Cyst vs Egg vs Follicle: Clearing up the confusion

My medical students often ask me how I decide what to post about. Usually, it’s just whatever pops into my head. Other times, like today, it’s on things that I have to explain over and over again to many different people. I realized that if something is inherently so confusing as to have the need to clarify it repeatedly, then it would do the world some good to put it down in words for everyone to read at their leisure.

Three different terms that are used in overlapping fashion are cyst, egg and follicle. The EGG is that all-important biological specimen that contains half a woman’s genetic information. In my simplified view of human reproduction, a woman randomly shuffles all her DNA (the stuff that influences her body to develop blue eyes, a cute little nose and a fondness for puppies) and packages exactly half of it into each egg. The goal then, is for the egg to unite with one of her husband’s sperm, which very coincidentally contains half of HIS genetic information. The final product is a baby, a tiny person with genetic information from both parents. We can’t see eggs on ultrasound, because eggs are smaller than a speck of dust. What we CAN see are FOLLICLES. Imagine a follicle to be like a water balloon with a speck of magic dust (the egg) inside it. A woman is born with about a million of these packed inside her ovaries. Each month after she reaches puberty, some of these follicles start to fill up with water like expanding water balloons. Most will grow just a little and then fizzle out. However, the biggest one, which gets crowned with the title of THE DOMINANT FOLLICLE will grow from microscopic size up to about 20mm, roughly half the diameter of a golf ball. Once it reaches that size, it should burst and let the egg fly out. Under ideal conditions, the egg gets slurped up into the Fallopian tubes where it can hopefully meet a nice eligible sperm and then off they go into the uterus to implant.

So when doing ultrasounds, I’ll often report to my patients, “Great news! You are growing two eggs in each ovary for a total of four, giving you four chances at pregnancy this month! Right now they are still a few days away from being ready.” When they ask how I know that, I answer “Based on the measurements.” This is where the patients naturally assume that by measurement, I am talking about the size of the eggs. But as you probably realize know after having paid attention to what I just wrote, I’m actually talking about the size of the follicles.

Now that we have clarified the relationship between an egg and a follicle, where does the term CYST come into all this? A cyst can be defined as a CLOSED SAC, sometimes filled with a substance like fluid, air or blood.. So actually, follicles are cysts. They happen to be GOOD cysts as opposed to bad cysts. So medically, CYST is the term we use to generically refer to many different sac-like structures, not just dust-filled water balloons. Cysts can be found in many places like in breasts, under the skin, in the cervix or in the brain. Furthermore, ovarian cysts can be the good kind (follicle) or bad kinds, such as endometriomas which are harmful cysts filled with fluid that looks like chocolate syrup.

Now here’s the tricky part. A follicle and a simple cyst look identical on ultrasound. So using just one single ultrasound, nobody can say for sure whether we see a good follicle or some other type of cystic structure. However, if you look again on ultrasound three days later and see that the water balloon has grown from 14mm to 19mm, then you get a pretty good idea that it is a growing follicle. If instead, your repeat ultrasound shows nothing, then it turns out that it was most likely a cyst which has gone away on its own.

The typical protocol for patients who are taking strong fertility drugs is to do a cyst check prior to starting the stimulation medications. This is an ultrasound done on about day #3 of the cycle. What we want to see is nothing. What we don’t want to see, but sometimes do, are cysts and if they are big enough, it tells us that this is not a good month to do a cycle, so we postpone things.

Finally, let’s go back to the idea of how it’s normal each month for a single healthy water balloon to grow to a mature size and ovulate its precious contents. Remember, I alluded to this being the dominant contender out of several contestants which start the race, meaning most women will start growing a lot of potential follicles each month, but without medications, they should just ovulate one (or occasionally two). The other follicles should quickly all fizzle out and not even grow to a visible size. However, there are some women with a common medical condition in which the ovulation process goes haywire. Instead of one dominant follicle emerging and the others all shrinking to nothingness, what happens is that a whole bunch of the follicles keep growing, but none really make it to mature size. So instead of seeing one large 20mm water balloon and nothing else, you see a whole bunch of 12mm follicles stagnated in their growth. Some of you have probably already guessed that this condition in which multiple small ungrowing non-ovulating follicles are seen in the ovary is called POLY-CYSTIC OVARY SYNDROME, but that’s a story for another day.