The fertilized egg remains in the fallopian tube for about three days. If it were to get into the uterus earlier, it would be too immature to adhere to the uterine wall and would pass on out of the uterus as a very early miscarriage, even before the mother would miss a menstrual period.

The tube acts as a preincubator, allowing the fertilized egg (embryo) to grow to just the right degree of maturity for its start in the uterus.

Scientist think that the hormones of the normal menstrual cycle cause this tightening and loosening of a tube's opening onto the uterus.

Upon its entrance into the womb,"home" for the next nine months, the baby has grown to be about twelve cells in size. The embryo, though is still too premature to stick to the wall of the uterus. It floats around in the uterine cavity for about two more days before it has grown surface cells that will stick to the uterine wall.

When it does adhere to the uterine wall, it begins to draw life sustaining oxygen, fluid, and nourishment from the mother's tissue instead of from the fluid in which it had been floating. This is called implantation.