Implantation occurs once a fertilized egg has adhered/implanted into the lining of your uterine wall. The medical community, including: the FDA, the American College of Obstetricians and Gynecologists (ACOG), and the National Institutes of Health are in agreement that a woman is not considered to be pregnant until implantation has occurred. So, medically speaking, successful implantation equals the start of a pregnancy.
It may be helpful to understand the concept of implantation when you take a quick look at the journey to pregnancy... the first step is conception. Usually as a result of sexual intercourse, sperm travel through the vagina, in past the cervix and up to the fallopian tubes where a sperm will most likely join with an available egg. If this fertilization has occurred, one can say that conception has taken place.
If you are aware of the day that you ovulate, sexual intercourse can take place anytime from about 5 days to a few hours before your egg is released. So, conception can take place during this time frame. Though, the establishment of a pregnancy takes several days -- because once an egg is fertilized, it will have to travel up into the uterus for implantation to occur. As the fertilized egg (known as a blastocyst ) makes this journey, it will continue to grow in size. At the same time, its cells will continue to divide and reproduce. It is thought that implantation of a fertilized egg is estimated to take place about 9 day (+/-) after ovulation. So, this journey of conception occurring, to the blastocyst traveling up into the uterus and then for implantation to take place within the uterine wall may take an estimated 7 to 14 days from the date you had sexual intercourse.
So, to recap the journey towards implantation and pregnancy... first conception occurs (a sperm fertilizes the egg). Next, it's time for implantation. Implantation refers to the process where the fertilized egg has become attached (or implanted) into the uterine wall. Implantation is the first “step” that triggers your body to start producing the hCG, or pregnancy hormone. Because pregnancy tests, both at home and doctor’s ones) screen for the presence of hCG to confirm a pregnancy, implantation must occur to elicit this hormone. If not, a pregnancy cannot be diagnosed through a pregnancy test.
Implantation refers to the process in which the fertilized egg, also known as the blastocyst, becomes attached to the wall of the uterus and starts to produce hCG. Because hCG is the hormone that pregnancy tests look for, it is necessary for implantation to occur before a pregnancy can be recognized through a pregnancy test.
The establishment of a pregnancy can take several days and is not completed until a fertilized egg has implanted in the lining of the woman's uterus. The medical community defines that you are officially considered pregnant once it has been established that the implantation of the fertilized egg has occurred. And, as mentioned above, this confirmation can typically take place by the use of pregnancy tests. If you receive a positive (i.e. pregnant) reading on a pregnancy test, then you can feel fairly confident that implantation has occurred since your body had started to produce the hCG hormone that was confirmed by the test. If you take a pregnancy test before implantation occurs, the test will tell you that you’re not pregnant since the trigger for the hCG hormone has not yet occurred, so this hormone will not be in your system.
So the bottom line: implantation defines the beginning of a pregnancy. A fertilized egg must make its way into the uterus and find a nice, cozy spot to attach to. If not, the fertilized egg will just be emptied out of your body, probably during a period. So conception does not automatically equal implantation. A trophoblast (type of tissue) actually develops from the fertilized egg and then surrounds it. This trophoblast joins the egg to the inside of the uterus and begins to push its way into the uterine lining. Then, the trophoblast, in fact, pulls the egg inside of the endometrium (uterine wall). This trophoblast will then occupy your blood vessels and divert your blood to the fertilized egg. At this point, you can officially say that implantation has occurred and a pregnancy has begun!