In vaginal delivery, the passage the baby takes from the uterus through the birth canal is largely affected by the size and shape of the pelvis, as well as the tissues it contains. The urinary bladder, portions of the pregnant uterus, part of the colon, and numerous nerves and blood vessels are all supported from underneath by layered sets of muscles in a hammock-like structure, all of which is surrounded by the bony pelvis.
The soft tissues of the pelvis or the presence of any abnormal masses or growths may block the passage of the baby through the birth canal. The most common abnormal masses arising from the tissue of the uterus are called uterine fibroids (also known as myomas). Most growths of this type pose no problems because they are mainly found in the upper part of the uterus. But any fibroids in the lower pelvis may block the descent of the fetus. Ultrasound evaluation of the size and position of fibroids and/or other masses of soft tissue may help determine if vaginal delivery is possible, though, the real test is labor itself.
The bony pelvis has four general shapes, which may occur separately or in combination:
- Gynecoid. This is the most common pelvic shape and is best for vaginal delivery. It has an oval-shaped inlet (wider from side to side than from front to back) with parallel sides, dull ischial spines, and a pubic arch that is 90 degrees or wider.
- Anthropoid. This shape has an oval inlet but is wider from front to back than from side to side compared to the gynecoid pelvis. The sidewalls are parallel or flare outward, and the back part is roomy enough to accommodate the back of the fetus's head. This may result in the baby being born face up.
- Platypoid. This type of pelvis has an oval-shaped inlet that is compressed from front to back. This may result in a fetus that traverses the pelvis with its head in a transverse or sideways position.
- Android. This is a male-type pelvis, with a small inlet that is somewhat heart-shaped. The sidewalls converge, the ischial spines are prominent, and the pubic arch is narrow.
The platypoid and android pelvis types are responsible for most obstructed labor.
The shape of the pelvis is usually assessed in early pregnancy during the initial pelvic exam. However, if labor is prolonged, the pelvis should be re-assessed, especially if your doctor is considering a forceps, vacuum, or cesarean delivery. As with fibroids, the real test of a bony pelvis is labor and delivery. Though some women may have an unfavorable bony pelvis, this by no means indicates that a successful vaginal delivery is unlikely. It merely provides additional information so that if the labor should become obstructed, an explanation is readily available.