Timing Is Everything
The fallopian tube is the tube-like structure that sits between the uterus and ovary. The fimbria are lush, finger-like projections on the end of the tube near the ovary. When a mature egg is ovulated, the fimbria must actively seek out this egg and pick it up from the surface of the ovary; the egg does not just fall into the tube.
Once the fimbria have picked up the egg, it is transported by little hair-like projections on the surfaces of the cells on the inside of the tube toward the uterus into the portion of the tube known as the ampulla. It is here that the sperm and egg meet and that fertilization will occur.
It doesn't take just one sperm to fertilize an egg. There are many protective cells surrounding the egg, and many sperm are lost while actively removing these cells in order to gain access to the egg. Only after these cells have been removed and a path cleared can a single sperm penetrate the egg. Once a sperm penetrates the egg, the protective layer around the egg immediately undergoes changes that prevent any further sperm from entering the egg.
After fertilization, the zygote, or early embryo, remains in the tube for another three or four days. While in the tube, continued development occurs. When the embryo is transported through the isthmic portion of the tube and into the uterus, it is usually about 20 to 40 cells in size. The embryo "floats" in the uterus for an additional couple of days before attaching to the wall of the uterus, a process known as implantation.
There are very few days in the normal menstrual cycle during which a couple may conceive. While sperm may survive for several days in the reproductive tract of the female, the egg is only healthy and capable of being fertilized for 24 hours at most. After that time, it is simply reabsorbed by the body. The process of egg development and ovulation determines all the timing of this fertile period. There are, really, only a few days at most in any given menstrual cycle during which conception can occur.
A normal menstrual cycle is defined as being 28 to 30 days in length (from the start of one period to the start of the next). While cycles of shorter or longer length may or may not be normal, cycles of this length are most common. For purposes of discussion, Day 1 is defined as the first day of normal menstrual flow. During the next 12 to 14 days, the events that result in the ovulation of a mature egg occur. This is called the "follicular phase," so called because the egg develops in a fluid-filled sac called the follicle. As the follicle (and the egg within it) develops, it produces hormones, the most important of which is estradiol (the primary form of estrogen).