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.
Word History:  'O frabjous day! Callooh! Perhaps Lewis Carroll would chortle a bit himself to find that people are still using the word chortle, which he coined in Through the Looking-Glass, published in 1872. In any case, Carroll had constructed his word well, combining the words chuckle and snort.