Lightening, part of the body's preparation for labour, occurs when the foetus descends into the pelvis late in pregnancy and is sometimes referred to as engagement or as the baby "dropping." It cannot be used as a predictor for when a woman will give birth, however, as it occurs at different times in different women and some women may not notice it happening at all.
Lightening can happen up to four weeks before labour actually begins. In women with previous pregnancies, lightening may not occur until labour begins. If you think you've experienced lightening, especially if you're feeling pelvic pressure, but your due date is more than four weeks away, call your doctor so you can be checked for preterm labour (references 1).
Lightening gets its name from the feeling of lightness or relief that many women experience after it occurs (references 3). With the baby moving farther from the rib cage and lower into the pelvis, you may be able to breathe easier and take deeper breaths, eat more in one sitting without feeling too full and find relief from heartburn (references 1). You might also notice more space between your breasts and uterus (references 2) or a change in the shape or appearance of your baby bump.
After lightening occurs, you may experience increased pressure in your pelvis, discomfort while walking, and the need to urinate more frequently as baby presses down on your bladder (references 2). Many women report feeling as though they're waddling when they walk (references 1) or that the baby could fall out at any minute, thanks to additional pressure on the rectum (references 3). However, it's possible to experience lightening without having any symptoms or even knowing a change has occurred.
While lightening may relieve some of your least favourite pregnancy symptoms, it can also introduce new ones. If you experience a great deal of pelvic discomfort after lightening occurs, try lying down with a pillow under your hips or performing pelvic tilts; stay active with swimming rather than walking to decrease pressure in your pelvis.
Don't be concerned if you're nearing your due date and you still don't think your baby has dropped into your pelvis. It doesn't necessarily mean labour is still weeks away because it happens differently in every pregnancy and there is no way to predict when it will occur. Lightening can happen gradually, without you noticing, or it may occur over the course of a single day (references 2).