A crowd of 300 million dashes in the dark at top speed to capture a rare and mysterious prize. Hundreds of millions will perish before they come close, but one will be victorious and form a union for eternity that will change the fate of humanity. It may sound like a big-budget Hollywood film, but it's the real-life drama that makes a baby! As it turns out, conception has nothing to do with birds, bees or storks, and everything to do with timing, luck, and a lot of heroic effort by cells so small they can't be seen without a microscope.

But, let's back up and start the story at the beginning:

When you were a 20-week-old fetus inside your own mother's womb, you already had all of the eggs you'd ever have, about 400,000 in all. Of these, only about 400 are destined to mature and be released over the course of your reproductive life. Even fewer will ever be fertilized. And only about 40 percent of those that are fertilized will implant in the lining of your uterus and grow into an embryo.

Of those implanted embryos, only 80 percent will grow into a full-term baby. In other words, the cells that came together to create your baby are luckier than a lotto winner on leap day. Conditions must be perfect and the timing must be exactly right for conception to happen in the first place.

Eggs, also known as ova, are the largest cell in the human body, about 1/175th of an inch or 1/7th of a millimeter.

Your eggs have been waiting for years inside a tiny bubble in your ovary called a follicle. Every month, about three to 20 eggs receive a hormonal signal that cues them to mature and prepare for ovulation. As ovulation nears, one follicle will become dominant and grow faster than the others, nearly tripling in size, and the others will stop growing.

As the follicle grows, it sends a hormonal signal to the lining of your uterus, called the endometrium, telling it to thicken with blood.

Ovulation happens when the winning egg bursts through the follicle and through the wall of your ovary. Some women experience sharp pains on one side when the egg bursts through the ovary wall, others don't feel anything. The egg then starts to travel down the fallopian tube, coaxed by tiny finger-like projections called fibrae.

Meanwhile, your partner's reproductive cells, sperm, are maturing to make it happen. Men make an average of 1.2 trillion sperm during their lifetimes, and release 300-500 with every ejaculation. Sperm take about 90 days to mature. They are so tiny that they can only be viewed with a powerful microscope - each one is about 4 microns long, or half the size of the average human cell.

Your partner's sperm count is key to making conception happen. While technically it only takes one sperm to fertilize an egg, sperm swim and work in teams. Alcohol, hot tubs, tight pants and exposure to certain drugs and chemicals can be fatal to sperm. Conditions must also be just right inside of your body. Your cervical fluid must be exactly the right consistency. If you have a 28-day cycle, this happens about 14 days after your period starts.

As your 24- to 48- hour window of fertility approaches, your cervical fluid will change texture from watery to creamy, then become slippery.

If you could look at the fluid under a microscope, you'd see microscopic channels that resemble the fronds of a fern. Sperm swim up these ladders in teams at the rate of seven inches per half-hour, occasionally stopping to rest, because swimming is hard work-- they have to beat their tails 800 times just to move 1/3 of an inch. If conditions in your vagina are too acidic or your cervical fluid isn't the right consistency, the sperm will quickly die. But if conditions are right, sperm form scouting parties to locate the egg.

Sperm appear to be able to communicate with one another, and if another man's sperm are present, they'll actually gang up on the intruders and fight them, ramming them with their heads and lashing with their whip-like tails. Some researchers have theorized that funny-looking sperm, such as those with two heads, may actually exist to sacrifice themselves in kamikaze attacks against intruders.

Every sperm carries a half-set of genetic information with it, including an X or Y chromosome which will determine your baby's gender. Sperm with the X, or female chromosome, swim more slowly but live longer. Sperm with the Y, or male chromosome, swim more quickly.

If you chart your cycle and know when you ovulate, you can actually improve your odds of conceiving a boy or girl by timing intercourse. Trying to conceive the day before ovulation improves the odds of having a girl, while waiting until the day after ovulation will favor the faster but more fragile male sperm.

Neither sperm nor eggs can live longer than 24 to 48 hours after they leave the comfort of the scrotum or ovary. But if all conditions are right, the sperm and egg will join in the fallopian tube.

When the strongest and fastest sperm reach the egg they become hyperactivated, ramming their enzyme-coated heads at it to break down and dissolve the egg's outer shell. Eventually, one lucky sperm will break through, and the egg instantly shuts down. No more sperm will be admitted.

The chromosomes of the egg and sperm then fuse together, assigning your future baby a gender, hair color, eye color and hundreds of other genetic characteristics. Identical twins or multiples may be formed if the egg then splits in two. Fraternal twins or multiples will be formed if more than one egg gets fertilized.

Over the next few days, the fertilized egg's cells multiply rapidly with the fibrae sweeping the tiny ball of cells, now transformed into a blastocyst, down the fallopian tube and into the top of the uterus.

Once there, the fertilized egg burrows into the lining of the uterus. The implantation of the fertilized egg is a tricky process, and it only succeeds about half of the time. If the fertilized egg implants, the site of implantation will be the place where the placenta attaches to the uterine wall.

After the blastocyst has implanted, it emits hormonal signals that tell the lining of the uterus to stay in place, instead of disintegrating and shedding as it normally would during menstruation.

Meanwhile, a few weeks from now you'll be worrying that your period hasn't come yet, and you may have an intuitive sense that something's up, or just feel bloated like a sure case of PMS, have a strange taste in your mouth, or your breasts may feel a little odd - the first faint clues that a baby is burrowing in who will take you on the adventure of a lifetime.

About the Authors
	Sandy Jones and Marcie Jones are nationally-known pregnancy and baby care experts and best-selling authors. Their pregnancy book: Great Expectations: Your All-in-One Resource for Pregnancy & Childbirth, is available from Barnes & Noble.com. Click here to buy your copy today!