This week begins your `real` pregnancy. The release of your egg is counted as day 14 and fertilization occurs either on that day or over the next 48 hours.
When they are born, baby girls have enough egg cells to last their whole life. In males however, sperm cells by the million are created on a daily basis and take approximately 61 days to mature. A mature sperm has a round head, a middle section and a long tail which they use to swim forwards. The head of the sperm contains the genetic material of the donor. One sperm cell lives for 48 hours and can travel at a speed of 4mm per minute.
Ovulation and fertilization
At the time of ovulation the cervix becomes softer and wider and the mucus becomes thinner, more elastic and less acidic, allowing the sperm cells to pass through more easily. At the time of ejaculation two hundred million sperm are released. They swim through the uterus to the fallopian tubes where they wait for ovulation to occur.
The mature egg cell (follicle) is the size of a cherry and `jumps` from the ovaries when ovulation begins. There are some women who claim to feel when ovulation begins. They say they experience a pain in the left or right of their abdomen. This is known as ovulation or central pain. It is possible on ovulation weeks you may have more vaginal discharge than normal.
The eggs cannot move on their own and are helped by tiny hairs on the inside on the fallopian tubes known as ciliae. The ciliae create waves in the direction of the uterus and the follicle travels along these tiny hairs. Because only one egg is released from one side at a time, only some of the sperm will find themselves at the correct place when an egg is released. There are also millions of sperm which never make it to through the uterus because they are defective or just poor swimmers. Only a few hundred will make the journey to the fallopian tubes. Once they egg arrives they will push with their heads to permeate the membrane into the egg cell. Only one sperm will be successful and once his head is inside, his tail will break off and the membrane will close making it impossible for any more sperm cells to enter. At this point the genetic characteristics of both egg and sperm merge and the sex will be determined.
If the sperm contains the X chromosome then the baby will be a girl, a Y and it will be a boy who grows in your womb.
It may be of interest to know that sperm containing the Y chromosome (the male) are generally the fastest swimmers but those with the X chromosome (the female) are bigger and likely to live longer. It has been suggested that if you have intercourse three or more days before ovulation and become pregnant then there is a higher chance that the baby will be a girl. This is because the sperm containing the X chromosome is more likely to survive long enough to wait for the egg to be released while those containing the Y chromosome are less likely to survive that length of time. However, if you have intercourse on or around the exact moment of ovulation then there is more chance the sperm containing the Y chromosome, being the fastest swimmers will reach the egg first resulting in your baby being a boy.
In the case of identical twins only one egg is fertilised but the egg splits into two embryos which both develop and grow into babies. Identical twins occur purely by chance and there is no evidence to suggest this is hereditary. Fraternal twins however occur when two eggs are released at the same time and both are fertilised. Having fraternal twins, unlike identical twins is hereditary and passed most commonly through the female side of the family. Fraternal twins can be either sex and their DNA (Deoxyribonucleic acid)will be different, while identical twins are always two boys or two girls and they share the same genetic information.