Approximately 250 million sperm enter the vagina, most of them die (99%) within 30 minutes. Once inside the sperm are killed off by deadly acids that protect the vagina from pathogens.
The Uterotubal Junction/Fallopian Tube
60,000 remaining sperm must navigate through the cervix located at the far end of the vagina. The cervix is usually plugged shut with mucus to defend against invading pathogens. However, during ovulation, estrogen flows from the ovary to the uterus, which softens the mucus to make it watery so sperm can navigate through. Sperm must single file through tiny channels in the cervical canal. Most of the sperm will die because they will get lost in the tens of thousands of passageways.
Approximately 3,000 sperm remain and must navigate through the uterus to the fallopian tube. Muscular contractions of the uterus will help to propel sperm in the direction of the correct fallopian tube on the side of the egg. Sperm are attacked by Leukocytes, or white blood cell army that is launched by the body's immune system. The remaining sperm must search for the doorway to the fallopian tube.
Once the sperm gets to the uterobal junction, or the doorway to the fallopian tube, the sperm must show the proper swimming characteristics and display the correct molecular recognition to be allowed to pass through. In the fallopian tube approximately 10-30 sperm remain. Once inside, sperm are safe from attacks. Sperm are provided with nourishment from the fallopian tube so they stay alive for several hours to days.
The remaining sperm must attempt to find the egg at the upper end of the fallopian tube. Capacitation: shed layers of protein to be able to propel faster in order to penetrate the egg. Timing must be perfect because a sperm will only live for a few hours after the capacitation process. The sperm that makes contact with the "outer coat" of the egg has a bag of enzymes on its head that explodes, which allows it to penetrate the "outer coat" of the egg. Once the sperm is inside, the genetic payload is delivered and the 23 individual chromosomes of the sperm and egg pair up. The sperm determines the sex of the baby, either an X carrying sperm that will develop into a girl (XX) or a Y carrying sperm that will develop into a boy (XY).
The zygote (union of sperm and egg) travels down the fallopian tubes towards the uterus, cells begin dividing. On day four the tiny bundle of cells is named a blastocyst. Five days later, the embryo breaks out of protective shell and implants in the wall of the uterus and begins growing.