WebMD explains that lightening in pregnancy is similar to the baby dropping during the third trimester. The baby settles or moves down further into the mother's pelvis.
According to WebMD, first-time mothers experience lightening approximately two to four weeks before the baby is scheduled for delivery. In mothers who have previously given birth to a child, lightening typically does not occur until labor starts. Lightening, however, is not a reliable indicator of the onset of labor.
Signs of lightening sometimes include a visual change in the exterior of the mother's belly because the baby appears to have dropped. More internal signs include easier breathing and less heartburn because of the repositioning of the baby. There is sometimes more pressure on the mother's bladder when lightening occurs.
Giving Birth Naturally explains that lightening can take place rather rapidly, over several weeks or not at all. The degree of lightening is related to the placement of the baby's head in the mother's pelvic area. The station of the baby's head refers to its placement: It varies from -5, meaning the highest place up, to +5, meaning the head is crowning. At +5, the woman is ready to deliver the baby. When a station is given as 0 that means that the baby's head is fully engaged.