HCG tests are more likely to produce false negatives than false positives. The longer after implantation a person waits to do the test, the more accurate it will be. HCG levels begin rising when an embryo implants in the uterus.
Implantation usually happens a week or so after ovulation. It can take several days for HCG levels to rise high enough for a test to detect the hormone.
Due to how long it takes for HCG levels to rise, it is possible for a woman to be pregnant and still get a negative test. A positive result usually appears after retesting a few days later.
False-positive results are rare. However, because home pregnancy tests are increasingly sensitive, some can detect very early pregnancies with low HCG levels.
This sensitivity means it is possible to have a positive test that a very early miscarriage then follows. A woman who delayed testing or who used a less sensitive test might not have known about the miscarriage.
In rare cases, a woman can have abnormally high levels of HCG even though they are not pregnant. The most common reasons for this include:
- a recent miscarriage
- using certain fertility drugs
- molar pregnancy, which is a pregnancy that implants but fails to grow, and which instead becomes a mass inside the uterus
Less common causes include:
- cancer, including tumors that secrete HCG
- endocrine disorders, especially pituitary gland issues
- unusual antibodies in the blood
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