In most females, their ovaries — tiny sacs just outside the ends of the fallopian tubes — contain immature ovum (eggs). For females in their teens and 20s, there are usually millions of them. During each fertility or menstrual cycle, one of those usually develops, is released by an ovary (each ovary takes turns releasing an egg every cycle) and pulled into one of the fallopian tubes. If there’s no ovum, or won’t be one within a few days, pregnancy can’t happen.
When an egg does get released, that cell starts slowly moving through the fallopian tube. When there’s an ovum in the fallopian tube, or one is soon to head that way, a couple other things are also usually going on. For one, cervical mucus, the fluid around the cervix, tends to become very watery and stretchy. The os, the opening of the cervix, gets softer and more open. For sperm cells to even have a chance of getting to the egg, both of these other things have to be happening. And both of these things, as well as having an available ovum, occur in a small window of time: all of this usually only happens for a few days in each cycle.
If a male ejaculates inside the vaginal opening, or very directly to the vulva, then we’ve got the making of a potential pregnancy because of semen and sperm cells. Semen and sperm are not the same thing. Semen is a fluid from the seminal vesicles, the prostate gland, and Cowper’s gland, which all comes through the urethra and out of the opening of the penis during ejaculation. Semen almost always contains sperm cells but it is not, itself, just made up of just those cells. Less than 5% of semen is sperm cells. The fluid of semen is how sperm cells can move out of the penis, and through the vagina. But it does more than that. It supplies nutrients and energy those cells need to be able to do anything at all, and it also neutralizes things that create problems for sperm like traces of urine and the vaginal environment.
Testicles usually produce millions of sperm every day. Sperm cells are very fragile and super sensitive. They are not resilient to things like being moved around from place to place — like from penis to hand, then hand to vulva –, to changes in temperature, or to environments besides the testes and special conditions designed to protect them.
During some parts of the fertility cycle in a woman, cervical mucus is too thick and pasty for sperm cells to be mobile, and the opening to the cervix isn’t open enough for them to get inside of it.
Sometimes, sperm cells have to wait within the vagina or other parts of the reproductive system because they got there too soon and an egg, and the other conditions needed, weren’t there for them yet. Sperm cells can potentially stay alive within the vaginal environment for up to 5 days.
Other times, there’s just not enough semen or viable sperm cells in that semen for pregnancy to happen. In a lot of ways the vagina, cervix and uterus aren’t friendly to sperm cells, similar to the way our bodies try to fight off unhealthy bacteria or other potentially harmful visitors. This helps ensure that only the strongest, best sperm make it through the vaginal canal.
Even in perfect conditions, less than a thousand sperm cells (out of the initial millions) will make it to the fallopian tubes, and only a few dozen of the sperm cells may wind up reaching the outer membrane of the egg cell. That’s one reason the male body produces so many sperm.
When sperm can get through the vagina, and through the cervical opening, what sperm cells remain will swim to either fallopian tube, which means only half wil potentially reach the egg. Once in the fallopian tubes, sperm cells are helped by contractions of the uterus to keep moving, and also by the cilia, super-tiny finger-like filaments in the fallopian tube that push the sperm cells towards the egg cell. Chemicals inside the reproductive system also help out, changing the sperm cells to give them more energy.
At this point, there are usually less than just fifty or so sperm left that surround the ovum and try to enter the sac.
If the ovum is healthy, sperm will push into the protective layers around it. The few remaining sperm cells then compete until one sperm gets to the egg first and fertilizes it, with the egg also taking part by pulling the sperm cell inside. When the egg does that, and secretes some special enzymes, it locks out any of the other sperm cells left trying to get in.
This point is called the fertilization of the egg. But we still don’t have a pregnancy yet.
About a day or so after fertilization, the egg cell and the sperm cell together become something new, a zygote. It starts to divide into many cells, staying in the fallopian tubes keeping busy with that for a few more days. That zygote then starts moving towards the uterus, while continuing to divide. It changes some more, becoming a hollow ball of cells called a blastocyst, and that is what implants in the uterine wall, attaching into the endometrium (the uterine lining, the same lining that shed during a period) and then becomes an embryo. Conception or implantation is the terms used most often for this stage of the process.
Many times, without any interference from anyone, fertilized eggs don’t divide or implant and a pregnancy does not occur, which can happen up to 50% of the time.
But if and when all the mentioned activities go smoothly, and the blastocyst does implant itself in the uterus, that’s when a pregnancy has occurred!
This process tends to take anywhere between five days to two weeks.