This is the week of your last menstrual period. The first day of menstrual bleeding is considered the official start date of pregnancy and the 40 weeks of pregnancy are calculated by this date.
Week Two: Fertilization
Every month, about three to 10 eggs receive a hormonal signal that cues them to mature and prepare for ovulation. At the same time, the uterus is forming a blood rich lining that is called the endometrium. This will thicken, upon fertilization, to form a sack strong enough to carry and cushion your baby throughout the entire pregnancy.
Week Three: Cells Join, Divide - New life begins
Once a month around the 14th day, one of the eggs travels down into the fallopian tube. This is ovulation. If in the next 36 to 48 hours just one of the 350 million sperm in the average ejaculate can get all the way from the vagina through the uterus and into the fallopian tube to penetrate the egg, the egg will become fertilized. Sperm can live for up to 5 days in this environment, therefore making pregnancy possible for up to 5 days after intercourse.
Week Four: Implantation
The fertilized egg immediately begins dividing and floats down into the uterus. Immediately upon fertilization your little one is set as a boy or a girl. Ladies, this is one that you can not take credit for since it is up to the sperm to determine the sex of your baby. Sperm carries either a "X" (girl) chromosome or a "Y" (boy) chromosome.
The sex of your baby is predetermined by the fathers sperm chromosomes.