Females are born with millions of immature eggs, hundreds of which will mature in their lifetimes. Each egg is about the size of a pinhead.
Women get their periods in cycles that occur about every 28 days, though it's normal for cycles to last 21 to 35 days, or even 45 days for young women [source: Womenshealth.gov]. Between periods, women ovulate, releasing a mature egg from one of the ovaries. Ovulation usually occurs about a week before (or after) a woman's period, although ovulation can be quite irregular and can occur even during the period. Generally, however, counting from the last menstrual period, most women will ovulate sometime between day 11 and day 21 [source: American Pregnancy Association].
Some women are able to feel an ache in the ovary area during ovulation. It's also possible to detect ovulation through a change in cervical secretion, which will be wetter and more slippery directly before and during ovulation. Ovulation usually causes a small dip in body temperature, followed by a spike, and women often measure their temperatures when trying to detect ovulation. Ovulation may also coincide with an increased sex drive, light spotting, a feeling of being bloated and even heightened senses, such as taste or smell.
Normally, one egg passes from either of the ovaries through the fallopian tubes. This only happens once per cycle. Sometimes two eggs (or, rarely, more) are released within a single 24-hour span. If both eggs are fertilized, it can result in fraternal twins.
The fallopian tube is where fertilization occurs. Each ovary is attached to a fallopian tube, and the opening from ovary to fallopian tube is about half an inch (13 millimeters) in diameter, but narrows down to a much smaller opening at the other end.
Inside the fallopian tubes are tiny hairs called cilia. They help pass the egg through the tube from the ovary toward the uterus. The entire journey takes several days, during which the egg exists in a perfect environment that provides it with nutrients.
Meanwhile, the uterus (prompted by signals released by the follicle that formed the egg) has formed an internal lining (endometrium) rich in blood and nutrients that's prepared to house and nurture the egg should it become fertilized. If no fertilization occurs, the egg disintegrates into the uterine lining that will soon pass from the body during a woman's period.
But what does occur during fertilization? Keep reading to find out.