The position of the fetus is usually not changed after the onset of labor, and it is difficult to correct an abnormal fetal position.
Pelvic entry is the late gestation period when the fetus is surrounded by amniotic fluid and fetal membranes, so that the head enters the pelvic cavity through the pelvic inlet of the mother, so that the body position is consolidated. Once the head of the fetus enters the pelvic cavity, the position is consolidated, and the space in the uterus restricts the change of the position of the fetus, so that the position of the fetus generally will not be altered again after it has been placed in the pelvic cavity.
Most pregnant women may enter the pelvis around 36-38 weeks of pregnancy, some pregnant women may enter the pelvis around 34 weeks of pregnancy, or may not enter the pelvis due to asymmetry of the cephalic plate, oversized fetus and other factors.
Failure of the fetus to enter the pelvis may cause premature rupture of membranes, which should be treated promptly with external inversion to correct the abnormal fetal position, or cesarean section if necessary.