Events in first stage of labour

The first stage is chiefly concerned with the preparation of the birth
canal so as to facilitate expulsion of the fetus in the second stage.
The main events that occur in the first stage are -

             a) Dilatation and effacement of the cervix.

             b) Full formation of lower uterine segment.

Dilatation of the cervix - 

Dilation of cervix is the process of enlargement of the external cervical os
(opening) from an orifice of a few mm to an opening large enough for the baby
to pass through. Dilatation is clinically evaluated by measuring the diameter of
the cervical opening in cm, with '0' (zero) cm being a closed external cervical os
(opening) and 10 cm complete dilatation.

Actual factors responsible are -

i) Uterine contraction and retraction -: The longitudinal muscle fibers of the upper
segment are attached with circular muscle fibers of the lower segment and upper
part of the cervix in a bucket holding fashion. Thus, with each uterine contraction,
not only the canal is opened up from above down but it also becomes shortened
and retracted. There is some co-ordination between fundal contraction and cervical
dilatation called ''polarity of uterus''.
ii) Bag of membranes -: The membranes (amnion and chorion) are attached loosely to
the decidua lining the uterine cavity except over the internal os. In vertex presentation,
the girdle of contact of the head (that part of the circumference of the head which first
comes in contact with the pelvic brim) being spherical, may well fit with the wall of the
lower uterine segment. Thus the amniotic cavity is divided into two compartments.
The part above the girdle of contact contains the fetus with bulk of the liquor called
hindwaters and the one below it containing small amount of liquor called forewaters.
iii) Fetal axis pressure -: During each contraction the uterus rears forward (become upright)
and the force of fundal contraction is transmitted to the upper pole of the uterus,
down the long axis of the fetus is applied by the presenting part of the cervix. This is
known as fetal axis pressure. It becomes more significant after rupture of the membrane
and during the second stage of labour.
iv) Vis-a-tergo -: The final phage of dilatation and retraction of the cervix is achieved by
download thrust of the presenting part of the fetus and upward pull of the cervix over
the lower segment.

Effacement or taking up of cervix -
Effacement is the process by which the muscular fibers of the cervix are pulled upward 
and merge with the fibers of the lower uterine segment. The cervix becomes thin during
the first stage of labour or even before that in primigravidae. In primigravidae, effacement
precedes dilatation of the cervix, whereas in multiparae, both occur simultaneously.
Expulsion of mucus plug is caused by effacement.

Lower uterine segment - 
Before the onset of labour, there is no complete anatomical or fuctional division of uterus.
During labour, the demarcation of an active upper segment and a relatively passive lower
segment is more pronounced. The wall of the upper segment becomes progressively
thickened with progressive thinning of the lower segment. This is pronounced in late first
stage, specially after rupture of the membranes and attains its maximum in second stage.
A distinct ridge is produced at the junction of the two, called physiological retraction ring
which should not be confused with the pathological retraction ring - a feature of obstructed 
labour. The lower segment is thus limited superiorly by the physiological retraction ring and
inferiorly by the fibromuscular junction of cervix and uterus.