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Fibroid Anatomy Explained

Locations, Types
& Size Classifications

Fibroid Anatomy explained locations, types, and sizes of fibroids

If you’ve been diagnosed with fibroids, your doctor has probably given you a brief explanation of how many fibroids you have, where your fibroids are located within your uterus, and how big (or small) they are. Your doctor may have even introduced you to specialized terms like ‘submucosal’ or ‘pedunculated’ to describe the specific anatomy of your fibroids. These fibroid attributes (location, type, and size classification) can have a major influence on your individual experience with fibroids, and can even preclude certain treatment options. Ask your doctor the right questions about fibroids.

Our fibroid specialists review what we know about fibroid anatomy and how it can affect symptoms and treatment options. We provide a detailed explanation of fibroid locations, fibroid types, and fibroid sizes so you can make sense of your diagnosis and your options.

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Uterine Fibroid Classification

Fibroids can be categorized in different ways, but the most widely adopted classification system is the one set by the International Federation of Gynecology and Obstetrics (FIGO). FIGO classifies fibroids based on location in the uterus. According to FIGO, 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 classification system, you’ll see that uterine fibroids are described as either pedunculated, submucosal, intramural, or subserosal.
Fibroid Anatomy with submucosal fibroid, intramural fibroid, pedunculated fibroid, and subserosal fibroid types labeled

FIGO Classification of Fibroids

Fibroid Type Description
Type 0 Pedunculated, intracavitary
Type 1 Submucosal, <50% intramural
Type 2 Submucosal, ≥50% intramural
Type 3 Contact with endometrium, 100% intramural
Type 4 Intramural
Type 5 Subserosal, ≥50% intramural
Type 6 Subserosal, <50% intramural
Type 7 Subserosal, pedunculated
Type 8 Other (e.g., cervical, parasitic)
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Uterine Fibroid Types

Pedunculated Fibroids

​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!
Fibroid Type Description
Type 0 Pedunculated, intracavitary
Type 7 Subserosal, pedunculated

Intramural 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].
Fibroid Type Description
Type 3 Contact with endometrium, 100% intramural
Type 4 Intramural

Submucosal 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].
Fibroid Type Description
Type 1 Submucosal, <50% intramural
Type 2 Submucosal, ≥50% intramural

Subserosal 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].
Fibroid Type Description
Type 5 Subserosal, ≥50% intramural
Type 6 Subserosal, <50% intramural

Other Types of 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.
Fibroid Type Description
Type 8 Other (e.g., cervical, parasitic)
Diagram comparison of fibroid sizes comparing to the size of pregnant uterus

Fibroid Sizes Explained

Uterine fibroids are very common. If you’ve never been diagnosed with fibroids, It’s actually very like that you have a fibroid and just don’t know about it yet. Because many women have small fibroids that are asymptomatic, uterine fibroids may go undetected. However, fibroids can grow in size. Estrogen and progesterone have been shown to promote fibroid growth. Fibroids usually start to shrink during and after menopause.

Fibroid Size Classification

Fibroid size can be described in a number of ways, either by estimating the fibroid volume, measuring fibroid diameter on imaging studies, or estimating 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.

Fibroid Size and Type

Fibroids can be described by their type (subserosal, intramural, submucosal, pedunculated) as well as their size. Because the myometrium is relatively thin, intramural fibroids tend to be small. If they are larger, they are inevitably classified as subserosal or submucosal [6]. In one study, intramural fibroids were significantly smaller than subserosal and submucosal fibroids [6].

Fibroid Size and Position

Fibroids can also be described by their position within the uterus. Positions commonly recorded are posterior (back of the uterus), anterior (front of the uterus), lateral (sides of the uterus), fundus (top of the uterus, furthest away from the cervix), and isthmus (bottom of the uterus, toward the cervix). In one study, fibroids were commonly found in the posterior, anterior, and lateral aspects of the uterus [4].
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How Does Fibroid Size & Location Affect Symptoms?

Many women with fibroids experience symptoms like heavy menstrual bleeding, pain or pressure in the pelvis. They might also have problems with pregnancy or infertility. Abnormal uterine bleeding is the most common symptom for uterine fibroids [9]. 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].

How Does Fibroid Size & Location Affect Pregnancy & Fertility?

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].
Woman with uterine fibroids holding pregnant stomach

How Does Fibroid Size & Location Affect Treatment Options?

Treatments for uterine fibroids vary from conservative medical therapy to minimally invasive interventions to surgical removal of the entire uterus. The optimal treatment for your uterine fibroids will depend on your desire to preserve fertility, your age, any other existing medical issues, ability to take time off for recovery, as well as other factors. Some therapies treat symptoms while others eliminate the fibroids completely. Some treatment options, while less invasive, may require subsequent treatment or re-intervention. These are all factors to consider when deciding which uterine fibroid treatment option is best for you, and some treatment options are only viable for certain types and sizes of fibroids.

Medications

The most conservative and first-line treatment option for uterine fibroids is medical therapy. Medicine can be effective if symptoms are mild, if a woman is in perimenopause (so treatment is only needed in the short term), or if pre-operative intervention to reduce the size of the fibroids would make a surgery less technically challenging [12, 13]. Nonsteroidal anti-inflammatory drugs (NSAIDs), hormones (estroprogestins or progestogens), and oral contraceptives can help with symptomatic bleeding and fibroid pain. Gn-RH analogues and selective progesterone receptor modulators (sPRMs) like ulipristal acetate (UPA) help treat bulk symptoms such as pelvic pressure, pelvic pain, and frequent urination [9]. Medical treatments are only used for the short-term due to the significant health risks associated with long-term use of these medicines [12].

Uterine Fibroid Embolization

Uterine fibroid embolization, also known as uterine artery embolization, is a minimally invasive procedure performed by an interventional radiologist or vascular specialist. During the procedure, small spherical beads are injected into the blood vessels that supply the fibroids with oxygen and other nutrients. The beads occlude the blood vessels, essentially starving the fibroids and causing them to shrink. UFE is an outpatient procedure with a recovery time around 1 to 2 weeks. The ideal UFE candidate is a premenopausal woman with heavy menstrual bleeding and/or severe menstrual pain caused by intramural fibroids who does not desire pregnancy in the future. 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 is also called fibroid ablation. It uses a concentrated energy source such as ultrasound, radiofrequency, or laser to cause fibroid tissue destruction. MRI or ultrasound imaging help guide where the energy source should be directed. Myolysis decreases menstrual bleeding and uterine fibroid size [12]. However, it can only treat one fibroid at a time, and large fibroids greater than 8 cm in diameter may not respond well [9]. In addition, myolysis targets the central portion of the fibroids even though studies have shown that fibroids tend to grow from their peripheries [9]. 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

In endometrial ablation, the lining of the uterus is destroyed. The fibroids themselves don’t necessarily shrink. Endometrial ablation helps decrease heavy menstrual bleeding and is most beneficial for women suffering from excessive bleeding due to fibroids [15]. Because endometrial ablation targets the inner lining of the uterus, it is most effective for submucosal fibroids (Types 0, 1, or 2) [16-18]. In addition, endometrial ablation is most amenable for fibroids less than 3 cm in diameter. 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 is a surgery that involves selectively removing uterine fibroids while keeping the uterus intact. It is considered a major surgery that may require a blood transfusion and a 1 to 3 day hospital stay. Blood loss and operative time are greater for myomectomy than hysterectomy [9]. The recovery time for myomectomy can be quite long, ranging from 2 to 6 weeks. However, myomectomy is considered a first-line conservative surgical therapy for symptomatic intracavitary fibroids (Types 0, 1, and 2) that are 4 to 5 cm in diameter [9]. 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

A hysterectomy is a surgery where the entire uterus or parts of it are removed along with any fibroids. Hysterectomy can be performed through the vagina, abdomen or laparoscopically. Like myomectomy, hysterectomy is a major surgery requiring a 2 to 5 day overnight stay in the hospital and a long recovery time of 2 to 6 weeks [13]. 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.
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More Resources
Woman discussing surgical vs. non-surgical fibroid treatment options with Fibroid Specialist
Surgical vs. Non-Surgical Fibroid Treatments: Which Is Best?

When it comes to fibroid treatment, we believe that less is more. A less invasive procedure means less trauma, less risk, and shorter recovery time. Learn more about why we opt for UFE, the least invasive treatment for uterine fibroids.

Woman discussing surgical vs. non-surgical fibroid treatment options with Fibroid Specialist
Uterine Artery Embolization
Uterine artery embolization is a minimally-invasive procedure that we offer at ProFibroidMD to relieve heavy bleeding, pain, and other symptoms in patients with uterine fibroids. Learn more about our specialty procedure.
Fibroid Specialist in Los Angeles
Fibroid Specialist in Los Angeles
Learn more about Los Angeles Fibroid Specialist Dr. Michael Lalezarian.
References [1] Munro MG, Critchley HO, Broder MS, Fraser IS. FIGO Working Group on Menstrual Disorders. FIGO classification system (PALM-COEIN) for causes of abnormal uterine bleeding in nongravid women of reproductive age. Int J Gynaecol Obstet 2011;113:1–2. [2] Cook H, Ezzati M, Segars JH, McCarthy K. The impact of uterine leiomyomas on reproductive outcomes. Minerva Ginecol. 2010 Jun;62(3):225-36. PMID: 20595947; PMCID: PMC4120295. [3] Guo XC, Segars JH. The impact and management of fibroids for fertility: an evidence-based approach. Obstet Gynecol Clin North Am. 2012 Dec;39(4):521-33. doi: 10.1016/j.ogc.2012.09.005. PMID: 23182558; PMCID: PMC3608270. [4] Lippman, Sheri & Warner, Marcella & Samuels, Steven & Olive, David & Vercellini, Paolo & Eskenazi, Brenda. (2004). Uterine fibroids and gynecologic pain symptoms in a population-based study. Fertility and sterility. 80. 1488-94. 10.1016/S0015-0282(03)02207-6. [5] Simms-Stewart D, Fletcher H. Counselling patients with uterine fibroids: a review of the management and complications. Obstet Gynecol Int. 2012;2012:539365. doi: 10.1155/2012/539365. Epub 2012 Jan 9. PMID: 22272207; PMCID: PMC3261489. [6] Mavrelos D, Ben-Nagi J, Holland T, Hoo W, Naftalin J, Jurkovic D. The natural history of fibroids. Ultrasound Obstet Gynecol. 2010 Feb;35(2):238-42. doi: 10.1002/uog.7482. PMID: 20069541. [7] Day Baird D, Dunson DB, Hill MC, Cousins D, Schectman JM. High cumulative incidence of uterine leiomyoma in black and white women: ultrasound evidence. Am J Obstet Gynecol. 2003; 188(1):100–107. [PubMed: 12548202] [8] Cramer SF, Patel A. The frequency of uterine leiomyomas. Am J Clin Pathol. 1990 Oct;94(4):435-8. doi: 10.1093/ajcp/94.4.435. PMID: 2220671. [9] Vilos GA, Allaire C, Laberge PY, Leyland N; SPECIAL CONTRIBUTORS. The management of uterine leiomyomas. J Obstet Gynaecol Can. 2015 Feb;37(2):157-178. doi: 10.1016/S1701-2163(15)30338-8. PMID: 25767949. [10] Ouyang DW, Economy KE, Norwitz ER. Obstetric complications of fibroids. Obstet Gynecol Clin North Am. 2006; 33:153–69. [PubMed: 16504813] [11] Exacoustòs C, Rosati P. Ultrasound Diagnosis of Uterine Myomas and Complications in Pregnancy. Obstet Gynecol. 1993; 82:97–101. [PubMed: 8515934] [12] Mas A, Tarazona M, Dasí Carrasco J, Estaca G, Cristóbal I, Monleón J. Updated approaches for management of uterine fibroids. Int J Womens Health. 2017 Sep 5;9:607-617. doi: 10.2147/IJWH.S138982. PMID: 28919823; PMCID: PMC5592915. [13] Khan AT, Shehmar M, Gupta JK. Uterine fibroids: current perspectives. Int J Womens Health. 2014 Jan 29;6:95-114. doi: 10.2147/IJWH.S51083. PMID: 24511243; PMCID: PMC3914832. [14] van der Kooij S.M., Hehenkamp W. Uterine fibroids (leiomyomas): Treatment with uterine artery. UpToDate. Accessed February 2021. [15] Stewart, E.A., Laughlin-Tommaso, S.K. Patient education: Uterine fibroids (Beyond the Basics). UpToDate. Accessed February 2021. [16] Singh SS, Belland L. Contemporary management of uterine fibroids: focus on emerging medical treatments. Curr Med Res Opin. 2015;31(1):1-12. [17] Davis MR, Soliman AM, Castelli-haley J, Snabes MC, Surrey ES. Reintervention Rates After Myomectomy, Endometrial Ablation, and Uterine Artery Embolization for Patients with Uterine Fibroids. J Womens Health (Larchmt). 2018;27(10):1204-1214. [18] Munro MG. Endometrial ablation: where have we been? Where are we going?. Clin Obstet Gynecol. 2006;49(4):736-66.

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