There have been a rash of web posts in the last few years claiming nearly magical properties for this "spot," with some people claiming that simply having the head of the penis enter into the fornix causes an instant orgasm for the woman that is more intense than any other. With claims like that floating around, it seems like a good idea to take a serious look at the subject.
AnatomyLet's start with some real anatomy, because there's a lot of bad pseudo-anatomy running around on the internet with respect to the fornix. This diagram from a med school anatomy course shows two views of the female internal genitals:
In the second view, the woman is facing to the left and she has a normal (forward-tilted) uterus. (Compare this with the first view in the image below.)
Notice that the fornix is not a "spot." It is the entire ring-shaped depression that goes all the way around the base of the cervix. What we call the "anterior fornix" or "the A-spot" is just the front-most part of the fornix. What we call the "posterior fornix" or "the P-spot" is just the rear-most part of the fornix.
Similarly, the lateral fornices (shown in the left hand drawing) are just the left and right quadrants of the fornix. In spite of the way they are usually drawn and talked about, the four "fornices" aren't separated from each other, they are just names for four arbitrary segments of a ring.
Is the uterus "tipped"?Now look at the location of the cervix and the location and depth of the anterior and posterior fornices in these two images from the Mayo Clinic's website, showing the two most common positions for the uterus:
In this case, the woman is facing to the right in both images, so the posterior fornix (or P-spot) is on the left, toward the rectum, and the anterior fornix (or A-spot) is on the right, right next to the bladder.
As you can see, the uterus isn't just flipped toward the rear in the second image. It also connects to the vagina at a different point. That means that the cervix is in a different location – more at the far end of the vagina instead of being on the front wall – and it is oriented at a very different angle. As a result, the anterior and posterior fornices are very different in location, depth, and angle.
In particular, notice that in the "normal" position for the uterus, shown on the left, the anterior fornix is quite a bit shallower and closer to the vaginal opening than the posterior fornix. But in the image on the right, both fornices are the same distance from the entrance, and the anterior fornix is actually somewhat deeper. This can have a major effect on whether a penis "fits" into either fornix in a particular position.
Roughly half of all women have a uterus that is tilted forward and rests on the bladder. Around 30% have one that is tilted back, resting against the rectum. And the rest have a uterus that is in-between, so it's not leaning against either the bladder or the rectum. (If you have a uterus and you don't already know which way it tilts, your GYN can probably tell you.)
These aren't the only variations. Sometimes the uterus has a bend in the middle (called retroflex or anteflex, depending on which way it bends). This can increase or decrease how far the cervix is displaced toward the front or back wall of the vagina.
Finally, women who are not on hormonal birth control and are not post-menopausal will often notice that the cervix is lower for several days around the time of ovulation. Sometimes this means an increased chance of painful cervix hits just on those days when your sex drive is greatest, which can be really annoying if your partner doesn't know how to adjust his position!
The position, shape, and orientation of the fornixSo there's quite a lot of variation in the position of the cervix and the angle at which it sticks into the vagina, whether it's on the front (anterior) wall of the vagina, or it's right at the end of the vagina, or it's somewhat on the back (posterior) wall. It can be more extremely forward or back than the above images show, or it can be anywhere in-between. And since the fornix is the space around the base of the cervix, variations in the location and angle of the cervix create large differences in the location and shape of the fornix as well.
Getting sexually aroused causes the uterus to lift up a bit into the lower abdomen, helping to lengthen and straighten out the vagina. The vagina also "tents," or gets larger at the inner end.
Both of these changes open up and alter the shape of the fornices a lot, but they don't change the relative position of the cervix very much. Tenting creates a fairly large potential space all around the cervix - including the space we call the fornix - and the width, depth, and shape of the fornix in an aroused woman's vagina will vary all the way around the cervix. Furthermore, its shape at any particular point varies a lot from woman to woman.
Genital "fit" in different positionsThen there's the question of penis width, length, and curvature. A shorter penis may be able to reach the anterior fornix but not the the posterior fornix, particularly if a woman has a longer vagina and a uterus that is tilted forward. If a woman has a shorter vagina, on the other hand, a longer penis can easily get trapped in the anterior fornix and poke painfully into the cervix if it is driven to full depth, and yet feel good if angled past the cervix into the posterior fornix.
Similarly, a penis with a large head (glans) may be too large to fit into the anterior fornix and hit the cervix instead. And a curved penis that is just the right length for a particular woman may fit snugly into one part of her fornix in one specific position and still miss her entire fornix in all other positions.
As this indicates, positions matter a lot. For most couples in most rear-entry positions, for example, the head of the penis presses against the front wall of the vagina and strokes across the g-spot into the anterior fornix. This is one reason why many women like doggie, and especially like pronebone, where the penis enters the vagina at a steep angle to the vaginal axis.
On the other hand, most couples find that it takes an extreme angle in a rear-entry position for the penis to reach past the cervix and connect with the posterior fornix. The woman can stand and rest her hands on the floor or a piece of furniture so that she is bent completely over, with her chest against her thighs. Or, if she is comparatively tall and flexible, she can drop her chest to the bed in doggie and raise her butt as high as possible to duplicate the same pelvic angle. Even in these cases, though, the man must enter low and angle his penis upward for it to work for most women. (An upwardly-curved penis helps.)
In missionary, the penis is normally aligned more or less with the main axis of the vagina, not pressing one part of the vaginal wall more than any other. However, if the woman's butt is raised – e.g., if she grasps her knees and pulls them toward her shoulders, lifting her hips off the bed – the penis can be angled toward the front wall, sliding across the g-spot and aiming toward the anterior fornix.
Conversely, the woman can extend her legs flat on the bed and the man can slide forward along the woman's body in a "pelvic override" or "coital alignment" position, forcing his penis to aim more vertically downward toward the bed. This causes the head of the penis to press against the back wall of the vagina and slide along it into the posterior fornix (if the penis is long enough). This works best when the uterus is "tipped" toward the rear.
Experimenting in the "cowgirl" positionCowgirl (woman on top) is in many ways the most flexible position in terms of angle, and it gives control to the person best able to determine if the penis is in some "magic spot." So if you're the female partner, get on top in the normal cowgirl position, with your vaginal opening centered around the base of his penis, and then shift your whole body forward or back, causing the head of his penis to press in the same direction.
Specifically, if you sit up straight and slide your body forward toward his waist about an inch or two, you'll press the head of his penis against the front wall of your vagina, where it will stroke your g-spot and slide into your anterior fornix. If, instead, you lean forward and slide your whole body backward a little bit toward his thighs, you'll press the head of his penis against the back wall of your vagina, and it will slip into your posterior fornix if it's long enough.
(As is true for the coital alignment position, this creates extra pressure and friction between the shaft of his penis and the front of your vaginal opening. This feels good and may pull your labia and clitoral hood downward, creating extra stimulation for the clitoris.)
You can also shift to the left or right. Don't tilt to the side; stay upright or tilted forward while you move your whole body sideways an inch or two. This will angle his penis toward one of your lateral fornices. If his penis is too long for your anterior fornix, and too short for your posterior fornix, and it keeps hitting your cervix in the middle and hurting you, try a sideways shift. Aiming his penis toward one of your lateral fornices might turn out to be the perfect answer.
Summing it upThere are many people for whom none of this matters. Whether the tip of the penis fits "just right" into the fornix at some point around its circumference makes no difference in how PiV feels to them.
However, some people really, really like that feeling, and if you're one of them, it's worth giving some thought to anatomical differences and the geometry that makes for a good fit for you and your partner. In particular, finding a good "fornix fit" can add almost electric intensity to "yab-yum," the period of motionless penetration that many couples who practice tantric sex include near the end of the ritual.
The PiV positions that allow for that "fitting snugly into the socket" feeling will depend on the length, angle, and shape of the vagina, the location of the cervix, and the length, width, and curvature of the penis. And all of these things vary widely from one person to the next.
This means that there are no "best" positions for every couple. You will just have to experiment with your partner. The best positions for the two of you will depend entirely on how your bodies fit together in different positions at different angles and depths.
To help you figure out what's going on in there, a reader recommended this website, http://beautifulcervix.com/ as "a great way to visualize exactly how things look internally." It's a great resource!
But you shouldn't rely just on pictures and generalizations. Knowing the location of your (or your partner's) cervix and the tilt of your (her) uterus can help you understand what's going on in there and really help you visualize which way to move. For most women, the cervix is in reach of a finger, so go have fun "playing doctor"!