Pregnancy may occur due to failure of?

Condom? pregnancy prevention pills? I think any preventive measure would count... unless you're looking for a specific one :S

Protection, sexual interaction, or not taking birth control 

They need to contract and relax

Answeralong with the tissue lining,the blood,tissue,and unfertilized egg leave the uterus,going through the vagina on the way out of the body,(menstrual period);