So! Withdrawal method! As promised (though just a smidge late).
Sounding like exactly what it is, the withdrawal method is when a man pulls his penis out of a woman's vagina just before ejaculation. The thought behind this method is, if the sperm are not able to enter inside the vagina, then the woman cannot get pregnant.
Problem is, it's not that simple.
For one thing, 'pulling out' is not as easy as it sounds. It takes quite a bit of power over the body to be able to not only know the exact moment when one should pull out, but also, to have the ability to do so when one's on the brink of orgasm.
Another important sidenote deals with those pesky sperm. Approximately 300 million sperm accompany a single ejaculation. Obviously, the withdrawal method (with perfect us) works well to take out a large chunk of the pregnancy risk. Unfortunately, ejaculation isn't the only time sperm come into play.
The Cowper's Gland produces fluid to neutralize the acidity caused by the urethra's (tube that the bladder empties urine outside the body, and also the exit of ejaculate) transport of urine. It also has a couple thousand sperm in it. When a man becomes erect (or sometime afterward, it's not on a clock of any sort), often times tiny droplets expel from the tip of the urethra, onto the glans (head of the penis). This is referred to as Cowper's Fluid or "pre-cum". In other words, even if a man pulls out before ejaculation, sperm (albeit a whole lot less) still infiltrate the vagina.
Effectiveness: With perfect use - 96%. Including user error - 81%