Lightening is a non-clinical term that describes the time when the baby descends into the pelvic cavity in preparation for birth. In clinical terms this is referred to as engagement.

It occurs in the third trimester. In head engagement, the fetal head descends into the pelvic cavity so that only a small part (or none) of it can be felt abdominally. The perineum and cervix are further flattened and the head may be felt vaginally. Head engagement is known colloquially as the baby drop, and in natural medicine as the lightening because of the release of pressure on the upper abdomen and renewed ease in breathing.

However, it severely reduces bladder capacity, increases pressure on the pelvic floor and the rectum, and the mother may experience the perpetual sensation that the fetus will “fall out” at any moment.

By William Smellie Public Domain image 

Cephalic presentation. (2016, September 17). In Wikipedia, The Free Encyclopedia. Retrieved 05:18, September 17, 2016, fromhttps://en.wikipedia.org/w/index.php?title=Cephalic_presentation&oldid=739815165