Ejaculation ~ Welcome to the Vagina

The journey begins with ejaculation, and while it does get the sperm started on their way, it's no joyride -- the sperm are catapulted into the vagina at amazing speed and under intense shearing forces. (Many sources say the sperm shoot out at 28mph/45kph, but I think that's an urban legend.)
If ovulation is not occurring or approaching, the sperm are doomed on their mission, and find themselves in very hostile surroundings. The vagina's acidity will cause the sperm to perish within hours. Those who continue towards the cervix will be trapped in thick, sticky cervical mucus, and any who manage to struggle all the way to the cervix will find the entrance to the womb firmly closed.
We can hardly blame the female reproductive tract for being so unfriendly -- after all, bacteria and other invading organisms would like to make a home there.
But as ovulation approaches, the vagina becomes more hospitable, becoming more alkaline and less acidic. Cervical mucus changes from a dense, impenetrable barrier into a watery, slippery fluid; at a microscopic level, the mucus actually forms tiny channels, like swimming lanes, guiding the sperm forward. This type of cervical mucus is called egg white cervical mucus (EWCM), because it is clear and slippery. It can also be amazingly copious, and you may have noticed it literally dripping from your vagina on the day before and day of ovulation, an indication that you are in the most fertile days of your cycle.
Even under the most favorable conditions, the sperm have a daunting distance to travel. Although only about 4 inches (10 cm) lie between the sperm and egg, the extreme itti-bittiness of the sperm make this distance a marathon. If the sperm were the size of a salmon, the trip would be 43 miles (69 km)! The sperm's whiplike tail, 10 times the length of its head, can propel it at about 3mm per hour.
Millions of sperm have been ejaculated to make the attempt; 20 to 150 million sperm in a single ejaculate is considered the normal range, but it could be as much as several times more -- even up to a billion sperm.
Why are millions of sperm needed to fertilize one egg?
Because they don't ask for directions. -- GoAskAlice.com
Sperm are manufactured constantly in the testes, regardless of how often a man has sex. Temperature is a critical factor in sperm production; normal body temperature is just a little too warm for optimal sperm production, and so the testes are housed in the scrotum, outside of the body at 2 to 3 degrees lower than normal body temperature. A too-hot scrotum can drastically impair sperm production, which is why tight pants are sometimes blamed for impaired fertility. Other factors, such as drug use, smoking, excessive drinking, obesity, and chemical exposure, have been linked to poor sperm production.
Of the millions of sperm launched into the vagina, not all are Top Gun quality, so to speak. Some will already be dead, and even in normal, healthy semen up to 40% may be defective, malformed or unable to swim forward properly. These are quickly left behind, eliminated from the gene pool.
Healthy sperm propel themselves upward in the vagina, nurtured by fluid in the semen, and bathed in favorable cervical mucous. Fluid in the semen also helps suppress the female autoimmune system, but white blood cells can still attack the helpless invaders at any moment. Sperm cells must also avoid getting trapped in the numberless folds and crevices throughout the reproductive tract.

Heat Seeking Guided Missiles

How do the sperm know which way to go? Recent research reveals that sperm have olfactory sensors -- a sense of smell -- and may be guided to the egg by chemical attractants. This "scent" emitted by the egg has yet to be detected, but research continues to attempt to do so, because it could yield new treatments for infertility, and new methods of birth control. Evidence also suggests that sperm are guided by heat, and that the upper part of the fallopian tube is warmer than the bottom, leading the sperm to the egg.
When sperm reach the top of the vagina, they encounter the cervix, the entrance to the uterus. Now open in honor of ovulation, sperm continue through the cervix and into the uterus. Only 1% of the millions of sperm ejaculated into the vagina will actually make it into the uterus. Once here, they get a helping hand. Contractions of the uterus convey the sperm to the fallopian tubes much faster than they could swim themselves.
The fallopian tubes, also called oviducts, lead from the uterus to the ovary -- and is where the sperm and egg will meet. A few inches in length, the inside of the fallopian tube is no larger than a pin head, but quite roomy for the tiny sperm.
After a journey lasting about 10 hours, only a few hundred sperm will make it this far. At last, the goal is in sight.

Conception ~ A New Human

One lucky sperm is the first to reach the egg; it emits enzymes that help it first bond to the surface and then penetrates the zona pellucida, the tough membrane surrounding the oocyte. The egg emits enzymes of its own, preventing any other sperm from penetrating.
Technically, it is only at this point after fertilization, that the egg matures from an oocyte to an ovum. The ovum casts off a polar body, a small, useless cell, in which the egg discards its extra replicated DNA, and results in the proper 23 chromosomes with a single strand of DNA each. This is the final phase of oogenesis (egg formation), and it's triggered only if and when a sperm fertilizes the egg; so most oocytes never become an ovum at all.
I always think of fertilization happening almost instantly; the sperm bores into the egg, their chromosomes combine, and voila, a zygote. In fact, fertilization can take 24 hours to complete, with nearly half that time just for the ovum to shed the polar body. During this time, the sperm loses its tail and its midpiece, until nothing is left but its package of chromosomes.

Day 1: A Zygote, 1-Celled Embryo

At last, the nuclei of the egg and sperm fuse; the egg and sperm cease to exist individually as they unite to form a zygote. The 23 single chromosomes from the egg combine with the 23 from the sperm, forming a complete set of 23 pairs, 46 in total; and at this moment, the life of a new and unique human being begins. This singular genetic blueprint, defining every trait of this new person, has been uniquely defined among a billion possibilities, and can never be changed; nor will it ever be repeated.
Recall that part of that genetic blueprint is contributed by the last pair of chromosomes, pair #23. The egg has contributed an X chromosome to this pair; and the sperm that fertilized the egg carries either an X or a Y as its 23rd chromosome. If the sperm contributed an X, the embryo will become a female; if it contributed a Y, the embryo will become a male.
The fallopian tube is lined with microscopic beating hairs called cilia, which gently propel the single-celled embryo toward the waiting uterus, a trip that will last a week or so.

Days 2-3: First Cell Divisions

In about 40 hours, the cell will divide, replicating its DNA for the first time. This event will be repeated trillions of times to create all of the cells needed to form a complete person, but each time, the DNA copy will remain the same, inherited from the original zygote created when the egg and sperm unite.
The new cells are called blastomeres, and the following day, they divide again for a total of 4 cells.

Day 4: Morula

The embryo has become a ball of about 32 cells, and is called a morula (from Latin for mulberry).
With each division, the cells grow smaller. We can now see why the egg cell needed to be so large to start with; the mass of cytoplasm must support all these early cell divisions before the developing embryo reaches the uterus.

Day 5: Blastocyst

The following day, the morula becomes a blastocyst as a fluid-filled core forms, pushing the cluster of cells to one side within an outer shell of cells.
For the first time, specialized cells have been formed: the outer layer of cells are destined to become the placenta, umbilical cord, and amniotic sac.
The inner cell mass will become the fetus, and will form all of the tissues, organs, and bones of the body. These are the stem cells, cell "blanks" capable of generating the over 200 types of specialized cells in the body, as well as reproducing more stem cells. (During embryonic stem cell research, this inner cell mass is extracted, destroying the embryo.)

Days 6-7: HCG Production

On days 6 and 7, the developing embryo begins to produce hCG (human chorionic gonatotropin). We would probably not be very interested in this fact, except that hCG is detectable in the mother's urine by pregnancy tests. Sensitive home pregnancy tests can detect even the very small amounts of hCG that will be present by a few days from now, reducing the dreaded 2WW -- the 2-week-wait to find out if you're pregnant.

Choreography by DNA

By day 7, the embryo arrives in the uterus, and may float there for a day or two before implanting in the lining, the endometrium. The endometrium has been thickening since early in the cycle, in anticipation of nurturing the growing embryo. A placenta begins to form.
By day 22, amazingly, a primitive beating heart exists. The embryo has just survived the most hazardous three weeks of its life. Sadly, as many as half of fertilized zygotes never reach this point, due to malfunctioning chromosomes, improper cell divisions, or failure to implant.
As you have seen, the embryo's development is a complex choreography of the creation of specialized cells in a certain order. All of this activity is directed by instructions in the DNA.
By the second month, sex organs begin to form. If the fetus has an XY chromosome, the instructions in the Y chromosome dictate forming male sex organs; if it has an XX chromosome, the instructions in the X chromosome are carried out to form female sex organs.