Which factor naturally opens the pelvic inlet in pregnancy?

• A. Initial descent of fetus causes sacral counternutation (bilateral extension)
• B. Relaxin loosening opens the pelvic ligaments
• C. Uterine growth expands the pelvic inlet
• D. Maternal posture alone opens the inlet

Answer: (A)

The opening of the pelvic inlet during early pregnancy and labor comes from dynamic movement of the sacrum as the fetus descends. When the head starts to descend, the sacrum undergoes counternutation (a backward tilt of the sacral base). This counternutation changes the shape of the pelvic brim, effectively increasing the space at the inlet and allowing easier engagement of the fetal head.

Relaxin loosens ligaments and aids overall pelvic mobility, but it doesn’t actively open the inlet in the moment of early descent. Uterine growth doesn’t widen the brim, and posture alone doesn’t create the active opening needed for entry. The key functional change that enlarges the inlet in this context is the sacral movement caused by fetal descent.