Size and position of the baby are the factors involved in problems with the passenger. Simply speaking, the passenger’s size must be somewhat smaller than the hole through the bones of the pelvis (the passage). Nature provides some mechanisms (both maternal and fetal) to provide for tight fits. The soft bones of the fetal head are able to “mold” and “overlap” without noticable damage to the baby. The maternal pelvis, since it is constructed of several bones, can flare a bit to increase the size of the hole. Even the mother’s position just before birth may further enlarge this opening (squatting increases the flaring of the pelvis). But even if the fit between fetal size and pelvic size is adequate, the position of the baby may present some difficulties.
The term, position, has a specific meaning to midwives and physicians However, position used here refers to any combination of factors involving how the fetus’s body is arranged in order to travel through the pelvis. Ideally, the position of the baby during labor and birth consists of the these factors:
- head presenting first
- head well tucked with chin on chest
- head angled such that the center of the head travels along the centerline of the pelvis
Any variation of these factors can increase the size of the presenting diameters of the head and make it more difficult for the head to descend through the mother’s pelvis. Depending on where one measures, the diameters of the fetal skull vary. The largest diameter of the the well-flexed fetal head is the biparietal diameter. If the head is not well-flexed (what is called “de-flexed” or “extended”), the larger occipito-frontal diameter presents. Thus, the de-flexed fetal head needs more room to descend. (see the Glossary for terms such as asynclitism, transverse arrest, extension)
Obviously, not all babies present head-first (the “cephalic” position). A baby in the transverse position is sideways, often with an arm presenting first. Babies in the transverse position cannot be born vaginally unless they turn or are turned. The transverse position in a full-term pregnancy is very rare. The breech position (butt first) is a bit more common (about 3-4 in 100 births). Babies in the breech position may be born vaginally. However, the risk of mishap (and damage to the baby) is greater with the breech position than with a baby in the cephalic position (head-first). These mishaps may occur due to the fact the the largest part of the baby (the head) comes last in breech births. If the head becomes stuck, the umbilical cord blood flow can become pinched off.