Friday Weird Science: Feeling the Vaginal Pressure, the idea of "insuck"

Dec 17 2010 Published by under Friday Weird Science

You know you've blogged too much when you have to search yourself to check whether you've written something up before. I cannot BELIEVE I haven't done this one yet!

I always wonder about laboratories that study orgasm and human sexual behavior. Do people stand around looking all official at lab meetings and talk with very serious demeanor about uterine and vaginal insuck? Or do they give in and let the hilarity reign? I would think, if the second, it'd be a really fun place to work.

Anyway, for a bit on Friday Weird Science I've been writing posts about the "function" or "role" of female orgasm. Does it have one? Does it have more than one? Does it matter? And while modern studies have generally focused on the possible pair bonding and hormonal aspects of female orgasm, older studies were after the physical angle. People assumed that female orgasm HAD to help the process of sperm and egg melding SOMEHOW.

But how?

Well, how about insuck? Fox et al. "Measurement of intra-vaginal and intra-uterine pressures during human coitus by radio-telemetry." J Reprod Fertil. 1970


For time immemorial, people have wondered what the female orgasm was for:

And at first, before thoughts of pair bonding and hormonal influence on behavior was strongly considered, people thought that the orgasm must do something physically to improve the odds of conception. After all, there's all those contractions...they must do SOMETHING, right?!

These scientists were determined to find out. They took one couple (married 11 years, none of this pre-marital insuck, thank you), and a radio pill. They placed the radio pill either in the vagina or the cervix to measure pressure, and placed the receiver under the mattress where the deed would be done.


That's the "radio pill" 9mm across. Poor woman had to have her cervix dilated. But it's not that bad, most modern intrauterine devices are of a similar size. Though it was ALSO connected to a "thread", so they could get it out again. Hope that didn't ruin the mood.

Anyway, they put it in, and got busy. As the deed was done, a pen attached by wire to the recorder located in the mattress was recording the vaginal or uterine pressure.


Here you can see the recordings of vaginal pressure from two rounds. In both instances, they noticed a big increase in vaginal pressure during female orgasm, and a FALL in pressure during male orgasm. In the uterus they observed something similar.


You can see a massive spike in pressure during the female orgasm, with a drop around male orgasm, and after that (the lower panel) the uterus returns to nice, even contractions (about 4 per minute).

The scientists concluded that what was happening here was something previously observed in horses, the phenomenon of "insuck". during sex, the mare's uterus goes to very negative pressure, essentially sucking the sperm into the uterus and thence into the fallopian tubes to facilitate fertilization. The authors of this paper think that the decrease in pressure following female orgasm in the human may have a similar effect (though "insuck" is not as dramatic as the guy who proposed that the penis acts as a 'plunging mechanism' to make sure that sperm are 'flung into the uterus').

But. This hasn't actually been proven. Masters and Johnson in fact tried this experiment using radio-opaque fluid in a cervical cap, but couldn't get transmission of the fluid into the uterus after orgasm (the researchers in this study blame the findings of Masters and Johnson on "artificial stimulation"). And while the authors think their idea of "insuck" is important, they have to admit it's not necessary. After all, female orgasm is not necessary for fertilization. But they took the high road, saying that "Our hypothesis must, therefore, be considered to explain only one, albeit the most efficient, method of fertilization."

So it's not THE answer, but it's the most efficient one. 🙂

As for me, I'm not sure I believe it. Try as I might, I cannot find a more modern replication of this study (with an n higher than 1, please). So until I see one, I'm not putting much weight on "insuck". Though it still sounds more fun than the 'plunging mechanism'.


7 responses so far

  • Seriously, where do you find this stuff?

  • AmoebaMike says:

    In order for "insuck" to really help, orgasms would have to be synchronized, i.e. once pressure has dropped and the male ejaculates, there's no longer any benefit.

    As seen in the graphs above, Hollywood's notion of male and female climaxing together doesn't happen often.

  • BDNf says:

    Well, this is not exactly what you're looking for and you're probably already aware of these but here it goes.

    The "upsuck" hypothesis has been tested by Baker and Bellis (1). They found some evidence that it could play a role but propose a third hypothesis. I haven't read the citing articles so I don't know if it's been contradicted or confirmed.

    The transportation of sperm is obviously not limited to the time of copulation nor around the orgasm but the "pump" mechanism remains pertinent : "These data support the view that uterus and Fallopian tubes represent a functional unit that is acting as a peristaltic pump and that the increasing activity of this pump during the follicular phase of the menstrual cycle is reflected by an increased transport into the oviduct ipsilateral to the ovary bearing the dominant follicle. In addition, they strongly suggest a critical role of oxytocin in this process." (2) (the title alone is appealing...)

    Furthermore, according to Levin, rapid transportation wouldn't be an efficient mode of fertilization : "Orgasm occurs with vaginal and uterine contractions. The latter have been misinterpreted as powering rapid sperm transport to facilitate fertilization, but such fast transport would lead to the tubal deposition of noncapacitated, incompetent spermatozoa. Vagino–cervico elevation, however, delays rapid sperm transport and allows the initiation of decoagulation and sperm capacitation before the elevation resolves. The fastest transport of spermatozoa from cervix to the fallopian tubes occurs in the nonaroused female by uterine/subendometrial smooth muscle peristalsis." (3)

    (1) Human sperm competition: ejaculate manipulation by females and a function for the female orgasm ( )

    (2) Sperm transport in the human female genital tract and its modulation by oxytocin as assessed by hysterosalpingoscintigraphy, hysterotonography, electrohysterography and Doppler sonography ( )

    (3) The Physiology of Sexual Arousal in the Human Female: A Recreational and Procreational Synthesis ( )

  • Jamey Mazor says:

    Merci à vous serve cette info. C’est toujours important de évaluer ce genre d’avis.

  • [...] about the second hypothesis? The idea of "insuck", as it were? The concept is this: over a long time now, people have wondered about the purpose of [...]

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