According to Dr. Lori Marshall, a Seattle, Washington-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.
Actual implantation occurs six to eight days after the egg is fertilized. As the placenta is created and grows, it continues to embed in the endometrium, but "we don't usually continue to call that implantation," Dr. Marshall says. "Implantation is really the initial event in establishing a pregnancy."
Generally, implantation lasts for a day or two, says Dr. Anthony Wakim, medical director of the IVF program at Magee Women's Hospital in Pittsburgh, Pennsylvania.
Implantation rates vary widely with the age of the female partner, Dr. Marshall says. "A fertilized egg probably implants only about 50 percent of the time; the rest of the time, they enter the uterus but don't implant at all or implant abnormally and don't continue as healthy pregnancies."
In some cases, implantation can occur yet be incomplete. "Sometimes women will have a biochemical pregnancy, with low levels of the pregnancy hormone B-HCG," Dr. Marshall says. "A clinical pregnancy with a sac and a heartbeat is never established. This is likely because the process of implantation did not occur completely."
Symptoms of Implantation
Some women actually experience implantation symptoms. How many does depend on who you talk to. Dr. Wakim, for example, says it's "very, very, very rare." Dr. Marshall says, "I have never seen a rigorous survey, but my clinical estimate is that about 5 to 10 percent of women experience some symptoms, usually bleeding or a little cramping." And if you talk to your friends, odds are a few of them will claim to have felt it. There's also something to be said for women's intuition.
Something to keep in mind: While cramping and bleeding can be scary, implantation bleeding is usually spottier and lighter in color than a normal menstrual period, and it doesn't last long, according to the Mayo Clinic. Some women mistake this light bleeding for a period and don't realize they're pregnant.
If you're feeling confused by spotting, cramping, or lighter-than-usual flow, don't rule out implantation. Wait an extra week and take a pregnancy test (or test again). If a normal period does not begin or if you don't receive a conclusive positive pregnancy test, talk with your doctor about your experience.