I've found myself mentioning the stuff below at various points over the past few months so thought I'd put it in a new thread for people to see. I'm not an expert, this is just stuff I've read/been told/found on the internet and happen to have collated. How do pregnancy tests work? Lots of women ask how early they can test for pregnancy. Home pregnancy tests (HPTs) work by detecting a particular hormone, HCG, in the urine. HCG has a similar structure to luteinising hormone (LH), but with an 'extra bit'. LH is detected by ovulation predictor kits (OPKs), which is why an OPK may pick up pregnancy but an HPT won't detect ovulation. However, whilst all women have a certain amount of LH present in their urine throughout the cycle (which is why an OPK is not a good substitute - some women will always get faint lines on OPKs), HCG is only produced in pregnancy and specifically, after the fertilised egg (blastocyst) has implanted. This means that HPTs will not detect pregnancy until after implantation. Implantation usually occurs between 7 and 10 days after ovulation. In most cases, the hormone takes 12-48 hours to appear in urine. The levels are low to begin with (1-5mi) and should double roughly every 36-48 hours. Most HPTs detect HCG at levels of 25mi+; some claim to detect at 10mi. First morning urine is least diluted, which is why it is recommended to test first thing in the morning (N.B. OPKs are usually directed for use later in the day - the LH surge normally begins early morning and takes 6hrs+ to appear in urine samples). How early can I test? Therefore, the earliest 'normal' time to see a BFP on an HPT is 7+1+2 days on a 10mi test (i.e. 9 days post ovulation). At the other end of the scale, it would be unreasonable to detect a BFP before 10+2+5 days on a 25mi test (i.e. 17 dpo). The average, according to Fertility Friend, is 14dpo - which fits well with these assumptions. Of course, there are exceptions to all rules, but it seems very probable that in cases of early BFPs (reported at 4 or 5 dpo), the tester was in fact mistaken about the day of ovulation. So how is it possible to 'feel' pregnant before implantation? Why isn't there a test for use at 1 or 2 dpo? Once the egg is fertilised (<12 hours from ovulation, as that is when an unfertilised egg starts to disintegrate), it starts to divide. This blastocyst produces a protein (and possibly lots of other chemicals, I couldn't find much research here!) known as 'early pregnancy factor', or EPF. EPF is an immunosuppressant, and it is thought it may be responsible for some of the very early pregnancy symptoms such as colds, sore throats, tiredness, etc. In fact, it is possible to test for pregnancy the day after ovulation, by testing for the presence of EPF in urine. The reason that these tests aren't commercially available (yet! I'm sure the big companies must be desperate to get these out!) is because the tests are very expensive to conduct and ethically problematic - they would tell you that the egg has been fertilised, but would not give you any idea as to whether the blastocyst would implant. It's thought that up to 50% of fertilised eggs simply pass out of the body during menstruation. If you were testing that early, the chances of disappointment would be so incredibly high that it's not worth doing. Many of us already know the disappointment of an early BFP, only for it to result in a very early miscarriage (<6 weeks). By the way, when I visited my GP the other day, he told me that his clinic used to use tests very similar to the internet cheapies many of us have been using (I took an unused(!) one in to show him, as I'd had several faint lines over a few weeks), but had stopped because they had had too many false positives and false negatives. His conclusion was that the unbranded ones are just not accurate enough. The clinic now uses 20mi clearview (same as ClearBlue) tests. I hope this helps. If you want to read more, there's a useful Wiki page on EPF here: http://en.wikipedia.org/wiki/Early_pregnancy_factor . I'm sure the knowledge won't stop us all POASing at 8 dpo , but it may help prevent unrealistic expectations and disappointment!