Parturition

PARTURITION

Parturition is the expulsion or delivery of the fetus from the mother’s body. It occurs at the end of pregnancy. The process by which the delivery of fetus occurs is called labor. It involves various activities such as contraction of uterus, dilatation of cervix and opening of vaginal canal.

PARTURITION

 

BRAXTON HICKS CONTRACTIONS

Braxton Hicks contractions are the weak, irregular, short and usually painless uterine contractions, which

start after 6th week of pregnancy. These contractions are named after the British doctor, John Braxton Hicks who discovered them in 1872. It is suggested that these contractions do not induce cervical dilatation but may cause softening of cervix. Often called the practice contractions, Braxton Hicks contractions help the uterus practice for upcoming labor. Sometimes these contractions cause discomfort. Braxton Hicks contractions are triggered by several factors such as:

1. Touching the abdomen

2. Movement of fetus in uterus

3. Physical activity

4. Sexual intercourse

5. Dehydration.

FALSE LABOR CONTRACTIONS

While nearing the time of delivery, the Braxton Hicks contractions become intense and are called false labor contractions. The false labor contractions are believed to help cervical dilatation.

STAGES OF PARTURITION

Parturition occurs in three stages:

First Stage

First, the strong uterine contractions called labor contractions commence. Labor contractions arise from

fundus of uterus and move downwards so that the head of fetus is pushed against cervix. It results in dilatation of cervix and opening of vaginal canal. Exact cause for the onset of labor is not known. This stage extends for a variable period of time.

Second Stage

In this stage, the fetus is delivered out from uterus through cervix and vaginal canal. This stage lasts for

about 1 hour.

Third Stage

During this stage, the placenta is detached from the decidua and is expelled out from uterus. It occurs

within 10 to 15 minutes after the delivery of the child.

MECHANISM OF LABOR

The slow and weak contractions of uterus commence at about a month before parturition. Later, the contractions gradually obtain strength and finally are converted into labor contractions at the time of labor. Exact cause for the onset of labor contractions is not known. It is strongly believed that the labor contractions are induced by the signal from fetus. And during labor, reflexes from uterus and cervix produce the powerful uterine contractions. Thus, uterus and cervix play an important role in labor.

Many hormones are also involved during parturition.

ROLE OF UTERUS

Once started, the uterine contractions cause the development of more and more strong contractions. That

is, the irritation of uterine muscle during initial contraction leads to further reflex contractions. It is called positive feedback mechanism. It plays an important role, not only in producing more number of uterine contractions but also the contractions to become more and more powerful.

ROLE OF CERVIX

Cervix also plays an important role in increasing the strength of uterine contractions. When the head of fetus is forced against the cervix during the first stage of labor, the cervix stretches. It causes stimulation of muscles of cervix, which in turn results in reflex contractions of uterus.

ROLE OF HORMONES

Hormones involved in the process of parturition:

Maternal Hormones

1. Oxytocin

2. Prostaglandins

3. Cortisol

4. Catecholamines

5. Relaxin.

Fetal Hormones

1. Oxytocin

2. Cortisol

3. Prostaglandins.

Placental Hormones

1. Estrogen

2. Progesterone

3. Prostaglandins.

Estrogen

Estrogen is continuously secreted along with progesterone throughout the gestation period. However, in

the later period, the quantity of estrogen released is much greater than that of progesterone.

Estrogen:

i. Increases the force of uterine contractions

ii. Increases the number of oxytocin receptors in uterine wall

iii. Accelerates the synthesis of prostaglandin from uterus.

Progesterone

Progesterone plays an important role in labor indirectly by its sudden withdrawal at the end of pregnancy.

Throughout the period of gestation, progesterone suppresses uterine contractions. It also inhibits the synthesis of prostaglandin (PGE2), which is necessary for uterine contraction. Progesterone inhibits prostaglandin synthesis by inhibiting the release of the enzyme phospholipase A, which is essential for prostaglandin synthesis. Sudden decrease in progesterone secretion at the end of gestation period increases the uterine contractions and PGE2 synthesis.

Oxytocin

Oxytocin:

i. Causes contraction of smooth muscle of uterus and enhances labor. During the later stages of

pregnancy, the number of receptors for oxytocin increases in the wall of uterus by the influence of

estrogen. Because of this, the uterus becomes more sensitive to oxytocin.

ii. Stimulates the release of prostaglandins in the decidua.

Oxytocin is released in large quantity during labor. It is due to neuroendocrine reflex. During the movement of fetus through cervix, the receptors on the cervix are stimulated and start discharging a large number of impulses. Impulses are carried to hypothalamus by the somatic nerve fibers and result in the release of a large quantity of oxytocin, which enhances labor. The release of more amount of oxytocin occurs due to positive feedback.

Relaxin

Relaxin is secreted from maternal ovary (corpus luteum) during the initial period of pregnancy. It is secreted in large quantity at the time of labor by placenta and mammary glands.

Relaxin:

i. Helps labor by softening the cervix and loosening the ligaments of symphysis pubis, so that the

dilatation of cervix occurs

ii. Increases the number of receptors for oxytocin in the myometrium

iii. Simultaneously suppresses the inhibitory action of progesterone on uterine contraction so that

the uterus starts contracting

iv. Facilitates the development of mammary glands.

Prostaglandins

In recent times, prostaglandins are considered to play a vital role in labor. Prostaglandins particularly

PGE2 facilitate labor by increasing the force of uterine contractions. The prostaglandins are secreted from

uterine tissues, fetal membranes and placenta. Their concentration is increased in maternal blood and

amniotic fluid at the time of labor. Prostaglandins increase the force of uterine contractions by elevating the intracellular concentration of calcium ions in the uterine muscles.

Catecholamines

It is believed that the circulating adrenaline and noradrenaline also might increase the uterine contraction

through alpha adrenergic receptors.

Cortisol

At the time of labor, hypothalamus releases large quantity of corticotropin-releasing hormone, which increases the release of cortisol from the adrenal cortex. Cortisol

enhances the uterine contraction and plays an important role in helping the mother to withstand the stress during labor.

 

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