated with reduced fertility outcomes, although not as significantly as submucosal fibroids [3]. In one study of 96 Italian women with fibroids, intramural fibroids were the most prevalent type identified on transvaginal ultrasound [4]. Type 3 fibroids and type 4 fibroids are considered intramural fibroids.

A submucosal fibroid is a type of intramural fibroid where part of the fibroid projects into the uterine cavity and another part resides in the smooth muscle of the myometrium. Because submucosal fibroids grow just underneath the uterine lining (the endometrium), they can reduce embryo implantation and pregnancy rates for couples using assisted reproductive technology to treat infertility. Women with submucosal fibroids may present with bleeding in between menstrual cycles. This is especially common with prolapsed submucosal fibroids [5]. In general, submucosal fibroids don’t seem to be as prevalent as subserosal or intramural fibroids [4]. Type 1 fibroids and type 2 fibroids are considered submucosal fibroids.

Subserosal fibroids grow on the outside of the uterus. Because they aren’t located near the endometrium or uterine cavity, they have fewer reproductive consequences [3]. However, large subserosal fibroids can put pressure on the abdomen, bladder or rectum, causing symptoms like pain, a feeling of fullness or being pregnant, bloating, pelvic pressure, urinary urgency or constipation. Like intramural fibroids, subserosal fibroids are also quite common [4]. Type 5 fibroids and type 6 fibroids are considered subserosal fibroids.

While most fibroids are benign, non-cancerous leiomyomas, some fibroids can be malignant. Leiomyosarcoma, a cancerous fibroid, is typically found in older, postmenopausal women who complain of new pain with or without bleeding in new or existing fibroids. Parasitic fibroids appear as abdominal masses but are found outside of and separate from the uterus. Cervical fibroids are found at the opening of the uterus and are rare. Type 8 fibroids are considered other types of fibroids (e.g., cervical, parasitic).

Fibroid size classification is done to either estimate the fibroid volume, measure fibroid diameter on imaging studies, or to estimate the size of the enlarged uterus as a result of a growing fibroid. In one study, women with one or more fibroids had a median fibroid size of 5 cm3 by volume [4]. Small fibroids have diameters less than or equal to 20 mm and they enlarge the uterus to appear as it would at the 4th week of pregnancy. Average, medium-sized fibroids range between 20 and 60 mm in diameter and cause the uterus to appear as if it were at 10 or 11 weeks gestation. Large fibroids are greater than 60 mm in diameter and enlarge the uterus, making it comparable to a womb at 12 to 16 weeks gestation.

Some large fibroids can exert pressure on surrounding organs and nerves, which can lead to more severe and noticeable symptoms. However, larger fibroids aren’t always associated with more symptoms like increased pain [4]. In one study, women with smaller intramural fibroids were more likely to report moderate or severe pain compared to patients without them [4].

Many women with uterine fibroids are able to become pregnant without a problem. However, certain types of fibroids can crowd the uterine cavity or distort the endometrium, causing complications. Some studies show that the risk of developing complications during pregnancy increases if a woman has uterine fibroids over 3 cm in diameter. However, women with large fibroids greater than 10 cm in diameter can achieve vaginal delivery approximately 70% of the time [3]. In general, complications such as miscarriage, c-section, preterm labor and postpartum hemorrhage can occur in approximately 10 to 40% of pregnancies when fibroids are present [10, 11].

Medications

Uterine Fibroid Embolization

The ideal UFE candidate is a premenopausal woman with heavy menstrual bleeding and/or severe menstrual pain caused by intramural fibroids. UFE may not be efficacious if the only fibroid symptoms you have are bulk-related (i.e. pelvic pain or pelvic pressure) [14]. In addition, UFE isn’t the most suitable option for very large fibroids or women with a high number of fibroids, as symptom recurrence is likely to occur [14]. Pedunculated fibroids with a narrow stem (the stalk has a diameter less than 50% of the diameter of the largest fibroid) are considered to be relative contraindications for UFE because these fibroids may detach into the peritoneal or uterine cavities and cause infection [14].

The probability of needing a new surgical procedure in 2 to 5 years after UFE ranges from 15 to 32 percent compared to 7% for surgical procedures like myomectomy and hysterectomy [12]. Though successful pregnancies have been reported post-UFE, there are studies that show lower pregnancy and higher miscarriage rates compared to other treatments like myomectomy [9].

Myolysis

Myolysis does impact future fertility and pregnancies. As fibroid tissue is destroyed with myolysis, scar tissue can form in the endometrium. While some women may achieve successful pregnancies, there could be potential complications. Myolysis is not always a definitive treatment. Surgical intervention may be required down the line [12].

Endometrial Ablation

Pregnancy is not recommended after endometrial ablation. A single endometrial ablation procedure may require follow-up intervention in the future. A recent study compared the re-intervention rates for women after they underwent endometrial ablation, myomectomy, or uterine artery embolization to treat fibroids. One year after the initial procedure, the need for a second procedure was 12.4% for endometrial ablation, 4.2% for myomectomy, and 7.0% for uterine artery embolization [16,17].

Myomectomy

Myomectomy isn’t without its flaws, and isn’t always ideal for women with multiple large fibroids. Failure rates range from 14.5% to 30% at 3 to 4 years’ follow-up [9]. Between 10 and 25 percent of women will require a second fibroid surgery [15]. Still, many women choose myomectomy in order to preserve their uterus for future pregnancies. Most women who have myomectomy are able to have children afterwards.

Hysterectomy

Hysterectomy is the most definitive treatment available for uterine fibroids, particularly for women who have no desire to conceive [12]. It can be very effective for patients with extremely large fibroids or a large quantity of fibroids. However, after hysterectomy, once the uterus or parts of it are removed, pregnancy is no longer possible.