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.
Given that it's the same wording, [2], [3], and now [4], and the pointed edit summary in that last edit, it's probably the same individual. I previously discussed it with that individual at User talk:Quiddity/Archive 10#Thagomizer. This is just a note, for future reference.