It is possible to detect pregnancy by taking a blood test one week before menstruation.
For women who normally have a menstrual cycle of 28~30 days, if they have sex during ovulation, 14 days before menstruation, that is, about 1 week before the formation of the fertilized egg, at this time the fertilized egg may have already been deposited and started to secrete human chorionic gonadotropin (HCG), but the level of HCG is relatively low, which is generally detectable by blood sampling, and there may be false negatives in the results of the early pregnancy test paper.
If a woman has normal sexual intercourse during the current menstrual cycle, even if pregnancy is not detected 1 week before menstruation, it is recommended that she should repeat the blood test or use the pregnancy test again 1 week later, if the result is still not negative, then the probability of pregnancy is relatively small.