FERTILIZATION

Fertilization occurs when a live sperm penetrates the newly released mature egg successfully and cell division starts. The woman has now conceived - the embryo is growing!

During sexual intercourse, millions of sperm are released into the vagina and move through cervix to uterus and fallopian tubes. Hormones during ovulation change the cervical mucus – it becomes watery allowing the sperm to swim up the vagina through the cervix. At other times the mucus is more viscous and unreceptive to sperm. Only about 2,000 sperm will be strong enough to reach the uterus and the fallopian tube.

The timing of sexual intercourse is very important for fertilization to occur. The egg can only be fertilized for about 18-24 hours after it is released, so sperm must be present in the fallopian tubes around the time of ovulation. Man's sperm can only survive approximately (average) 3-5 days so a woman's fertile time will only be around 5-6 days every month, with the most fertile time being the 2-3 days up to and including ovulation.

Research suggests that even if intercourse takes place at the right time a normal healthy couple may still only have a 30-50% chance of fertilization and pregnancy occurring.

Once fertilized, an egg normally takes a few days to travel into the womb where it will need to implant itself in order for a viable pregnancy to begin. If this implantation happens the pregnancy hormone called Human Chorionic Gonadotrophin (hCG) starts to be produced.
This hormone is vital for the maintenance of the lining of the womb and the new embryo and the amount of hormone produced rises rapidly, doubling approximately every two days, reaching a peak 60 to 80 days after conception. Soon after conception tiny amounts of this hormone can be detected in a woman's urine and this is what the early pregnancy test kits measure.

Chances for getting pregnant for healthy couples who had intercourse during ovulation are 50/50. So don't be discouraged if pregnancy does not occur after the first month of trying.

Fertilization Important Steps

Ovulation. The ovulation process is crucial for fertilization to take place. Ovulation is fully developed and controlled by hormones – it is pretty complicated hormonal interactions involving a variety of endocrine glands. Tubal function must also be adequate or the ovum will not be picked up by the fallopian tube to be fertilized within the ampulla. Women are fertile for about 24-48 hours around the time of ovulation (before and after ovulation).

Sperm. In most ideal conditions which include ideal vaginal, cervical and uterine environments; fertile eggs, strong sperm health, etc. - sperm may be able to survive during 6-7 days. Sperm can remain fertile in the female reproductive tract for about 48 hours, although this can be quite variable.

Fertilization. Following ovulation, the ovum is picked up by the fimbria of the fallopian tube. It remains in the ampulla portion of the tube (in average during 18-24 hours). Healthy sperm usually “wait” in fallopian tube for healthy egg (in average 48 hours). One sperm penetrates the zona pellucida (egg). If fertilization does not occur, the ovum disintegrates and is destroyed by the tube.

Implantation.  After fertilization occurs, the ovum remains in the fallopian tube for about 72 hours. During this time there several cellular divisions are happening, but the size of the fertilized ovum does not increase. Around 72 hours the zona pellucida fragments and falls away. The ovum enters the uterine cavity for 60 to 72 more hours, and the central cavity begins to form. A definite cell mass is formed on one side of the blastocyst by the time implantation occurs. The trophoblast cells burrow into the endometrial stroma to form syncytiotrophoblast. Primitive amniotic and chorionic cavities begin to form, and a germ disk is recognizable soon after implantation.

Fertilization - Important facts to remember

Fertilization happens in one of the fallopian tubes, which carries an egg from an ovary to the uterus. It can take up to seven days after intercourse for sperm and egg to join and become a fertilized egg. Usually, it's because the sperm gets into the fallopian tube before the egg is released.

How Long After Ejaculation Sperm Live (From WikiAnswers): Sperm exposed to room air on clothing, bed linens or toilet seats lose motility (the ability to swim) rapidly. Once the semen dries out, the sperms are usually dead.
If the semen is near the woman and no fertile fluid is present, they will die within a few hours outside the body. If semen is spilled on the external genitals (not in the vagina) they can swim to enter the vagina. Thus they travel from outside the body to inside.

Inside the body sperms can survive for 72 hours or more. Half of them perish in 1 to 5 hours journey inside. Note that this half is more than 60 million sperms. That said, you should take basic caution after sex that you've cleaned up any big splashes on her or around her.