Towards the end of pregnancy, the baby will normally descend and dip into the woman's pelvic cavity, which is a process known as engagement. The baby will normally descend head-first, although it can be bottom or feet-first if the baby is in the breech position.
In the normal head-first position, relation to the brim is the terminology used to describe how much of the baby's head can still be felt resting above the pelvic cavity. This in turn reveals how much, if any, of the baby's head is engaged.
To make this assessment, the baby's head is divided into fifths. The lower the number, the lower the baby is. For example, a baby that is 5/5 has not yet descended into the pelvis and is resting above the pelvic cavity. A baby described as 4/5 (four fifths) has the top part of its head in the pelvis.
This continues until birth is imminent, by which time the baby will be described as 0/5 in relation to the brim.
The baby's descent into the pelvic capacity usually happens by 38 weeks, although it can happen as early as 33 weeks.
However, some babies will not descend into the pelvis until labour begins. This can happen with second and subsequent babies; and can also happen to women with well toned abdominal muscles. This is because the tummy area is much tighter, which affects the angle of the baby and its descent into the pelvis.