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Most women know that a sensation that feels like menstrual cramps coming just a day or two early about two weeks after unprotected intercourse can be a sign of pregnancy. Spotty bleeding at the same time is further indication that conception has taken place. Not every bleeding toward the expected end of a woman's cycle, however, is a sign of pregnancy. Here's why, and how to tell the difference.

Telling the difference between implantation bleeding and other sources of bleeding is largely a matter of timing. Women aren't, despite what some men seem to believe, fertile all the time, just waiting to receive a man's sperm for the process to begin. A woman's ovaries release an egg at about the midpoint of a woman's menstrual cycle. She may experience some midriff pain about the time the egg is released. 

That egg is only viable for about a day after it is released from the ovary into a Fallopian tube. For it to be fertilized, sperm has to be "swimming" up the cervix toward the Fallopian tube to reach the egg at the right time for conception. If there aren't sperm on the way when the egg is released, chances are that it won't be fertilized and it will just go into the uterus and disintegrate.

If the egg is fertilized, the resulting zygote begins multiplying cells rapidly while it is still at the opening of the fallopian tube. After three or four days, it leaves the Fallopian tube and is implanted in the lining of the uterus. At this point, which is already in the third week since the last time the woman has had her period, there may be slight, spotty bleeding. However, it takes a few more days for the blood to travel out of the uterus through the cervix and vagina where it can be noticed. By this time, it's almost time for the next menstrual period. So how can a woman tell whether she's had implantation bleeding or her period is just early?

  • In implantation bleeding, the blood is dark, rusty red or brown, not bright red like fresh blood. That's because it takes time for the blood to flow through the uterus.
  • the volume of blood in implantation bleeding is usually less than the volume of blood in menstruation. Usually implantation bleeding just causes spotting and lasts only a few hours to maybe as long as a day. "Heavy" implantation bleeding, however, can last as long as four or five days. However, this bleeding could also be due to something entirely different, such as endometriosis. However, there are cases in which doctors have suspected endometriosis when in fact a baby was on its way.

Bleeding and abdominal pain aren't reliable signs that a woman is pregnant. A better although still imperfect way to determine whether a woman is pregnant is a pregnancy test for a hormone called human Chorionic gonadotropin, or hCG.

A woman's uterus produces this hormone to cause thickening to provide a place for the embryo to grow. While the embryo is still in the Fallopian tube, production of this hormone is nil or at least undetectable. When the embryo is implanted in the lining of the uterus, tiny amounts of hCG can be detected by the most sensitive blood tests.

It's possible to detect hCG as early as four days after conception through a blood test administered by a doctor. Most physicians, however, will not test women for pregnancy until they have missed a period or if there are medical indications (heavy bleeding, for example) that pregnancy needs to be ruled out before treatment for another condition can begin. It's possible to use a urine test to detect hCG as soon as eleven days after conception, but any test that is less sensitive than 10 mIU/ml (most tests are only sensitive to 40 mIU/ml), that is, that requires a concentration of over 10 mIU/ml to detect pregnancy, just won't detect pregnancy in the first week after conception.

Women don't have to wait until the day their period is due to test for pregnancy. Be aware that both false positives (tests showing a baby is on the way when there is no pregnancy) and false negatives (failure to detect changes in hCG) are common during the first month of pregnancy. Implantation bleeding is also an unreliable pregnancy test, but taken into consideration along with urine or blood tests there is a better idea of whether pregnancy has occurred.

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