Parturition Process

Parturition Process

1. Parturition is the birth process which is accompanied with labor pains . 

 2. It is a neuro - endocrine mechanism which involves rise in estrogen : progesterone ratio and increase in oxytocin receptors in myometrium of uterine wall . 

 3. The fully developed foetus gives signals for the uterine contractions by secreting Adrenocorticotropic hormone ( ACTH ) from pituitary and corticosteroids from adrenal gland .  

4. This triggers release of oxytocin from mother's pituitary gland , which acts on uterine muscles of mother and causes vigorous uterine contractions . 

 5. This leads to expulsion of the baby from the uterus . 

 6. Parturition involves three stages, viz. Dilation stage, expulsion stage and after birth or placental stage.

Oxytocin During and after delivery of a baby, oxytocin affects two target tissues: the mother’s uterus and breasts. During delivery, stretching of the cervix of the uterus stimulates the release of oxytocin which, in turn, enhances contraction of smooth muscle cells in the wall of the uterus ; after delivery, it stimulates milk ejection (“letdown”) from the mammary glands in response to the mechanical stimulus provided by a suckling infant.

The function of oxytocin in males and in nonpregnant females is not clear. Experiments with animals have suggested that it has actions within the brain that foster parental caretaking behavior toward young offspring. It may also be responsible, in part, for the

feelings of sexual pleasure during and after intercourse. 

Hormones of Pregnancy

During the first 3 to 4 months of pregnancy, the corpus luteum in the ovary continues to secrete progesterone and estrogens, which maintain the lining of the uterus during pregnancy and prepare the mammary glands to secrete milk. The amounts secreted by the corpus luteum, however, are only slightly more than those produced after ovulation in a normal menstrual cycle. From the third month through the remainder of the pregnancy, the placenta itself provides the high levels of progesterone and estrogens required.

As noted previously, the chorion of the placenta secretes human chorionic gonadotropin (hCG)  into the blood. In turn, hCG stimulates the corpus luteum to continue production of progesterone and estrogens—an activity

required to prevent menstruation and for the continued attachment

of the embryo and fetus to the lining of the uterus .By the eighth day after fertilization, hCG can be detected in the

blood and urine of a pregnant woman. Peak secretion of hCG occurs at about the ninth week of pregnancy .

During the fourth and fifth months the hCG level decreases sharply and then levels off until childbirth.

 The chorion begins to secrete estrogens after the first 3 or 4 weeks of pregnancy and progesterone by the sixth week. These hormones are secreted in increasing quantities until the time of

birth . By the fourth month, when the placenta is fully established, the secretion of hCG is greatly reduced, and

the secretions of the corpus luteum are no longer essential.A high level of progesterone ensures that the uterine myometrium is relaxed and that the cervix is tightly closed. After delivery, estrogens and progesterone in the blood decrease to normal levels.

Relaxin, a hormone produced first by the corpus luteum of the ovary and later by the placenta, increases the flexibility of the pubic symphysis and ligaments of the sacroiliac and sacrococcygeal joints and helps dilate the uterine cervix during labor. Both of these actions ease delivery of the baby.

 A third hormone produced by the chorion of the placenta is

human chorionic somatomammotropin (hCS) also known as human placental lactogen (hPL). The rate of secretion of hCS increases in proportion to placental mass,reaching maximum levels after 32 weeks and remaining relatively constant after that. It is thought to help prepare the mammary glands for lactation, enhance maternal growth by increasing protein synthesis, and regulate certain aspects of metabolism in both  mother and fetus. For example, hCS decreases the use of glucose

by the mother and promotes the release of fatty acids from her adipose tissue, making more glucose available to the fetus.

 The hormone most recently found to be produced by the placenta is corticotropin-releasing hormone (CRH), which in nonpregnant people is secreted only by neurosecretory cells in the hypothalamus. CRH is now thought to be part of the “clock” that establishes the timing of birth. Secretion

of CRH by the placenta begins at about 12 weeks and increases enormously toward the end of pregnancy. Women who have higher levels of CRH earlier in pregnancy are more likely to deliver prematurely; those who have low levels are more likely to deliver after their due date. CRH from the placenta has a second important effect: It increases secretion of cortisol, which is needed for maturation of the fetal lungs and the production of surfactant

What is labor at the time of Parturition

Labor :

Labor is the process by which the fetus is expelled from the uterus

through the vagina, also referred to as giving birth. A synonym for

labor is parturition.

 The onset of labor is determined by complex interactions of several placental and fetal hormones. Because progesterone inhibits

uterine contractions, labor cannot take place until the effects of progesterone are diminished. Toward the end of gestation, the levels of estrogens in the mother’s blood rise sharply, producing changes That overcome the inhibiting effects of progesterone. The rise in

estrogens results from increasing secretion by the placenta of corticotropin-releasing hormone, which stimulates the anterior pituitary gland of the fetus to secrete ACTH (adrenocorticotropic hormone).

In turn, ACTH stimulates the fetal adrenal gland to secrete cortisol

and dehydroepiandrosterone (DHEA), the major adrenal androgen. The placenta then converts

DHEA into an estrogen. High levels of estrogens cause the number

of receptors for oxytocin on uterine muscle fibers to increase, and

cause uterine muscle fibers to form gap junctions with one another.

Oxytocin released by the posterior pituitary stimulates uterine contractions, and relaxin from the placenta assists by increasing the

flexibility of the pubic symphysis and helping dilate the uterine

cervix. Estrogen also stimulates the placenta to release prostaglandins, which induce production of enzymes that digest collagen fibers in the cervix, causing it to soften.

 Control of labor contractions during parturition occurs via a positive feedback cycle . Contractions of the uterine myometrium force the baby’s head or body into the cervix, distending (stretching) the cervix. Stretch receptors in the cervix send nerve impulses to neurosecretory cells in the hypothalamus, causing them to release oxytocin into blood capillaries of the posterior pituitary gland. Oxytocin then is carried by the blood to the uterus,

where it stimulates the myometrium to contract more forcefully. As

the contractions intensify, the baby’s body stretches the cervix still

more, and the resulting nerve impulses stimulate the secretion of yet more oxytocin. With birth of the infant, the positive feedback cycle is broken because cervical distension suddenly lessens.

 Uterine contractions occur in waves (quite similar to the peristaltic waves of the gastrointestinal tract) that start at the top of the uterus and move downward, eventually expelling the fetus. True labor begins when uterine contractions occur at regular intervals,

usually producing pain. As the interval between contractions shortens, the contractions intensify. Another symptom of true labor in some women is localization of pain in the back that is intensified by walking. The most reliable indicator of true labor is dilation of the cervix and the “show,” a discharge of a blood-containing mucus

into the cervical canal. In false labor, pain is felt in the abdomen at

irregular intervals, but it does not intensify and walking does not

alter it significantly. There is no “show” and no cervical dilation.

 True labor can be divided into three stages (:1 Stage of dilation. The time from the onset of labor to the

complete dilation of the cervix is the stage of dilation. This stage, which typically lasts 6–12 hours, features regular contractions of the uterus, usually a rupturing of the amniotic

sac, and complete dilation (to 10 cm) of the cervix. If the amniotic sac does not rupture spontaneously, it is ruptured

intentionally. 2 Stage of expulsion. The time (10 minutes to several hours)

from complete cervical dilation to delivery of the baby is the stage of expulsion.

3 Placental stage. The time (5–30 minutes or more) after delivery until the placenta or “afterbirth” is expelled by powerful uterine contractions is the placental stage. These contractions also constrict blood vessels that were torn during delivery, reducing the likelihood of hemorrhage.

 As a rule, labor lasts longer with first babies, typically about

14 hours. For women who have previously given birth, the average

duration of labor is about 8 hour although the time varies enormously among births. Because the fetus may be squeezed through the birth canal (cervix and vagina) for up to several hours, the fetus is stressed during childbirth: The fetal head is compressed,and the fetus undergoes some degree of intermittent hypoxia due to compression of the umbilical cord and the placenta during uterine contractions. In response to this stress, the fetal adrenal medullae secrete very high levels of epinephrine and norepinephrine,the “fight-or-flight” hormones. Much of the protection against the stresses of parturition, as well as preparation of the infant for surviving extrauterine life, is provided by these hormones. Among

other functions, epinephrine and norepinephrine clear the lungs

and alter their physiology in readiness for breathing air, mobilize

readily usable nutrients for cellular metabolism, and promote an increased flow of blood to the brain and heart.

 About 7% of pregnant women do not deliver by 2 weeks after

their due date. Such cases carry an increased risk of brain damage to the fetus, and even fetal death, due to inadequate supplies of oxygen and nutrients from an aging placenta. Postterm deliveries may be facilitated by inducing labor, initiated by administration of oxytocin (Pitocin), or by surgical delivery .