Usually, it's because the sperm gets into the fallopian tube before the egg is released. Once there, the sperm wait for an egg to arrive. After fertilization, it can take 5-10 days for the fertilized egg to begin to attach itself to the lining of the uterus. Pregnancy begins when this happens.
The arrangement of spikes originally had no distinct name; the term Thagomizer was coined in 1982 by cartoonist Gary Larson in his comic The Far Side, and thereafter became gradually adopted as an informal term within scientific circles, research, and education.
The term paruresis was coined by Williams and Degenhart (1954) in their paper Paruresis: a survey of a disorder of micturition in the Journal of General Psychology 51:19-29. They surveyed 1,419 college students and found 14.4% had experienced paruresis, either incidentally or continuously.