The third week of your pregnancy is actually the first week after conception.
In 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 starts 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.