Hope this helps some of you...

implantation bleeding occurs during implantation of the egg cell

it occurs towards the end of the implantation process . The trophoblast is a highly specialized tissue. It develops from the fertilized egg and surrounds the egg. It attaches the egg to the endometrium and as it is continuously growing it invades the endometrium. As it eats its way into the endometrium it pulls the egg into the endometrium. It ( the trophoblast) also invades the mothers blood vessels and divert her blood to cavities inside the trophoblast. These cavities are than filled by maternal blood becoming blood lakes. Most of the lakes are connected. The trophoblast on the outer surface of the egg are thinner and slower growing but also contains cavities that are connected to the deeper cavities. Some of these outer cavities communicate or open into the uterine cavity. Blood that leaks from these cavities are causing implantation bleeding. It occurs shortly before the egg is completely implanted and completely covered with endometrium.

Typical implantation bleeding occurs about four weeks after the last menstrual period. It is usually less and darker than the person's normal menstruation. The diagnosis of typical implantation bleeding is made in retrospect. The duration of the pregnancy is four weeks more than expected, because the last period recorded by the patient was the implantation bleeding and not her last menstruation.

Implantation bleeding has no adverse effects on a pregnancy.

Some experts doubt the existence of implantation bleeding . If you search for "implantation bleeding" you will come across sites that doubt if it ever occurs. The fact however remains that some women ( we estimate about 1%-2%) has a bleeding about 4 weeks after the last menstrual period with a pregnancy duration 4 weeks longer than expected.