Uterine Fibroids: How Ultrasound Maps Their Size, Location, and Impact

Uterine fibroids (leiomyomas) are benign smooth muscle tumours of the uterus. They are the most common benign gynaecological tumour, affecting approximately 20–40% of women of reproductive age and up to 70% of women by the age of 50. Despite their prevalence, many women are unaware they have fibroids until they are identified incidentally on an ultrasound scan.

Fibroids range in size from a few millimetres to over 20 cm. A uterus containing multiple large fibroids can reach the size of a 20-week pregnancy. Symptoms depend largely on the number, size, and — critically — the location of the fibroids within the uterus.

The FIGO (International Federation of Gynaecology and Obstetrics) classification system categorises fibroids by their relationship to the uterine cavity and outer surface. This classification is clinically important because the location of a fibroid determines its symptoms and the most appropriate treatment approach.

Submucosal fibroids (Types 0–2) have the greatest impact on menstrual bleeding and fertility. Even small submucosal fibroids can cause heavy menstrual bleeding (HMB) and implantation failure. Ultrasound accurately identifies submucosal fibroids and measures the degree of cavity distortion — information that is essential for planning hysteroscopic resection.

A fibroid mapping scan at our Marylebone clinic provides a systematic assessment of all identifiable fibroids, including: number and location (FIGO classification); size (three dimensions for each fibroid); echogenicity (solid, cystic, or mixed); vascularity (Doppler assessment); relationship to the endometrial cavity; and overall uterine size and shape.

Transvaginal ultrasound is more sensitive than transabdominal scanning for detecting small submucosal fibroids and accurately measuring their relationship to the uterine cavity. For very large uteri or multiple fibroids, a combined transabdominal and transvaginal approach is used. Our HCPC Registered sonographers provide a written report with FIGO classification for all identified fibroids.

Heavy menstrual bleeding (HMB) affects approximately 1 in 3 women with fibroids. The mechanism is multifactorial: submucosal fibroids increase the surface area of the endometrium, impair normal uterine contractility, and alter local prostaglandin and growth factor signalling. Intramural fibroids that distort the uterine cavity also contribute to HMB.

"Submucosal fibroids increase the surface area of the endometrium and impair uterine contractility — even a small submucosal fibroid can cause disproportionately heavy bleeding."

NICE guideline NG88 on heavy menstrual bleeding recommends pelvic ultrasound as the first-line investigation for women with HMB and a palpable uterine mass, or when pharmacological treatment has failed. At our CQC Compliant clinic, we provide a same-day written report that can be used to guide treatment discussions with your GP or gynaecologist.

The impact of fibroids on fertility depends on their location. Submucosal fibroids that distort the uterine cavity reduce implantation rates by approximately 50% and are associated with increased miscarriage risk. Intramural fibroids greater than 4 cm may also reduce fertility, though the evidence is less clear. Subserosal fibroids that do not distort the cavity have minimal impact on fertility.

For women undergoing IVF, pre-treatment fibroid mapping is recommended to identify submucosal fibroids that may be amenable to hysteroscopic resection before embryo transfer. Our Weymouth Street clinic works alongside fertility clinics in Central London to provide timely fibroid mapping scans with same-day written reports.

Fibroids typically grow slowly during the reproductive years and regress after the menopause. Serial ultrasound monitoring — typically every 6–12 months — is recommended for women with symptomatic fibroids who are being managed conservatively, and for women with fibroids who are trying to conceive.

Rapid fibroid growth (more than 1 cm per year) or new symptoms in a postmenopausal woman warrant further investigation to exclude the rare possibility of uterine sarcoma. Ultrasound alone cannot reliably distinguish between a rapidly growing fibroid and a uterine sarcoma — MRI is required in this scenario.