Normally, menstruation is very regular 28 days and the ovulation period is 14 days before the next menstruation. A pregnancy test must be taken 12-14 days after ovulation, which means that a weak positive reaction can be seen only when you are about to have your period. Pregnancy tests are sometimes not very accurate and may show false negatives or false positives depending on the environment or quality. A blood test is the most accurate, as long as there is an increase in HCG during the blood test, pregnancy will be confirmed.