I took an hCG injection about five days ago. How long does it usually take for the hCG to leave your system? I took a home pregnancy test and it was positive. Could this be a false positive from the injection?

G.R.

Pregnancy tests look for the presence of the hormone hCG (human chorionic gonadotropin). In a normal pregnancy, hCG is produced at very low levels immediately after the embryo implants in the wall of the uterus. Doctors often measure Blood levels of hCG in the earliest stages of pregnancy; their rise over the course of several days can indicate that the pregnancy is proceeding properly.

As it happens, hCG is structurally similar to another hormone, called luteinizing hormone (LH). One of the functions of LH is to trigger ovulation. During a normal menstrual cycle, the pituitary gland sends out a burst of LH when you are ready to ovulate. This LH surge signals the ovary to release the egg and stimulates the ovary's production of the hormone progesterone.

Because of its similarity to LH, hCG can accomplish some of the same functions in the body. LH is not available for use in medical treatment, but hCG is easily harvested from the urine of pregnant women. The readily available hCG hormone has thus been an important tool to battle infertility for many years. A woman taking ovulation induction medications (fertility drugs to encourage the development of eggs) will typically be monitored with ultrasound exams to determine when the eggs are mature enough to be released. At that time, she will be given an injection of hCG, which mimics the action of the LH surge and triggers the release of the eggs.

After such an injection, the level of hCG in the body will fall by half every 36 hours. This means that after an injection of 5,000 units of hCG, enough hormone will remain in the body to show up as a positive reading on most pregnancy tests for eight to 10 days, and a 10,000-unit dose of hCG can trigger a positive test result for a day or two longer than that. Therefore, if a low-level positive result on a pregnancy test is seen during the 10-11 days after injection, the test may be measuring falling levels from the injection and not signs of a pregnancy. It is impossible to say where the hCG being measured comes from.

If, after 12 days, a pregnancy test is positive, quantitative measurements of hCG in the blood, obtained 48 hours apart, can verify whether the positive pregnancy test indicates a "real" pregnancy, and not just a false indication of pregnancy based on the remnants of the injected hCG.