Hi Welcome to HEALTHCARE MAGIC.. I have gone through your query and can understand your concern... First of all chances of pregnancy on last day of expected menstruation are very less. You should not worrying about getting pregnant after having unprotected sex on last day of your menstruation. Sometimes periods could be delayed by reasons such as hormone disturbance, ovarian problem, etc. But if you had unprotected sex before 19th this could result in pregnancy & thereby delayed periods. In that case you should go for a pregnancy test. If you are pregnant then nothing could prevent pregnancy in you except going for an abortion. However withdrawal method should not be employed for prevention of unwanted pregnancy. It has high success rate because the pre-ejaculation fluid can contain some viable sperms, which could cause pregnancy. Refer to a gynaecologist for more queries Hope this information helps