How does pregnancy happen?
Pregnancy= Fertilization+ Implantation. If one sperm makes its way into the fallopian tube and burrows into the egg, it fertilizes the egg. The egg changes so that no other sperm can get in. This fertilized egg stays in the fallopian tube for about 3 to 4 days, but within 24 hours of being fertilized, it starts dividing fast into many cells. It keeps dividing as it moves slowly through the fallopian tube to the uterus. Its next job is to attach to the lining of uterus; this is called implantation.