1. Malpresentation of the fetus: A situation where the fetus is not in the normal "head down" position is called a malpresentation, or more specifically a vertex or cephalic presentation, and a c-section is usually necessary. This is not always the case, however; certain abnormal presentations, such as the "frank breech" in which the baby's rear end comes first and the legs are completely extended, can potentially deliver vaginally. Your doctor may assess the size of your pelvis using a physical exam or an x-ray to help determine whether or not this will be possible in your case.
2. Failure to dilate or failure of descent: If you get into the active phase of labor (when your cervix is dilated 4 cm or more) but you then have no change in your cervix for two hours with adequate contractions (measured by a small catheter placed through the vagina and into the uterus), you are experiencing what is called "failure to dilate" and you may need a c-section. If you are completely dilated and the baby is not delivered after one or two hours, this is known as "failure of descent." Generally, this means that your baby's head is too big to fit through your pelvis (known as cephalo-pelvic disproportion or "CPD"). If the baby's head is accessible, your doctor may perform an "assisted" vaginal delivery with forceps or a vacuum. If the head is not accessible or your doctor feels it is unsafe to assist the delivery, a c-section is required. Lastly, if your labor is being induced artificially and your cervix is closed after 18-24 hours of attempted induction, a c-section is often needed.
