Engagement or "lightening" is the movement of your baby down into the pelvis that occurs toward the end of your pregnancy. This lowering of the baby into the pelvis typically occurs prior to labor, although for some women the baby will not engage until the very start of labor. Engagement is not a reliable indicator of when to expect labor.
When the baby begins to drop, you will notice that your abdomen will change shape, shifting down and forward. Your abdomen may even seem smaller, as the head of the baby enters the birth canal. This position creates increased pressure on your bladder, causing the need to urinate more frequently. You may also feel aches or pains in your pelvic joints and perineum as well. You may be able to breathe more easily after the baby drops, as there is now more room in your upper abdomen for your lungs to expand.
Lightening is measured in "stations", indicating exactly where the crown of the baby’s head is in relation to the pelvic bone. When the crown of the baby’s head is right at your pelvic bone, the baby is at 0 station, also called "fully engaged."