It is best to take an analysis for hCG after the start of the delay, after 3-5 days. Suppose additional studies were carried out in the process of pregnancy planning (an ovulation test was performed, a control ultrasound study), and you know exactly when ovulation and fertilization occurred. In that case, you can donate blood no earlier than ten days after that. Suppose the study is carried out too early when the level of the body has not yet developed the hormone necessary to determine pregnancy. In that case, the result will be falsely negative, and the analysis will still need to be taken again after 3-5 days.