At the end of the third trimester, the baby settles, or drops lower, into the mother's pelvis. This is known as dropping or lightening.
Dropping is not a good predictor of when labor will begin. In first-time mothers, dropping usually occurs 2 to 4 weeks before delivery, but it can happen earlier. In women who have already had children, the baby may not drop until labor begins.
You may or may not notice a change in the shape of your abdomen after dropping. You may notice that your breathing becomes easier and heartburn occurs less frequently after dropping occurs. But the increased pressure on your bladder after dropping occurs may make you have the urge to urinate more often.
Usually the baby's head is the first (presenting) part to enter the pelvis, but the feet or a shoulder can present first.
During the last month, your doctor will estimate how far the baby's head has moved down into (engaged) the pelvis. This is measured in "stations." A baby is at –3 station when the head is above the pelvis and at 0 station when the head is at the bottom of the pelvis (fully engaged). The baby is at +3 station when the head is beginning to emerge from the birth canal (crowning).
Medical Review:Kathleen Romito MD - Family Medicine & Adam Husney MD - Family Medicine & Kirtly Jones MD - Obstetrics and Gynecology