The most important information about fertilization: What exactly happens when an egg is fertilized by a sperm? How long can sperm cells survive? And what is a Y spermatic cord?
We want to make a baby! If that’s true, then you should have as much sex as possible around ovulation. And then? You need to be patient before you can find out with a pregnancy test whether it worked. But maybe you want to know exactly how fertilization works? We explain all the important facts here.
On the way to fertilization: off we go with the mass start
In the beginning, there are an incredible number of them: Around 100 million sperm threads make their way to the egg together after ejaculation. But 90 percent of the tiny, tail-whipping sperm cannot escape the germ-hostile sperm environment and die. The rest wiggle their way through the cervical canal into the uterine cavity (womb). But which sperm will make it to fertilization?
Step two: Refuel and align first
Sperm may be motile, but they can’t get far without help: they need the mucus at the cervix and in the uterine neck, which fills them up with special sugars. The secretion also guides the sperm; it bundles them together and pushes them upwards as if on a rail. Without this guidance, the sperm would tail around in circles. And never fertilize the egg.
Before fertilization comes the merciless selection
Even in normally fertile ejaculate, deformed sperm can be found, those with two heads or with missing tails. The secretion in the cervix sorts out the damaged sperm threads: two-headed ones cannot thread themselves into the correct path (because they are too big), tailless ones bend off because there is no “tail rudder” to provide stability.
Some sperm go nowhere
At least half of the sperm make a wrong turn out of the uterus – into the fallopian tube, which does not contain a fertilizable egg in this cycle. The tissue absorbs sperm that have strayed. They will no longer play a role in fertilization.
Only eight hours for fertilization
The egg cell, the largest cell in the human body (with a diameter of a tenth of a millimeter), has a very limited lifespan. It is ready for fertilization for just eight hours. Nevertheless, there is a chance that egg and sperm cells will unite on around five days of the cycle: sperm can survive in the fallopian tube for up to five days and wait for the egg. More on this topic: How long do sperm survive?
The bridal dance in front of the egg
A few hundred sperm threads make the journey and pass the selection process, eventually buzzing around the egg cell ready for fertilization. It is not the fastest sperm that makes it to fertilization, but the one selected by the egg: its outer shell sends out attractants. The helmet on one of the waiting sperm heads reacts most strongly to this. And the egg cell now opens its tough shell for this sperm thread. At the same moment, the path to the inside is blocked for all other sperm. This is because mating requires only two cell nuclei, one female and one male.
The calm after the storm
Slowly, the nucleus of the sperm and the nucleus of the egg drift towards each other. They separate their shells, their chromosomes form 23 pairs – the cipher for a new, unique human being.
And finally: gender is determined
Egg cells always carry the sex chromosome X in their cell nucleus, sperm cells can be equipped with an X or a Y. If an X sperm is used, it becomes a girl; if it has a Y sperm, it becomes a boy. For a long time, Y sperm were thought to have the ability to win quickly. Sex at ovulation, it was said, would bring the lighter, faster, tail-whipping boy sperm threads to the fore. Since it has been possible to watch conception (with IVF), fertility doctors have discovered that speed and lower weight do not give the Y sperm an advantage. More and more studies indicate that the woman’s hormonal climate affects the egg’s envelope. This envelope selects the sperm, which then comes into play.