FIRST STAGE OF LABOUR

Physiology of first stage of labour

The first stage of labor is primarily focused on preparing the birth canal to facilitate the expulsion of the fetus in the second stage. The duration of labour can significantly differ and is influenced by factors such as parity, birth interval, psychological well-being, fetal presentation and position, maternal pelvic shape and size, and the nature of uterine contractions. The active phase of the first stage is typically completed within 6 to 12 hours for primi-gravida women and within 5 to 6 hours for multiparous women.

Following physiological changes are occurs in first stage of labour

1. Duration
2. Uterine contraction (action)

• -Fundal dominance
• -Polarity of the uterus
• -Contraction and retraction of uterine muscle
• -Formation of upper and lower uterine segment
• -Formation of retraction ring
• -Taking up of cervix (effacement of cervix)
• -Dilatation of cervix

3. Mechanical factors

• -Fetal axis pressure
• -Formation of bag of forewater and hind waters
• -General fluid pressure
• -Rupture of membrane
• -Appearance of show

1. Duration

The duration of labour exhibits considerable variability and is subject to influences by parity, birth interval, psychological well-being, fetal presentation and position, maternal pelvic anatomy, and the nature of uterine contractions. It is reasonable to anticipate that the active phase of this first stage to be completed within 6 to 12 hours.

2. Uterine action

Fundal dominance:

Each contractions start in the fundal region near one of the cornua and spread downwards across being stronger and persisting longer in the upper region. The fundus and mid-zone remain hard throughout the period of contraction.

Polarity of the uterus

Polarity is the term used to describe the neuromuscular harmony that prevails between the two poles or segments of the uterus throughout labour. During each uterine contraction, these two poles act harmoniously. The upper pole contracts strongly and retracts to expel the fetus; the lower pole contracts slightly and dilates to allow expulsion to take place. If polarity is disorganized then the progress of labour is inhibited.

Contraction and retraction of uterine muscle

Uterine contraction are involuntary, controlled by nervous system and by endocrine influence. They usually recur with rhythmic regularity, and intervals between them gradually diminish from 15 minute more or less at the beginning of the first stage to 2 or 3 minute at the end of the first stage.

Retraction is when muscle fibers retain some of the shortening of the contraction instead of becoming completely relaxed . It assists in the progressive expulsion of the fetus; upper segment of the uterus becomes gradually shorter and thicker and its cavity diminishes.

Formation of upper and lower uterine segment

Upper uterine segment, having been formed from the body of the fundus, is mainly concerned with contraction and retraction; it is thick and muscular. Lower uterine segment is formed of the isthmus and the cervix, about 8-10 cm in length and prepared for distention and dilatation. The muscle content reduces from the fundus to the cervix, where it is thinner. When the labour begins, retracted longitudinal fibres in upper segment pull on the lower segment causing it to stretch; this is aided by descending presenting part.

Development of retraction ring

The ridge forms between the upper and lower uterine segments; this is retraction ring. The physiological ring gradually rises as the upper uterine segment contracts and retracts and the lower uterine segment thins out to accommodate the descending fetus. Once the cervix is fully dilated and the fetus can leave the uterus, the retraction ring rises no further. It is present in every labor and is perfectly normal until it is not marked enough to be visible above the symphysis pubis. In normal labor it is not visible because the fetus is gradually being expelled through the dilating cervix . In obstructed labor, where fetus can not descend to pass through the cervix , the lower , segment must stretch to accommodate it, because the fetus is being pushed out of the shortened upper segment . Retraction ring is termed in case of invisible and bandl’s ring when it becomes visible.

Cervical effacement

Effacement refers to the inclusion of the cervical canal into the lower uterine segment. It takes place from above downward; the muscle fibres surrounding the internal os are drawn upwards by the retracted upper segment and the cervix merges into the lower uterine segment. The cervical canal widens at the level of the internal os, where the condition of the external os remains unchanged.

Dilatation of cervix

The external os begins to enlarge from a circular opening. It is aided by upward traction, exerted by the retracted muscle fibres in the upper segment exerts pull on the margin of the weakened area-the cervix and makes the os enlarge. The well flexed head will, when closely applied to the cervix, aid dilatation.

In primigravida , external os may be closed at the beginning of labour or it may admit the tip of one finger and does not dilate until the cervix had been taken up, but the internal os dilates during the process of taking up of the cervix. In multipara, the external os usually admits one finger prior to the onset of labour and dilataion of the external and internal os proceeds simultaneously with taking up of the cervix.

It is the process of enlargement of the os uteri from a tightly closed aperture to an opening large enough to permit the passage of the fetal head. Dilatation is measured in cm and full dilatation at term equates to about 10 cm. A well-flexed fetal head closely applied to the cervix favours efficient dilatation.  Pressure applied evenly to the cervix causes the uterine fundus to respond by contraction and retraction.

3. Mechanical factors

Formation of bag of forewater and hind waters

The membrane (amnion, chorion) are attached loosely to the decidua , a lining the uterine cavity except over the internal os. In vertex presentation, the girdle of the head being spherical, may well felt with the wall of the lower uterine segment. Thus the amniotic cavity is divided into two compartments. The part above the girdle of contact contain the fetus with bulk of the liquor is hind water and below it containing small amount of liquor is forewater. With the onset labor, the membrane attached to the lower uterine segment are detached and with the rise of intrauterine pressure during contraction there is herniation of the membrane through the cervical canal.

General fluid pressure

While the membranes remain intact, the pressure of the uterine contractions is exerted on the fluid and, as fluid is not compressible, the pressure is equalized throughout the uterus and the fetal body; known as general fluid pressure. When the membranes rupture and a quantity of fluid emerges, the placenta and umbilical cord are compressed between the uterine wall and the fetus during contractions and the oxygen supply to the fetus is diminished. Preserving the integrity of the membranes, therefore optimizes the O² supply to the fetus and also helps to prevent intrauterine and fetal infection, especially in longer labours.

Rupture of membrane

The amniotic sac should remain intact until the cervix is fully dilated, but this by no means always happens the membrane may rupture days before labor begins or during first or second stage and in some instance not until the head is being born. Towards the end of the first stage the bag of membrane receives very little support because of the extensive dilation of  cervix. It is also subjected to increased force of the strong uterine contraction. If there is badly fitting presenting part,  fore water are not cut off effectively and the membranes rupture early, but some cases this happens for no apparent reason.

Fetal axis pressure

During each contraction uterus rises forward and the force of the fundal contraction is transmitted to the upper pole of the fetus down long axis of the fetus and applied by the presenting part to the cervix. When the fetus lies longitudinally in flexed attitude there is a tendency of straightening out of the fetal axis due to contraction of circular muscles of the body of uterus. This allows fundal contraction to transmit through the podalic pole into the fetal axis and allows mechanical stretching of lower segment and opening up of cervical canal. Fetal axis pressure cannot operate in presence of excess liquor or in cases of transverse lie where smooth dilation occurs.

Appearance of show

This is the blood stained mucoid discharge seen a few hours before or within a few hours after labour has started. The mucous is the thick, tenacious substance which formed the cervical plug-the operculum during pregnancy. The blood comes from the ruptured capillaries of the decidua vera where the chorion has become detached and from the dilating cervix.

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