Two problems with the baby itself, called passenger problems, cause abnormal labor and require intervention:

  • the baby may appear to be in jeopardy, leading to immediate delivery to avoid injury; and
  • the baby may be blocked from passing through the birth canal because of its size or position within the uterus (presentation).

 

Presentation

Presentation, or how the baby is orientated to pass through the birth canal, depends on which part of the baby is closest to the cervix before delivery. Ninety-five percent are in a cephalic (head-down) presentation and about 4% are in a breech (buttocks-down) presentation. One in 300 is in a transverse presentation, or lying across the opening to the birth canal, and one in 1,000 is in a compound presentation, where the baby's arm or leg is next to the main presenting part, usually the head. More common in preterm labor, breech, transverse, and compound presentations are considered abnormal and are associated with labor problems. Currently, non-cephalic presentations should be delivered by cesarean section.

Size

Most babies are proportionate in size to their mothers, but sometimes a fetus is too large to pass through the pelvis of the mother. This situation is known as cephalo-pelvic disproportion (CPD). There is no good way to know the size of a baby prior to the onset of labor, though estimation by the mother, doctor, or by means of ultrasound imaging should be attempted to evaluate the size of the baby both before and during labor. When the baby is too large to pass through the pelvis, labor dystocia occurs and is manifest by abnormal cervical dilation or descent of the baby through the pelvis.

The baby's weight, unless it exceeds nine pounds, does not usually determine whether the fetus is considered too large for vaginal delivery. Diabetic mothers, whose babies have large bodies compared to their heads, are important exceptions. The risk of shoulder dystocia, where the shoulder of the fetus becomes stuck in the birth canal, is substantially increased. But babies rarely suffer serious consequences as a result of shoulder dystocia. They normally have minor injuries or none at all, and only few have long-term problems.