Lightening: Not to be confused with a discharge of atmospheric electricity, lightening refers to the sensation that a pregnant woman feels when the baby drops. This is the time when the presenting (lowermost) part of the fetus descends into the maternal pelvis.
Lightening classically occurs 2 to 3 weeks before labor begins. However, it may not occur in women who have had two or more prior viable pregnancies (and so are multiparas) until labor actually begins.
Lightening is also called "engagement" because the presenting part of the fetus is then engaged in the mother's pelvis.