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 spiked tail of Stegosaurus (Wikimedia Commons). Way back in 1982, a famous Far Side cartoon depicted a group of cavemen clustered around a picture of a Stegosaurus tail; one of them points to the sharp spikes and says, Now this end is called the thagomizer...after the late Thag Simmons..