A four-day delay in menstruation with a test strip usually does not detect pregnancy. Pregnancy can be detected by delaying menstruation for more than 7-10 days, or an indeterminate amount of time for women who usually have irregular menstrual cycles. It usually takes about a week for the sperm and egg to unite, after which the fertilized egg will implant in the uterine cavity and the body will secrete human chorionic gonadotropin after implantation. A blood test for human chorionic gonadotropin can be done about 10 days after the sperm and egg unite to determine if you are pregnant. Women who are sexually active and have regular menstrual cycles can have a blood test for human chorionic gonadotropin 7-10 days after their period has been delayed to determine if they are pregnant. For irregular menstruation, you can check for pregnancy after 7-14 days of delayed menstruation.