When asked about implantation – the kind related to pregnancy, that is – many women ask for clarification. So if you had the same reaction upon reading this, you're not alone.

Christine Anthony of Evanston, Ill., asked for clarification, but it turns out her definition was right on target: "Do you mean when women are aware of the timeframe when the fertilized egg actually embeds itself in the uterus ... the absolute first step in pregnancy establishment?"

For a more medical definition, here's one from Dr. Lori Marshall, a Seattle, Wash., based fertility expert: "[Implantation] is the process that involves attachment of the embryo to the lining of the uterus. The embryo penetrates through and embeds into the endometrial lining."

In other words, without implantation, there can be no pregnancy. It's a complex process that involves signals sent back and forth between the embryo and lining of the uterus, Dr. Marshall says. A lot of things have to occur: The embryo must extrude or hatch from the membrane surrounding it; the uterine lining must be receptive to implantation. This requires preparation with estrogen and progesterone and only occurs for about a four-day window.

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