For a woman to become pregnant:
- intercourse must take place around the time when an egg is released from the ovary (ovulation)
- the systems that produce eggs and sperm have to be working at optimum levels
- the fallopian tubes must be open and healthy.
The first key fact to understand is that women are born with around two million eggs (the largest cell in the body) and by the time they hit puberty around 300,000 remain and that decline continues. Once a month, every month from puberty to menopause, women ovulate, releasing eggs in the middle of their monthly cycle. The whole process is controlled by two glands in the brain – the hypothalamus and pituitary – which tell hormones in your body to trigger certain physical responses. The egg is released from a mature follicle in the ovary and travels down the fallopian tube. Fertilization may occur in the fallopian tube if sperm are present.
Eggs live (and can be fertilized) for 12-24 hours after being released, and sperm can stay alive and active in your body for up to 72 hours after ejaculation, so you don’t have to have intercourse at the exact moment of ovulation to get pregnant.
Men create sperm in the testes; they produce an average of 100 million sperm each and every day. Over approximately three months, these sperm travel through a system of tubes called the epididymis, maturing along the way before being released during ejaculation. Interestingly only around 4% of sperm in an average ejaculation are considered normal and capable of fertilizing an egg. Nevertheless, it just takes one sperm to fertilize the egg for you to become pregnant.
After the sperm is deposited into the upper vagina via ejaculation, they must travel through the cervical mucus into the uterus and then into the fallopian tube before they can meet with the egg. Sperm make this long journey under their own steam and with some help from upward contractions of the uterine walls. During the trip, sperm prepare themselves to meet the egg by subtle alterations of their heads (acrosome) and movement patterns. When they meet the outer membrane of the egg, the sperm start to burrow through it and then enter the egg itself. At the moment the first sperm successfully penetrates the egg, a reaction is triggered that makes the egg resistant to all other sperm. This single sperm is absorbed into the egg, where the genetic material contained in its head fuses with that of the egg. Fertilization is now complete.
After fertilization, the combined egg and sperm – now known as an embryo – develops in the fallopian tube for the first three days, then travels down into the uterus. By the fifth day it will become a blastocyst, a hollow ball of cells surrounding a cyst-like cavity. Once the blastocyst breaks free from its shell, or hatches, it is ready to adhere to the surface of the endometrium (implantation).
If fertilization does not take place, or if the fertilized egg does not attach itself to the endometrium lining of the uterus, it breaks down, the endometrium is shed and period follows.