The start of labor can occur at any time whether or not your baby has dropped into the pelvis. In full-term first pregnancies, the baby often does drop -- also called lightening -- several weeks before labor begins, but this might not happen with subsequent babies. If you go into preterm labor, whether it's your first baby or your fourteenth, your baby probably won't drop before the start of labor.
Lightening -- sometimes called fetal engagement -- means that your baby's head, or other presenting part, if he's coming bottom first, has dropped into the pelvic outlet. The presenting part moves down to the narrowest part of the pelvis, where the ischial spines protrude slightly. Doctors often refer to this as zero station. The stations of labor describe the progression of the baby through the pelvis, starting at minus 4 and ending with the delivery of the baby at plus 4 station, when the head is on the perineum.
Why Babies Drop
At the end of pregnancy, several factors make it easier for your baby to drop further into the pelvis. The ligaments and tissues in the pelvis soften and become less taut under the influence of the hormone progesterone. The increased amount of blood circulating in the pelvic blood and uterine blood vessels also softens the tissues, making it easier for the uterus to descend into the pelvis. The baby's continued growth also leads to the need to expand upward, outward or downward.
How to Tell
When your baby drops, the first thing you notice is that it's easier to breathe. The second thing you might notice is how much more often you need to urinate, with the increased pressure on your bladder. Heartburn might decrease and you may feel you can eat more. The increased pressure from the baby's head on your pelvic bones also worsens the "pelvic waddle"-- the characteristic walk of late pregnancy. You might also experience more lower back pain after your baby drops.
When a Baby Doesn't Drop
It might seem more logical that second or later babies would drop into the pelvis sooner than firstborns, because you've already given birth once. But it's because you've already had one baby that the second might not engage before contractions start. Your abdominal muscles aren't as tight the second or third time around, which allows the baby to protrude out further rather than dropping down lower. Other factors that can keep a baby from dropping, such as an unflexed head -- flexing decreases the amount of space the baby's head takes up. If you have a small pelvis and the baby has a large head, he might not drop until the forces of labor push his head down.
Problems During Labor
If your baby doesn't drop by the time you start labor in a full-term first pregnancy, you could have an increased risk of having a Cesarean delivery. A Kaiser Permanente Medical Center study published in the January 2004 "Journal of American Obstetrics and Gynecology" found that women whose head-down firstborns hadn't dropped by 41 weeks had a 12.4 times higher risk of a Cesarean birth than first-time mothers whose babies had dropped. In women who have had a previous baby, failure to drop by the time cervical dilation is complete is abnormal, Certified Nurse-Midwife Helen Varney reports in her textbook "Varney's Midwifery."