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, coined in the 1980s by ecologist Eugene F. Stoermer, takes its prefix from the Ancient Greek word for human because its proponents believe the influence of humanity on the Earth's atmosphere and crust in the last few centuries is so significant as to constitute a new geological epoch.