While false positives are rare, there are are many possible causes of a positive pregnancy test.
1. Pregnancy
By far the most common cause of a positive pregnancy test, pregnancy is usually detectable only after a fertilized egg has made it to the uterus. This generally occurs roughly two weeks after ovulation or four weeks after your last menstrual period.
2. Molar Pregnancy
Molar pregnancies are not viable and do not involve an actual embryo, but they cause a positive pregnancy test. Instead of forming the beginnings of a baby, the fertilized egg forms a fast-growing cluster of placenta cells. A molar pregnancy may mimic the early stages of a normal pregnancy, but there is no baby.
3. Ectopic Pregnancy
Like molar pregnancies, ectopic pregnancies are not viable. The fertilized egg implants in the fallopian tubes or another part of the body instead of the uterus. An ectopic pregnancy is a medical emergency; the blastocyst must be removed promptly to prevent the woman from suffering serious complications.
4. Recent Miscarriage
If you have had a miscarriage-- even a very early one-- within the last several days, your urine may still contain trace amoungs of hCG. Miscarriages that occur very early may seem identical to an ordinary menstrual period.
5. Certain Medications
Do not dismiss a false positive pregnancy test as the result of a painkiller, antidepressant, herbal supplement or blood pressure medication. While very few drugs can cause a false positive pregnancy test, it is important to tell your health care provider about any medications you do take. The "pregnancy hormone" hCG may be an ingredient in a few prescription fertility drugs.
6. Evaporation Lines
A few cheap pregnancy tests may display a false-positive result when moisture crosses the test. This will not cause a very clear positive result, but it can create a very faint grayish line where the test might have displayed a positive result. Retest if your results are ambiguous.
7. Waiting too Long
If you take a pregnancy test and don't look at it until several minutes later, it may display a false positive result. Be sure to read your test within 2-5 minutes (or whatever the manufacturer recommends) and disregard any results that appear after this time.
8. Invalid Test
If your pregnancy test is past your expiration date or has been exposed to extreme hot or cold, disregard the results. Do not use a pregnancy test that has, for example, sat in your car on a hot day. Re-test if you aren't sure about your results.
9. Choriocarcinoma
Choriocarcinoma is a serious, aggrsesive form of cancer affecting the placenta. It is usually preceded by a normal pregnancy, ectopic pregnancy, miscarriage or molar pregnancy. This disease is highly curable if it is detected early, so be sure to mention your false positive pregnancy test to a health care provider.
10. Other Tumors
Very rarely, a positive pregnancy test may be the first indication of a tumor in the ovaries, uterus or endometrium. The amount of hCG secreted by these tumors will vary dramatically depending on the type of tumor. If you have a family history of hormone-sensitive cancers, be sure to mention this to your obstetrician or midwife when you follow-up on the results of your test.