"Lightning" is not the same as "lightening." The former is an electrical discharge from a thunderstorm. The latter is the descent of the uterus into the pelvis to line the baby up with the vagina at the end of a pregnancy.
Unless you're pregnant, you probably don't want to see lightening.
Lightening is derived from lighten, to make lighter in weight or color (as in, the sky was lightened), then abbreviated to lightning. For lightening of the pregnancy, also called engagement or dropping, the word is not abbreviated. There is a related term, often conflated through the visual effects of Highlander that is expressed with lightning, called Quickening. Perinatally, quickening is used to describe the sensation of the first movements felt of the fetus in pregnancy.
