Home > General OB/GYN Topics, Obstetrics > Amniotic Fluid Ferns at ANY gestational age

Amniotic Fluid Ferns at ANY gestational age

Throughout my career heard so many reasons why the fern slide didn’t fern in the apparently ruptured patient.

“She’s only 19 weeks, they don’t fern this early.”

“There’s blood in the sample, that makes it not fern.”

“She has chorio, that makes it not fern.”

Over the years, this has driven me a bit crazy, because the real reason that the slide is not ferned is not any of these reasons.  The reason there is no fern on the slide is because the fluid on the slide is not amniotic fluid.


Clearly there are some reasonable questions to be asked about what will cause or prevent ferning in a vaginal pool sample.   Does gestational age affect it?  Does blood affect it?   How long does one need to wait?

Asking these questions is great.  What is not great is making crap up about the answers when these questions have been clearly answered in the literature.

Ferning of amniotic fluid contaminated with blood.  Rosemond RL, Lombardi SJ, Boehm FH. Obstet Gynecol. 1990 Mar;75(3):338-40.

“Amniotic fluid was obtained from 38 patients between 16-38 weeks’ gestational age who underwent amniocentesis. Amniotic fluid was immediately mixed with freshly obtained, heparinized fetal cord blood in varying concentrations (blood to amniotic fluid 1:5, 1:10, 1:20). The slides were examined microscopically for the presence of ferning. All samples were fern-positive, but many had atypical ferns described as “skeletonized.” As the concentration of blood to amniotic fluid increased, the number of atypical ferns increased (32 of 38 at 1:5, 22 of 38 at 1:10, and nine of 38 at 1:20). We conclude that the presence of blood may alter the morphology of the fern, but does not act as a contaminant that would affect the accuracy of the test.”

Amniotic fluid arborization: effect of blood, meconium, and pH alterations. Reece EA, Chervenak FA, Moya FR, Hobbins JC.. Obstet Gynecol 1984 Aug;64(2):248-50

“Thirty-six specimens of amniotic fluid across gestational ages (16 to 42 weeks) were tested. The fern test was unaffected by meconium at any concentration and by blood at dilutions of 1:10 or greater. When blood and amniotic fluid were mixed in equal amounts, ferning was not present. Arborization of amniotic fluid was unaffected by pH alterations.”

The ferning and nitrazine tests of amniotic fluid between 12 and 41 weeks gestation.  Bennett SL, Cullen JB, Sherer DM, Woods JR Jr.  Am J Perinatol. 1993 Mar;10(2):101-4

“Samples were obtained from amniocenteses between 14 and 42 weeks. Part I: Of 112 samples allowed to dry on a slide for 3 minutes only, 86.6% were ferning positive and 100% were nitrazine positive. Flame-drying increased the presence of ferning to 96.4%. Part II: 363 samples were allowed to dry completely for up to 10 minutes. All samples were ferning and nitrazine positive….These tests may be reliably performed at gestations of 12 to 41 weeks.”

MORAL:  Amniotic fluid will fern at any gestational age, in varying pH, and in the presence of a moderate amount of blood.  If you have fluid on the slide, and the slide doesn’t fern, you need to seriously doubt the diagnosis of ruptured membranes.

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  1. s. smith
    January 5, 2012 at 7:35 pm

    Personal experience tells me otherwise! I felt a distinct “POP” in my lower abdomen the night before I delivered, which felt (sorry to be gross) like someone snapped an industrial sized rubber band at my cervix. It was very disconcerting, much more so than a jabbing kick from my baby. Something told me to precariously get myself upstairs to the bathroom, and when I sat down to see what would happen, a GUSH of fluid came out. When I wiped, it was watery, mixed with mucousy, and possibly a tinge of pink? As I wiped, even more came out. Now look. I know what peeing feels like. I didn’t lose feeling in my urethral exit point just because I was close to delivery. I could feel when I actually peed after that, and it felt nothing like the GUSH, which felt like it came from my vagina…like I had a reverse douche! Anyway, I went in to the hospital, and when they asked me for a urine sample, I obligingly went to the bathroom, sat down, and as I put the cup below myself, and relaxed, GUSH!!! Again, same fluid material came rushing out…all over my hand, the cup, it almost squirted everywhere! It was a LOT. I told the interns and the nurses, and all of them said, no, that in fact they tested a swab from my vagina and that there was no ferning and two other tests, (didn’t specify) showed that I must have peed. Well, I would have liked them to do a darn pee test b/c what went in the cup neither looked like, nor smelled like pee, and I still had to pee after that big explosion. I think that research is timely, costly, and laborious, and that many doctors may be looking for the easy way out by simply saying, “hey, the test says what the test says!” When will doctors listen to women, several of us, who know our buttholes from our peeholes, from our vaginas….and are clear about where the fluid is coming from (sorry for the vernacular)? Just because you don’t have any other answer to offer does not mean that there is NO OTHER EXPLANATION. I’m not making crap up. I am aware of Bartholin’s glands and I am aware of excitatory lubrication expulsion for women, but look….this was definitely NOT that. I challenge doctors to do some meaningful work to get to the bottom of this, because belittling women by patronizingly telling them they’re not experiencing what they most definitely ARE experiencing, and telling them that for the first time in their lives, they must obviously be peeing and have absolutely no awareness that they are doing so is ludicrous! Give women some credit! I know what fluid coming out of my vagina feels like, the same way I can distinguish what fluid streaming out of my urethra feels like. I believe that the tests that you refer to are not sensitive or specific enough, and do not account fully for differences in individual chemistries. How many people do you know who were told that they did not have ruptured membranes, after they knew for certain they had, and then got an ultrasound showing less than 1cm of fluid in all measured areas????? An no fluid is some areas? This made me furious at my delivery! How can we see doctors perform miraculous surgeries like separating conjoined twins, but we can’t get to the bottom of this? You are resting on and are very assured of what you are saying but I challenge you to investigate this more. There is absolutely MORE to this story.

  2. s. smith
    January 5, 2012 at 7:42 pm

    And by the way, the studies you are citing???? They are dated as follows: 1990, 1984, 1993. Seriously??? I will go and look now to see if they are RCTs or if they are reviews of other people’s work from even earlier, which would mean, they are basically duplicating information and offering nothing new to the field. Meaning, not truly investigating anything…only perpetuating old information. Sorry to seem so confrontational, but this drives me, (as you said yourself in your writing above) CRAZY!

    • January 7, 2012 at 2:24 am

      Ms Smith – Thanks for your comments. Amniotic fluid does fern at any gestational age, though one still has to use their head if one doesn’t see that fern. If most of the fluid has come out and one cannot get a real pool of fluid to sample, one may not get enough actual amniotic fluid to create a fern on a slide. My article was talking primarily about second trimester samples, where the controversy exists.

      You’re right that the literature on the topic is old. Nothing new about it has been published, as it is a pretty basic topic that is not of great interest. The matter of whether amniotic fluid ferns is pretty much settled, so there is little reason to do new research on the topic. RCTs don’t really have a role in the evaluation of a diagnostic test. RCTs are for evaluation of interventions, not tests.

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