Least chances of pregnancy is with -

park's textbook of preventive and social medicine 23rd edition. *Oral contraceptive of combined type are almost 100 effective in preventing pregnancy.some women do not take it regularly so the actual rate is lower.mechanis of action:prevent the release of ovum from ovary.this is achieved by blocking pituitary secretion of gonadotrophin.progestogen only preparation render the cervical mucous thick and scanty and there by inhibit sperm penetration.