For pregnancy to occur, every step of the human reproduction process has to happen correctly. The steps in this process are:

One of the two ovaries releases a mature egg.
The egg is picked up by the fallopian tube.
Sperm swim up the cervix, through the uterus and into the fallopian tube to reach the egg for fertilization.
The fertilized egg travels down the fallopian tube to the uterus.
The fertilized egg implants and grows in the uterus.