Pregnancy tests work by detecting human chorionic gonadotropin (HCG), a hormone normally produced only in pregnancy. HCG can be measured in blood or urine. Blood tests are more sensitive, but the accuracy of a pregnancy test also depends on other factors.
A woman's body begins to produce HCG when the fertilized egg implants into the wall of the uterus. The timing of the events that lead up to implantation vary greatly. Ovulation usually occurs 14 days before the menstrual period, but even in women with very regular cycles, the timing may vary by a couple of days. The next event, fertilization, can also vary, depending on the timing of intercourse. Finally, the time between fertilization and implantation can be between 6 and 12 days.
Other factors that determine the accuracy of a pregnancy test include:
- the brand (some are more sensitive than others)
- the time of day you test your urine (first thing in the morning is best)
- the care with which the test is performed (instructions must be followed precisely)
Find out more about this book:Your Developing Baby, Conception to Birth: Witnessing the Miraculous 9-Month Journey (Harvard Medical School Guides)