For fertilization (and pregnancy) to occur, sperm must travel from Point A—the male reproductive system—to Point B—the female fallopian tube, where an egg is waiting. Simple, right? Not quite.
First, think about the math. The distance a sperm must travel to meet an egg is only about 6 inches/15 centimeters long. But that journey is 3,750 times the length of a sperm cell. It’s as if a person had to swim 40 miles to reach a destination. And the journey isn’t exactly a friendly and welcoming one; the course is filled with obstacles to be overcome. In other words, of the 50 to 250 million sperm (an accepted range for fertility) that start the journey toward the egg, the one that successfully fertilizes it is a true champion. And by the time this champion completes its mission, it is very different from the sperm cell that began it.
The Vagina: A Not-Very-Friendly Entranceway
Chemically speaking, the vagina is hostile territory for sperm; the environment is acidic (pH 4.2), which impedes sperm movement. But the seminal fluid not only provides the sperm with energy for the journey, it also helps change the vaginal environment to a more sperm-friendly pH 7.2. To do this, the semen (the majority of the seminal fluid) changes twice. First, after being deposited in the vagina, it becomes thicker and coagulated for about 20 minutes, which minimizes sperm loss. Then it liquefies, which helps the sperm swim into the uterus.
The Cervix: A Maze to Master
“The cervix is almost like a puzzle, a maze,” says Michael A. Thomas, M.D., professor and director of reproductive endocrinology at the University of Cincinnati Academic Health Center. There the sperm confront two obstacles.
The first obstacle is the cervical mucus, which changes during a woman’s cycle. For most of the cycle the mucus is thick and viscous, trapping sperm and preventing them from proceeding further. But close to ovulation the mucus becomes thin and stretchy, helping the sperm to cross the cervix and enter the uterus.
The second obstacle are cervical fibers, which are so close to each other during most of the cycle that they act as a barrier to prevent sperm from passing. But close to ovulation the gaps between the fibers widen and create tunnels that orient and guide sperm into the cervix. Fewer than one million of the original 50 million-plus sperm make it this far.
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The Uterus: A Steep Climb
Once into the uterus, sperm need to climb to reach the fallopian tubes (also called oviducts). Many sperm die along the way, and only a few thousand reach the top. There, two roads exist, since there are two oviducts, each leading to an ovary. But only one ovary releases an egg each cycle. Some sperm will head toward the ovary that did not release an egg. Only a few hundred sperm will survive and choose the path toward an egg.
Throughout this journey, the sperm are undergoing a drastic transformation. Freshly ejaculated sperm are incapable of fertilizing an egg. The newbie sperm must go through a process called “capacitation” within the female body. Chemicals in the sperm’s head change or are removed, and the sperm’s tail movement increases. “These steps help the sperm mature and render them able to fertilize an egg,” explains Dr. Thomas.
The Fallopian Tube: Victory
In the fallopian tube, the sperm that remain await the egg. As the ovulated egg makes its way down the tube, these sperm surround it. Many sperm try to enter. One champion succeeds.
A version of this article originally appeared in the July/August 2009 issue of Conceive Magazine.
