The relationship between brim and pelvis during childbirth can be a bit confusing. When we say that 4/5ths of the head is in the pelvis, it means that a significant portion of the baby's head has descended into the pelvic area. This indicates progress towards delivery. However, it doesn't necessarily mean that only 1/5th is left outside. It could also mean that 1/5th of the head is still above or outside of the pelvic brim. The midwife may be able to feel this portion during an examination. It's important to note that these fractions are not exact measurements but rather estimates used by healthcare professionals to assess progress during labor. Each woman's anatomy and labor progress differently, so it's best to consult with your healthcare provider for personalized information about your specific situation.