This calculator shows possible implantation dates based on when you ovulated (either actual, or calculated based on last period): that is, approximately when your egg, if successfully fertilized, would attach to the endometrium, after which the pregnancy hormone is produced and starts to double quickly.
From Fertilization to Implantation
During the first phase of the menstrual cycle, the Follicular Phase, follicles start to develop in one of the ovaries, stiumulated by the hormone FSH, courtesy of the pituitary gland. At some point, one follicle becomes dominant, probably having had enough of life behind ovary walls, and, with help from a surge of LH, breaks out of the ovary (ie. ovulation).
Now alone out in the fallopian tube, she waits. And waits... With good-timing, and a little/lotta baby dust, if just one of hundreds of millions of sperm manages to find its way and fertilizes the egg, a zygote comes to fruition.
Over the next 5 to 6 days, the newlyweds have two primary tasks: move from the oviduct into the uterus; And, during this journey, transform from the 1-celled fused zygote to a multi-celled blatocyst which can implant into the uterine wall. All this, and more, as we go through our day, wondering if it's time to stock up on pregnancy tests...!