Ovulation
Ovulation occurs during the middle of a woman's menstrual cycle. The egg, when it is ripe for fertilization, travels from your ovary down the fallopian tube. The fallopian tube is about six or seven inches long, and leads from the ovary to the cervix and birth canal. Your egg can live for about a day after it's been released from the ovary, and waits at the opening of the cervix for a sperm cell to arrive and fertilize it
Male Ejaculation and Fertilization
After sexual intercourse, when the male reaches climax, muscle contractions in the penis send a small amount of semen into the woman's vagina. The average male ejaculation contains at least 40 million sperm. Most of these will die in your vagina due to the highly acidic environment, which serves to weed out the weaker sperm cells that wouldn't be able to survive. A single sperm cell--except in the case of twins--penetrates your egg, and a protein shield then forms around the egg to block any other sperm from entering once it's been fertilized.
Nurturing the Fertilized Egg
After an egg is fertilized by a sperm cell during conception, the fertilized egg is called a zygote. The zygote makes it's way down the fallopian tubes to the uterus where five to eight days after fertilization it attaches to the lining of the uterus wall and begins the stage of blastocyst development.
Blastocyst development is complete by day 10 or 12 after fertilization, and the zygote then moves on to the embryo stage. The embryo develops until about the end of the eighth week of pregnancy, and then it is considered a fetus until the baby is born.
Week 2-3
Around two to three weeks after fertilization, the fertilized egg has implanted in the uterine wall and cells have begun to form an umbilical cord. Growth of the embryo during this time centers around an axis that will become the spine and spinal cord. The embryo also starts to become more elongated and take on more of a human shape as it begins forming organs such as the heart and gastrointestinal tract.
Weeks 4-5
At this stage, the woman's body produces hormones to stop the menstrualcycle . The organs continue to develop and the heart starts to beat and pump blood through the circulatory system. Some tissue formation begins that will develop into the spine and other bones, and limb buds appear where the arms and legs will eventually form.
Weeks 6-8
By this time, the embryo is about a half inch long and has eyes, slits where the mouth and nose will form and even hands with discernible fingerprints. Most of the organs will complete development at this time except for the brain and spinal cord. Neural development will have advanced enough to make the embryo capable of some movement, but at this stage it is considered to have a "reptilian brain" with only the most primitive of brain functions. Hair starts to form during this stage and some of the facial features. The tail disappears and the embryo looks more like a human.
