How does a sperm cell encounter the ovum?

When men and women make love, the man's penis enters the woman's vagina. When the man ejaculates, sperm cells are released into the woman's vagina where they travel up to her Fallopian tubes. If there is an ovum in one of the tubes, it can be fertilised by one of the sperm cells so forming an egg which will grow into a baby. If there is no ovum to receive the sperm cell, then there will be no baby.