The simple answer to this question, of course, is: whenever you want! With cheap and effective pregnancy tests sold at dollar stores, it's fairly accessible for people to test whenever and as often as they want.
But wouldn't it be great not to waste time, money, and hope on taking a home pregnancy test when it's not even possible to tell if you're pregnant? Let's take a look at the timeline of conception and the mechanics of home pregnancy tests to better understand the best time to take a pregnancy test.
From the time of intercourse or insemination, it can take from a few hours up to 5 days for fertilization to occur, depending on when you ovulate, or release an egg. Sperm can live for approximately 5 days. An released egg is viable for 24 hours. After fertilization, it can take between 6-12 days for implantation to happen. Implantation is when the fertilized egg implants itself into your uterine wall. Once the fertilized egg implants, the hormone HCG (human chorionic gonadotropin) enters the blood stream. It is this hormone that is responsible for a positive pregnancy test.
From the time of implantation, HCG hormone levels are approximately 1-2mIU (milli international unit) and double every 48-72 hours. Any HCG level above 25mIU is considered pregnant, while less than 5 is considered not pregnant. The average pregnancy test detects 20-25mIU of HCG. Therefore, taking a pregnancy test 3-4 days after implantation is the earliest you could possibly test positive with a urine pregnancy test. If you're counting the time since ovulation, this would be between 9-16 days past ovulation -- at the earliest.
It's important to know that not all home urine pregnancy tests are the same. Each one detects pregnancy, or HCG levels, at different mIU levels. Some early pregnancy tests will detect lower HCG levels and can give you results earlier, while others are not as sensitive. This list seems to offer a fairly comprehensive list of brands and HCG levels.
If 9-16 days past ovulation is the earliest you can test, what's the ideal time to test? Anything beyond 16 days past ovulation, or from the beginning of when you would expect your period. The closer to that time and in the first week after a missed period, the more accurate your pregnancy test will be. If you receive early negative tests, wait until you're closer to your expected period. While early false positives may happen as well, they are less likely. Usually, a positive pregnancy test indicates pregnancy. It's time to find an amazing care provider!
Sometimes, an early positive pregnancy test followed by a later negative pregnancy test can indicate that the pregnancy has ended, as the levels of HCG are not increasing as expected, though this is not always the case. Contact your care provider to discuss your results and potentially schedule a blood test for pregnancy.