The major event that occurs during the first stage of birth is

Adilation of the cervix.
Bdelivery of the foetus.
Cbursting of the membranes.
Dexpulsion of the afterbirth.

Verified Answer :

a)dilatation of the cervix

AL2006

From that list, the first event is almost always dilation.  It's followed
by bursting, delivery, and expulsion of the placenta.

In a small fraction of cases, the bursting may occur before dilation.