A blood draw one day after your period is delayed will not necessarily detect pregnancy. Normally, the fertilized egg will start to produce human chorionic gonadotropin after it has been laid, and pregnancy can be detected by checking for human chorionic gonadotropin about 1 week after fertilization. If you have regular menstrual cycles of 21-35 days, a blood test for human chorionic gonadotropin can be done to determine whether you are pregnant if you have delayed menstruation. The menstrual cycle is generally affected by various factors such as environment, diet, emotion, endocrine and so on. Normally, a delay of up to 7 days in menstruation is considered normal. If menstruation is delayed for more than 7 days, you can use pregnancy test paper at home to check whether you are pregnant, if there are two red lines as deep as each other, you can initially consider that you are pregnant, but for those who have delayed their menstruation for one day, the pregnancy test paper can not be detected in general, and the accuracy rate is low, so you need to go to the hospital and take blood to measure the human chorionic gonadotropin to make sure that you are pregnant or not. If the result is higher than normal, pregnancy can be confirmed. This method is currently the most accurate means of testing, and generally do not need to fast before the blood draw.