Because it's hard to know exactly when conception occurred, it's accepted obstetrical practice to calculate the age of the embryo and fetus from the first day of your last menstrual period. Age calculated this way is called "gestational age," and it’s the way age is calculated during your pregnancy and in this calendar as well. In the first 10 days or so of our calendar, the mother ovulates and her egg starts to travel down the fallopian tube, where it encounters the sperm.
"Fetal age" is the actual age of the fetus from the date of conception. Gestational age is about 2 weeks more than the actual fetal age.
The second week of your pregnancy is actually the time when the egg travels down in the Fallopian Tube far enough to meet the sperm and conception is ready to happen.
The first 2 weeks of pregnancy, your egg was actually in the ovary and preparing for ovulation. At ovulation, it entered the fallopian tube, where it was fertilized by a sperm: conception. Conception takes place, then, on the first day of the third week of your pregnancy. Conception begins as a marathon, a race to the finish--with hundreds of millions of competitors. A single male ejaculation may contain up to 500 million sperm, but only one of them can succeed in penetrating the egg that lies ensconced in a fallopian tube, waiting to be fertilized. When that sperm finally enters the egg, instantaneously an electrical jolt alters the egg's outer membrane so that it becomes impermeable to the other sperm, which literally fall off of the egg's surface.

Inside the egg, the sperm's entrance literally shakes things up. The egg protoplasm starts to shimmy violently. The nuclei of the sperm and the egg, each containing 23 chromosomes, move towards each other and shed their membranes. Within 12 hours the nuclei have merged to form a complete set of 46 chromosomes: a blueprint for an entire human being. Every inherited characteristic the individual will have--his or her sex, eye color, skin tone, personality-is delineated in this one, tiny egg, now called a zygote. As the zygote divides, every daughter cell will carry a perfect copy of this blueprint.