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Any section of the mother's pelvis can be abnormally small in diameter, and may prolong labor or make it inefficient. In other situations, the baby may be too large to navigate the pelvis, causing dystocia. In most cases, the baby is given time to mold the head and pass through. If attempts at pain relief, uterine stimulation and maternal rest do not lead to progress, a cesarean section can be performed. While any mother would rather be spared those hours of labor, they are necessary to test the pelvis.
Because a full bladder or a full rectum also can impede progress and obstruct the passage, mothers are encouraged to empty their bladders at least every two hours during labor. Caregivers no longer routinely use enemas, because the bowel has a natural tendency to empty in the early phase of labor. However, an enema may be requested or recommended if it would make the mother more comfortable.
Your Baby's Job During Labor
Babies come in many sizes. In normal gestation, weight is determined by a combination of genetics and the mother's nutrient intake. Head sizes may vary by one to two centimeters, which can make the difference in negotiating the midplane of the pelvis. Women who gain more than 40 pounds during pregnancy and women with uncontrolled diabetes have babies who are larger than average.
Fetal position is closely tied to uterine tone, pelvic structure, cord length, placental implantation site, the number of children the mother has had and the health of the fetus. The baby who engages into the pelvis in a posterior position or in a breech presentation may not fit as well. The baby's head ideally should be well flexed so the smallest part of its skull presents first. When the fit is not favorable or larger parts of the fetus present first, there's a greater risk of dystocia.