Understanding the Conception Process

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.

As long as a sperm cell is alive in the fallopian tube, it's capable of fertilizing an egg. If there's no egg in the fallopian tube, there's no chance of fertilization.

The fallopian tubes are about 4 inches (10 centimeters) long and transport the egg from the ovary to the uterus. They also provide sperm that make it that far with nutrients and a safe environment, the same kind the egg enjoys as it passes through. Of the millions of sperm cells that initially enter the cervix, there may be anywhere from one to a couple hundred that arrive at the fallopian tube [source: Regan].

Eggs will survive about a day after they're released from the ovaries. If not fertilized, they'll break down. It's only during this day that a woman can become pregnant, though it may be a result of a sexual encounter days earlier, since sperm can survive in the fallopian tubes for a few days.

When an egg does pass through, the sperm have receptors that allow them to smell the eggs, which are surrounded by cells releasing the sweet scent: progesterone. Sperm cells most definitely become very active when an egg is present. In fact, progesterone makes sperm become so active that they slough off layers of proteins. Both the surge in activity and the loss of proteins enables sperm to pierce the egg. This process is called capacitation. Once this occurs, the sperm only have a few hours to live. Only a few -- perhaps half a dozen or less -- sperm cells will ever share proximity with the egg [source: National Geographic].

How does the sperm cell actually penetrate the egg? The head of the sperm, once making contact with the exterior of the egg, will more or less "pop," releasing enzymes that allow it to cross through the barrier.

Once a sperm cell penetrates the exterior of the egg, fertilization occurs -- its DNA payload is delivered as the sperm is absorbed by the egg. The genetic blueprint of the child is now set in stone. Once a single sperm enters the egg, the egg's protective protein covering changes and doesn't allow other sperm to enter.

From one egg that has been fertilized by one sperm cell, encoded genetic information coupled with cell growth will eventually create an entire human being.

But first, as we'll learn in the next section, that fertilized egg must find an ideal location where it can undergo radical change.

Conception is the fertilization of the egg by a sperm cell. If you're a sperm cell entering the vagina, there's about a 1 in 250 million chance you'll be the sperm cell that reaches the egg. Once that occurs, the fertilized egg is known as a zygote.

The initial follicle from which the egg was formed (the corpus luteum) will release progesterone for approximately 14 days [source: N.V. Organon]. This prompts the buildup of blood and nutrients in the uterine wall (endometrium).

Information encoded in the collective DNA of the egg and sperm cell instructs the zygote to continue its development into an embryo. Within 24 hours after forming, a zygote will begin a process of cellular division that will soon lead to exponential cell growth. Very early on, it will develop into a solid cluster of cells, but then it forms a blastocyst, a hollow structure that the mass of cells continues to develop within. The mass of cells inside this structure will be the embryo, while the outer wall will become the placenta and other nutrient-providing tissues needed by the fetus.

The zygote then makes a four-day journey down the fallopian tube toward the uterus, aided again by the tiny hair-like structures lining the tube. After about five days, the zygote will have made its way into the uterus. In one more day's time, this mass of cells will "hatch" from its thin-walled sac.

Now, the blastocyst is ready to make direct contact with the endometrium. When it does, both the endometrium and the blastocyst will exchange hormones, allowing the blastocyst to connect to the uterine wall, a process known as implantation. Women may experience light bleeding or spotting during this process, though it should only last about 48 hours.

As this process occurs, the cervix will be closed with a mucous plug. With the zygote safely in the blood- and nutrient-rich uterine lining, the pregnancy has taken a big step forward. At this point, some of the zygote's cells form the placenta, while others form the developing embryo.

Within about three weeks, the first nerve cells will develop. As the weeks and months roll by, explosive cell growth will occur, and extremely specialized and astoundingly different cells will be created.

Interestingly, if the fetus is female, she very quickly develops millions of immature eggs that she'll carry into adulthood, at which point she can begin this cycle all over again.

See the next page for lots more information on conception.