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.
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August 6th, 2008 at 10:56 pm
Thank you so much for this awesome explaination. I was confused about follicle vs eggs in my upcoming infertility / probably IUI quest. I really appreciate you taking the time to explain it.
September 11th, 2008 at 10:20 am
Thank you also. I have an IUI scheduled for Saturday and didn’t know what the difference was. Thanks!
October 3rd, 2008 at 1:21 pm
This was a GREAT explaination!! Thank you so much for taking the time to explain this process!
November 10th, 2008 at 7:44 pm
As the others said, thank you for this informative post. I do have a question…as my particular scenario was not represented in your examples. I had a 13mm “leftover follicle” at my CD3 ultrasound. The R.N. in the room told me it was fine to proceed with Femara this cycle. Unfortunately, my CD13 ultrasound revealed that same follicle still measuring 13mm and no other follicles had matured (the next largest was .7mm). My RE is suggesting I move on to injects. But I am concerned that the poor response this month was due to this residual follicle, and we may be jumping the gun. What does it mean if the follicle does not change in size over a 10 day period? Thank you so much!
November 12th, 2008 at 11:46 pm
First of all, it’s not known for sure that it did not change in size. It’s possible that the follicle you saw on D#13 was a different one from the cyst that you saw on D#3. The decision to move on to injectables should be based on your age, how many cycles of Femara you’ve already done, your total duration of infertility, the suspected cause or causes of your infertility and many other factors, not specifically on this one particular cyst/follicle. Good luck!
November 13th, 2008 at 2:42 pm
I had a sonogram done on day 7 of my cycle which was done as a routine exam for my first visit with the fertiltiy doc. Everything looked great and I had 13 to 17 follicles in each ovary which according to the doctor was great for a 31 year old. I went in to have another sonogram on day three of my cylce and this time there appeared to be a very small cyst. They did a bllod test to check my FSH levels and as long as that came back okay then they werent going to be concerned with the cyst since it didnt show up on my previous sonogram. The bloodwork did come back fine so I am going to start the chlomid tomorrow but am a liitle concerned since I have heard that if you have a cyst and you take chlomid, the cyst will be the only thing that grows. I would love to hear your thoughts on this. Thanks for everything you post as it has been very helpful and informative.
November 13th, 2008 at 2:48 pm
No need to respond as I have read further through some of your other post which has now answered my question.Thanks.
February 19th, 2009 at 4:55 pm
I got an ultrasound on CD 21 and was told there was one potential follicle and I would ovulate CD 24.(I thought I had already ovulated on CD 16 due to other signs).
CD 24 repeat ultrasound - showed an oblong shaped ’something’ and it seemed like I was ‘leaking’. Hence - ob decided I had ovulated (follicular size was probably 20 mm). My period started 9 days later though. Usually it starts around 14 days after ovulation.
Did the ob read the ultrasound incorrectly?
February 19th, 2009 at 5:51 pm
Axe, unfortunately, it’s impossible to tell from a verbal description and a “probable” measurement what the reality was for you this past cycle. There are some possible scenarios ranging all the way from normal ovulation and incorrectly interpreted ultrasound all the way to abnormal follicular progression and a correctly interpreted series of ultrasound exams. It’s impossible to go back in time and know for sure. I would advise the focus NOW to center on what action to take from here on in order to achieve a baby, assuming that is your goal.
Good luck!
February 19th, 2009 at 5:55 pm
Thank you
February 27th, 2009 at 2:50 pm
Thank you so much Dr. I am a novice at all of this…a 38 year old female who has tried for about 2-3 years with no luck. I am so glad that I found your blog. I went for a 2nd ultra sound today, I have been on HMG since Monday. Had a rapid progression in the left ovary that they were concerned could be a CYST, so they sent me for bloodwork. Heard from the nurse today, who said that are my hormone levels was only 175 and they are “choosing to ignore the 26 and look to a 13 in the right and a 12 in the left.” I had NO IDEA what she meant. I mean I assumed they were measurements, but I did not even think to ask her to clarify. I googled “13 ultra sound egg size” or something like that, and your blog came up. I feel much better now. Thanks for explaining in laywomans terms. It is much appreciated!! Follow up again on Monday, I am hoping the 12 and the 13 have the potential “speck of magic dust” to make a baby! Thanks again!
February 27th, 2009 at 6:13 pm
Anadrama, I can only guess that your RE feels that the 26mm is probably not a normal follicles but rather a dud because the E2 (estradiol) was only 175. So they are ignoring it and banking on the continued health of the 13mm and the 12mm to grow to a mature size.
March 6th, 2009 at 9:20 am
Hi doctor,
My wife and I decided to see a fertity doctor after trying to get pregnate for 2 years now with no succees. We are on our 3rd cycle of clomed and IUI with no luck. So now he wants to start injections. How do the injections differ from useing clomed what are the pros / cons? My other question is he only does an ultrasound to tell us if there are follicals present, how many and the size but doesn’t do another utrasound to see if they are growing larger. Should he be doing another one before we go threw the IUI procedure. As you know IUI’s are not cheap so I think we should pospone the IUI if the follical does mature and releases an egg.
Thank you
A confused husband
March 6th, 2009 at 10:00 am
Those are great questions. A through answer would take a long time to explain. The quick mini-version answer is that injectables generally increase the egg quantity and the egg quality (in terms of likelihood of pregnancy). And, yes, most cycles are done with more than just a single follicle-check ultrasound. The bigger question should be why are your RE and staff not answering these questions for you? =) Good luck!
March 6th, 2009 at 10:22 am
Thank you for your quick response, I really didn’t know that most cycles are done with more than just one follicle -check ultrasound. I found your blog when trying to do a search on (how to tell if a follicle matured and released an egg). That’s why I posted my question on your blog, I never asked our RE because I didn’t know. but now I do, so I will bring it up at out next visit before our next IUI.
Thanks again
March 7th, 2009 at 4:50 pm
Thank you doctor - I have been searching to get some answers and this was a very very informative article. Just a couple of questions for clarification.
I developed hair loss (and hypothyroidism) following 2 yrs of severe GI issues. Last year the hair loss got worse, acne, some facial hair increase. Testosterone (both been within and outside range), Estrogen, progesterone, Day 3 LH:FSH ratio normal but day 14 was 2:1, and DHEA-s slightly elevated.
Also I was told to take estrogen since it was on the lower end of normal and I was told it helps with hair loss but hair loss got worse - not sure if there is a connection.
Day 3 showed 10 < 10mm follicles on left and 12 on right ovary.
My question is whether just the ultrasound report would that be conclusive of PCOS? If I understand from your article - Day 3 should have shown fewer 10mm follicles?
I have been to 4 different OB-GYN and reproductive endocrinologists but questions are not entertained during the consults - am just told to take birth control pills or Clomid and that’s it.
Thank you for reading
March 8th, 2009 at 12:42 am
Pam,
The general recommendation in this situation would depend on factors such as age, BMI, family history of diabetes and whether or not you have symptoms of insulin resistance.
I’d suggest going to the RE whom you feel is most knowledgeable and ask them about testing for insulin resistance.
March 8th, 2009 at 12:20 pm
Thank you doctor I will ask for testing of insulin resistance. It’s very tough asking for anykind of testing and I am at my wit’s end trying to get some answers.
Would you be please kindly clarify the statement below (which I see over and over again) - can’t reconcile that with my low estrogen, normal range progesterone and being told I have PCOS. Can one have low estrogen and PCOS?? And then lose more hair by getting estrogen supplementation and eating soy?
” Another medical term for PCOS is chronic anovulation. In PCOS, the hormone estrogen is present at normal or high level without the balancing effects of the hormone progesterone”
March 12th, 2009 at 1:06 pm
hi doc..
i wanna ask sumthing about my confusion…
am 32yrs old lady ,have 9yrs old girl n married for 11yrs..last year i had en ectopic pragnency n my ryt side tube was ruptured n it was cut off,,,so now am having my left syd n i m having a bioconuate uterus..so last year we tried to have a baby n it was unluky,,so frm last month we tried n have scan done n it shows that my follicles are not dominent,my huz counts also very less.so it is possible to have a baby in this situatio…thanks
March 13th, 2009 at 12:07 pm
hi doctor can u help me please to get rid of these confusions plx,,am very worried
March 13th, 2009 at 12:16 pm
Rosy, I don’t have nearly enough information to render a specific opinion. It sounds like you have a sperm problem, an egg problem and a tubal problem. I can tell you that your best strategy is to find a good reproductive endocrinologist, who can put together the best plan. Without fully interviewing you and possibly doing an ultrasound exam, it’s too difficult to advise you of anything else. Good luck!
March 13th, 2009 at 10:30 pm
Thank u so much.now v r planning to visit a doc frm forughn country.n v r hoping that everything will b olryt..doc ur blog is interesting n its very informative…thankx
March 14th, 2009 at 8:35 am
if my follicles are nt growing is it a concern doctor..any madicines for that?
June 24th, 2009 at 6:53 pm
I took 150 mg chlomid (4th cycle) from day 5-9. I went in on day 16 for an ultra sound. My doctor said I have a cyst on each ovary. One 3.5cm and the other 4.2cm. He gave me a shot to trigger ovulation. He said they are very large but still thought I would ovulate. What is your thoughts due to the size of my cysts? Thank you!
June 24th, 2009 at 6:54 pm
I do have PCOS…
July 3rd, 2009 at 3:36 am
its my 1st cycle n i ve been inejected with lucrine for 2 wks. Yesterday, i went for blood test n scan. Basically, they said my blood test is ok. but they found 1 cyst on each side of my ovaries size 19mm.
So my doc wanted me to continue with lucrine injections for a week n come back for blood test n scan again.
what should i be expected for good results doc. “the cysts” should grow(which means it is not cyts but follicles) or the cysts to goes off by itself.
pls reply to me doc, i long yearn for a bb.
thk you
July 25th, 2009 at 2:06 pm
Hi there,
I went for follicle scan on cd3 and nurse saw 7 small follicles in left ovary and a 2cm ‘cyst’ that was dominating it. I had some pain in left side on cd8 and 9 and some pink spotting on the evening of cd9, which is v unusual for me.
Is it possible that the cyst could have been a follicle about to ovulate??
thank you
July 29th, 2009 at 6:49 am
If you read this post, you’ll have a better understanding of the difficulty in distinguishing a follicle from a cyst. But in general, when a cystic structure is seen on ultrasound, you would take into account the context (time in the cycle, change over time of the cystic structure) to determine whether it’s a follicle or not.
September 16th, 2009 at 11:15 pm
Hi dr,
TTC#1 for 9 months with no luck. My second cycle to go for u/s and and on my CD 11 we found follicles measuring 2.2,2.3 and 2.3 cms in my lt ovary. i thought they will be giving a hcg inj to rupture but my gyn asked me to wait till tomorrow i.e 2 days to see whether it grows, a follicle or cyst. I am so concerned , may be this is due to pco? kindly clarify
September 16th, 2009 at 11:17 pm
the dr, dint mention them as cysts .. i dont knw.. kindly help!!
September 25th, 2009 at 6:11 pm
Hi Dr
thanks for explaining the follicle v egg to me. In my ultrasound before insemination, my Dr saw a follicle 19mm. There was only one follicle, is that enough to get pregnant?
September 28th, 2009 at 11:45 pm
Awesome Explaination :)!
October 12th, 2009 at 9:21 pm
Thnks for this useful post.Yesterday was my day 12 and I have taken HCG injection.My follicle size is 26mm.when I scan on day 10,it was 17mm.My dosctor told me to take injection this time and told it may be a cyst.is it cyst?Is it good follicle size?Please answer doctor…
October 25th, 2009 at 5:45 pm
hello!!! i was at my u/s on day 12 of my cycle and it showed i had many follicles. many of them way too small–one was at 24mm and the other at 26mm. that seems REALLY big to me! is that too big for fertilization? RE said they weren’t cysts. i was on femara AND bravelle injectibles this month.
October 25th, 2009 at 6:55 pm
26mm is not too large to achieve a pregnancy. Good luck!
October 26th, 2009 at 11:15 am
thank you! that’s what my RE said too, but you know us women who are TTC, we go crazy sometimes and have to get many opinions and viewpoints!
i feel like i am going to start my period…..which is a bummer, but another question if you wouldn’t mind….i had lots and lots of pain (cramping) during my ovulation this month after i took my trigger shot…..was ovulation SUPER painful because of all the fertility meds or b/c of another factor?? i hope the medicine did not excite my ovaries too much.
November 14th, 2009 at 3:27 pm
Hi Dr. I really enjoyed this post. I was on Clomid cycle #1 50mg last cycle and only registered 3.5 progesterone on CD21. My RE decided to bump up dosage to 100mg and begin monitoring cycle #2. On CD2, one 10mm “cyst” was found on my left ovary. My RE was away and so the on-call doc told the nurse it was “borderline” but that I should sit this cycle out. Everywhere I search online, it seems 10mm is not big enough to be too concerned about at this stage. Is this doctor concerned the Clomid will, in effect, promote the growth ONLY of this 10mm follicle (which has no egg inside), instead of growing viable little guys? Couldn’t my right ovary potentially produce viable eggs? Of course I don’t want to put medicine in my body and hope in my heart unnecessarily and I will follow the doctor’s orders, but I wondered if you think this is an over-cautious approach?
November 14th, 2009 at 3:56 pm
Doctor, a quick PS. My baseline hormone levels were “perfect” and there were no enlarged follicles on my left ovary. Just the 10mm guy hanging out on my left side.
November 20th, 2009 at 7:37 am
hi, i’ve enrolled for ivf and waiting to start lucrine injection…i hv one question, if i’m pregnant when i’m taking lucrine, do u think that the baby will hv problem? thxs…
December 28th, 2009 at 4:53 pm
Hi,
I just had a BFN on an IUI cycle. I went in for CD3 scan, big follicle still left. They offered me BCP. I have very concerned, I am 41 and do not want to loose a month.
Is it imperative to take the BCP?
Can I get preg naturally with that follicle ?
They are leaving it up to me, since there are no certainties.