Human chorionic gonadotropin (hCG) is a hormone produced by the placenta of a pregnant woman.
Early in pregnancy, the level of hCG increases in the blood and is eliminated in the urine.
A pregnancy test detects hCG in the blood or urine and confirms or rules out pregnancy.
Urine tests can be falsely positive in those that are taking the medications... chlorpromazine, phenothiazines and methadone .
Spurious evaporation lines may appear on many home pregnancy tests if read after the suggested 3–5 minute window or reaction time, independent of an actual pregnancy.
False positives may also appear on tests used past their expiration date.
A woman who has been given an hCG injection as part of infertility treatment will test positive on pregnancy tests that analyses hCG, regardless of her actual pregnancy status.
However, some infertility drugs (e.g., clomid) do not contain the hCG hormone.
Some diseases of the liver, cancers, and other medical conditions may produce elevated hCG and thus cause a false positive pregnancy test.
These include choriocarcinoma and other germ cell tumors, IgA (Immunoglobulin A) deficiencies, heterophile antibodies, enterocystoplasties, gestational trophoblastic diseases (GTD), and gestational trophoblastic neoplasms.
EPF can be detected in blood within 48 hours of fertilization. However, testing for EPF is expensive and time-consuming.
Most chemical tests for pregnancy look for the presence of the beta subunit of hCG, or human chorionic gonadotropin, in the blood or urine.
hCG can be detected in urine or blood after implantation, which occurs six to twelve days after fertilization.
Quantitative blood (serum beta) tests can detect hCG levels as low as 1 mIU/mL, while urine test strips have published detection thresholds of 10 mIU/mL to 100 mIU/mL, depending on the brand.
Qualitative blood tests generally have a threshold of 25 mIU/mL, and so are less sensitive than some available home pregnancy tests.
Most home pregnancy tests are based on lateral-flow technology.
With obstetric ultrasonography the gestational sac sometimes can be visualized as early as four and a half weeks of gestation (approximately two and a half weeks after ovulation) and the yolk sac at about five weeks' gestation.
The embryo can be observed and measured by about five and a half weeks. The heartbeat may be seen as early as six weeks, and is usually visible by seven weeks' gestation.
The accuracy of a pregnancy test is most closely related to the day of ovulation, not of the act of intercourse or insemination that caused the pregnancy.
It is normal for sperm to live up to five days in the fallopian tubes, waiting for ovulation to occur.
It could take up to 12 further days for implantation to occur, meaning even the most sensitive pregnancy tests may give false positives up to 17 days after the act that caused the pregnancy.
Because some home pregnancy tests have high hCG detection thresholds (up to 100 mIU/mL), it may take an additional three or four days for hCG to rise to levels detectable by these tests — meaning false positives may occur up to three weeks after the act of intercourse or insemination that causes pregnancy.
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