Human Chorionic Gonadotropin (HCG) is a hormone produced during pregnancy, often called the “pregnancy hormone.” Medically, HCG injections are primarily used in fertility treatments to trigger final egg maturation and induce ovulation, commonly known as the “trigger shot.” It also supports early pregnancy by encouraging progesterone production. Since pregnancy tests detect HCG, understanding how long the injected hormone remains detectable is important for accurately monitoring treatment outcomes.
Understanding HCG’s Half-Life
The elimination of injected HCG from the body is governed by a concept called the half-life. The biological half-life is the time it takes for the concentration of a substance in the body to be reduced by one-half.
For injected HCG, the mean half-life typically falls within the range of 24 to 36 hours. This means that if a person receives a certain dose, only half of that amount will remain in their system about a day and a half later. The remaining half is then reduced by half again during the next half-life period, and so on.
The hormone is metabolized primarily in the liver, where it is broken down into its component parts. These fragments and a portion of the intact hormone are then cleared from the body, mainly through the kidneys and excreted in the urine. This gradual decay process means the hormone concentration fades over several days.
Factors That Affect Clearance Rate
Several individual factors influence the actual rate of clearance. The most direct variable is the initial dosage of the injection administered. A higher dose, such as 10,000 International Units (IU), requires more half-life cycles to clear than a smaller dose, like 5,000 IU.
Individual metabolic rate also plays a role in how quickly the hormone is processed. People with a faster metabolism may clear the HCG slightly quicker than others.
Impaired kidney or liver function can slow the metabolic and excretory processes, extending the time the hormone remains in the bloodstream. Additionally, body mass index (BMI) can influence the hormone’s distribution and concentration in the body.
Calculating the HCG Washout Period
The “washout period” is defined as the time required for the injected HCG to fall below the detection threshold of standard pregnancy tests, which is typically less than 5 milli-International Units per milliliter (mIU/mL). For a standard 10,000 IU trigger shot, the HCG usually takes 10 to 14 days to fully clear from the system.
For example, if a 10,000 IU injection is administered, and the half-life is approximately 30 hours, the concentration would drop to 5,000 IU after 30 hours. After another 30 hours (60 hours total), it would be 2,500 IU, and the decay continues exponentially. It takes multiple half-life cycles for the concentration to drop low enough to be undetectable by a pregnancy test.
At 10 days (240 hours), which is roughly eight half-lives in this scenario, the initial concentration of HCG is greatly reduced, often approaching the lower limit of detection. Because of individual variation, medical professionals often advise waiting the full 14 days to ensure the hormone level is negligible. Following the specific guidance provided by a healthcare provider regarding the expected washout period is the most reliable approach.
Avoiding False Positives on Pregnancy Tests
The presence of residual injected HCG is the primary cause of a “false positive” result on both home urine tests and early blood tests. The test is accurately detecting HCG, but it is detecting the medication rather than a new pregnancy.
Patients are generally advised to wait until after the estimated washout period, typically around 14 days post-injection, before taking a test. Alternatively, some individuals choose to perform a process called “testing out the trigger,” where they take daily pregnancy tests to monitor the line fading.
A line that fades completely, then reappears and darkens, suggests a new, true pregnancy is being detected. Only a test performed after the complete washout period, or a blood test showing a rising HCG level, can reliably indicate a true pregnancy.