Second stage of labour

Physiological changes during second stage of labour

The second stage of labour begins with the full dilatation of the cervix and concludes with the fetus being expelled. This stage focuses on the descent and delivery of the fetus through the birth canal. The forceful contractions of the uterus are further facilitated by the voluntary contractions of the abdominal muscles, known as “bearing down” efforts.

Increase uterine contraction

Uterine contractions increase in intensity, accompanied by painful involuntary contractions and retractions in the uterine muscles. These retractions cause the uterine muscles to gradually shorten after each contraction as the baby descends into the mother’s pelvic cavity. Importantly, the muscles do not fully return to their initial length even after the contraction subsides and the muscle relaxes.

Abdominal pressure

As the second stage begins, the abdominal muscles and diaphragm, which serve as accessory muscles during labour, contract forcefully to facilitate the expulsion of the fetus. The diaphragm, a resilient muscle that separates the abdominal cavity from the chest, is involved in this process. The contractions of both the diaphragm and abdominal muscles are evident in a clinical context as “bearing down” pains.

The woman in labour takes a deep inspiration and holds her breath, thereby fixing the diaphragm in a lower position and then contracts the abdominal muscles. This action increases the intra abdominal pressure, compressing the uterus and helps in increasing the expulsive force. In the beginning, this secondary power is voluntary and the woman can withhold the urge to push. But in the later part of the second stage, the urge and the pressure becomes involuntary and synchronizes with the uterine contraction.

Distension of the pelvic floor

As the fetus moves into the vagina dilating the vagina cavity, the structures in front as well as those behind the uterus changes their position. The structure in the front of the bladder and the urethra get pushed upwards and forwards. This results in inability to pass urine by the woman in labour.

The structures behind the uterus are the rectum, the anus and the perineum. These get displaced downwards and backwards. The result is that woman gets desire to pass stool and perineum becomes stretched and thinned out. The anus opens up as the head descends.

Expulsion of the fetus

As the fetal head descends, the pelvis’s soft tissues undergo displacement. Internally, the bladder is pushed upward into the abdomen, posing a risk of injury during fetal descent. Consequently, the urethra stretches and thins, reducing its lumen. Posterior, the rectum becomes flattened into the sacral curve and the pressure of the advancing head expels any residual fecal matters. The elevator ani muscles dilate, thin out and are displaced laterally and the perineal body is flattened, stretched and thinned.

The fetal head becomes visible at the vulva, advancing with each contraction and receding between contractions until crowning takes place. The head is then born. The shoulders and body follows with the next contraction, accompanied by gush of amniotic fluid and some times in the blood.  The second stage ends with birth of the baby.

Phases of second stage of labour

Early phase (passive)

 From full dilatation until the urge to bear down, the soft tissue of the vagina and pelvic floor gradually stretches and thins due to the pressure exerted by the advancing fetal head. The woman may not feel a strong urge to push until the head has descended enough to press on the rectum and perineal tissue. The head will then become visible.

Active phase or perineal phase (expulsive phase)

From the onset of pushing efforts to crowning of the presenting part. Once the fetal head is visible a maternal urge to push because fetal head is low, causing a reflex need to bear down.

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