Home pregnancy tests check levels of human chorionic gonadotrophin, or hCG, in the urine. HCG is produced by the growing placenta if an embryo successfully implants. Normally, a home pregnancy test will be positive at the time of a missed period or even a day or two before. However, because hCG is administered during in vitro fertilisation( IVF) cycles, testing too early may give a false positive result.
The Reason for hCG Administration
HCG is given to mature the eggs within follicles so the eggs can be easily removed. Normally, eggs ovulate out of the follicles when luteinizing hormone, or LH, rises as follicles reach a mature size and estradiol, the dominant female hormone, rises. An LH surge is undesirable during an IVF cycle because the timing of ovulation after an LH surge isn't precise, and eggs could ovulate before retrieval is done. LH release is suppressed by medications during the IVF cycle.
Clearing hCG After Injection
HCG is given via injection approximately 35 hours before an egg retrieval is done. HCG levels are tested the next day to make sure the hCG level has risen enough to mature the follicles, according to the Geneva Foundation for Medical Education and Research. Embryos are typically transferred to the uterus three to five days after egg retrieval, and implantation generally occurs six to 10 days after egg retrieval. But hCG isn't produced for a few days, according to the Advanced Fertility Center of Chicago.
HCG remains in the system for as between 5 and 14 days, according to the Advanced Fertility Center of Chicago. Positive hCG levels can be checked by very sensitive HPTS (which may register as little as 12 milli-international units per millilitre of hCG) as early as 10 to 11 days after an LH surge. Since an LH surge hopefully doesn't occur in an IVF cycle, but instead is simulated by the hCG injection, there may be an overlap of several days where all the hCG hasn't yet cleared the patient's system, but where an HPT might normally be done. Testing at this point may result in a false positive result.
Waiting to Test Until hCG is Cleared
As hard as it is to do, waiting a full 14 days after hCG injection to do an HPT will help avoid the disappointment of a false positive test that becomes negative a few days later, when all the hCG is cleared from the body. Some women have circumvented the wait by buying HPTs in bulk and testing daily, starting a few days after embryo transfer, to see if the hCG clears and the test becomes negative. Once the hCG is excreted and an HPT is negative, a positive result a few days later indicates a new source of hCG and a true positive result.