Understanding how sexual reproduction works may help you meet your goals, whether those include getting pregnant or avoiding pregnancy. So in case you missed this subject in your high school health class or need a refresher, a sexual reproduction primer follows.
Sexual reproduction in people involves a man and a woman. The man contributes a sperm cell, and the woman contributes a cell called an egg. Sperm cells are continuously made in the testes, glands that reside in the scrotum just beneath a man’s penis.  When a man and woman engage in vaginal sexual intercourse, the man puts his penis inside the woman’s vagina or birth canal. He can then ejaculate or release a substance called semen from his penis. Semen contains mostly water, as well as nutrients and, in highly fertile men, up to 500 million sperm cells. 
If the man and woman are not using a barrier method of contraception, such as a condom or a diaphragm, semen enters the vagina via ejaculation. Side note: Another way of getting semen into the female reproductive system is artificial insemination—where previously collected semen is introduced into the vagina, cervix, or uterus directly. Then, sperm cells start swimming. In the case of typical sexual reproduction, they swim up the vagina, through the cervix, which is the opening of the uterus, into the uterus or womb, and then into the fallopian tubes—special cylinders of tissue that connect the uterus with the two ovaries, the organs where eggs mature. Unlike sperm cells, which are made constantly, the one or two million eggs a woman is born with are all the eggs she will have in her life. 
It’s All in the Timing
In fertile women, one ovary releases a mature egg—usually just one, but very occasionally two—into the fallopian tube during each menstrual cycle in a process called ovulation. If a woman is ovulating or has recently ovulated, the sperm have a chance of meeting that egg, usually inside the fallopian tube into which it was released. If an egg is not present, the sperm eventually die, but they can live inside the woman’s body for up to five days, meaning that even if a woman has not ovulated before vaginal intercourse, she could still get pregnant. 
Fertilization Starts Everything
When sperm cells get close to the egg, the egg releases a hormone called progesterone, which make the sperm hyperactive. When a sperm cell comes in contact with the egg’s surface, the sperm head releases chemicals that digest the layers of jelly that protect the egg. These chemicals also induce a reaction in the egg that blocks any other sperm cells from getting in. Once the sperm makes it completely inside the egg, the egg is considered fertilized; this event is also known as the moment of conception. In rare occasions where a woman’s ovaries have released more than one egg, it is possible that each egg could meet a sperm cell and be fertilized, which would result in fraternal twins. 
Both sperm cells and eggs have half the amount of genetic material, called DNA, that a regular human cell has. Therefore, when they come together during fertilization, their union leads to one cell—the fertilized egg, also called a zygote—with exactly the right amount of DNA. And if you could examine the zygote’s DNA at this stage, you would already be able to tell whether it has the potential to develop into a male or female human. Females have two bundles of DNA called X chromosomes and males have one bundle called an X chromosome and one called a Y chromosome. Each parent contributes one of these so-called “sex chromosomes” to the fertilized egg. 
The zygote then starts to move down the fallopian tube into the uterus—a several day trip—as it makes copies of its DNA and divides: first into two cells, then four, then eight, and so on. As it becomes a larger bundle of cells, it is called an embryo. Very rarely, at an early stage of division, the embryo will split into two embryos, which can lead to identical twins. Once the embryo reaches the uterus, it has about 100 cells and will implant into the soft and cushy uterine wall. If conditions are right, it will continue to grow and develop into a baby.