Welcome to e health forum.
The Depo provera shot has to be given during the first 7 days of menstrual cycle, for it to effective and prevent pregnancy during that cycle.
As long as the shot is taken, within 12 -13 weeks of the last Depoprovera shot, contraceptive effect will be present. Depoprovera has a efficacy of is said to be over 99.9% effective against pregnancy.
Hence if the last due date, of the shot as per you, is 20th april, then the chances of pregnancy, while on the shot, is very slim.
I hope this helps.