What Are Uterine Fibroids — And Why Location Matters More Than Size

If you've been diagnosed with uterine fibroids, one of the first things you probably did was search for how big is "too big." It's a natural question — but it's not actually the one that matters most. When it comes to fibroids and fertility, where a fibroid grows tells us far more than how large it is.

Let's break down what fibroids actually are, the three main types, and why location — not size — drives the conversation about treatment.

What Is a Uterine Fibroid?

A uterine fibroid (also called a leiomyoma or myoma) is a benign, non-cancerous growth made of muscle and fibrous tissue that develops in or on the uterus. Fibroids are extremely common — many people have them without ever knowing it, especially if they're small and not affecting the uterine cavity.

Fibroids can range from the size of a pea to the size of a grapefruit, and a single person can have just one or several at once, sometimes of different types simultaneously.

The Three Types of Fibroids — And Where They Sit

Fibroids are classified by their position relative to the uterine wall and cavity. This classification (part of the broader FIGO system used clinically) is the single most useful piece of information for understanding fertility risk.

1. Submucosal Fibroids

These grow just beneath the lining of the uterus (the endometrium) and bulge into the uterine cavity — the space where an embryo would implant. Because they physically intrude into that space, submucosal fibroids are the type most closely linked to fertility issues, including implantation difficulty and miscarriage risk.

2. Intramural Fibroids

These develop within the muscular wall of the uterus itself, without necessarily reaching the cavity. Intramural fibroids are the most common type overall. Whether they matter for fertility depends heavily on size and whether they distort the shape of the cavity — a small one tucked into the wall often has no impact at all.

3. Subserosal Fibroids

These grow on the outer surface of the uterus, sometimes even on a stalk (pedunculated). Because they sit outside the cavity entirely, subserosal fibroids are generally the least likely to interfere with fertility — even when they appear large on imaging.

So Why Does Location Matter More Than Size?

Here's the key concept: fertility risk comes from disruption to the uterine cavity, not from the fibroid's overall volume.

A 6cm subserosal fibroid sitting on the outside of the uterus may never need treatment before you conceive. Meanwhile, a 1.5cm submucosal fibroid — tiny by comparison — can distort the cavity enough to affect implantation and typically does warrant treatment first.

This is why, when you're told "you have a fibroid," the far more useful question is: "Where is it, and is it touching my cavity?" — not just "how big is it."

The Bottom Line

Not all fibroids are created equal, and a diagnosis alone doesn't tell you much. What matters is classification:

• Submucosal → closest attention, often treated before fertility treatment

• Intramural → depends on size and cavity involvement

• Subserosal → usually no impact, rarely treated for fertility reasons alone

If you've been told you have fibroids and aren't sure which type — or what it means for your fertility plans — that's exactly the kind of question worth bringing to a consult, where we can look at your specific imaging together.

In the next post in this series, we'll cover when fibroids actually need to be treated before starting IVF — and what that timeline looks like.