At a glance......
- 1 Onset of Labor
- 2 Stages of Labor
- 3 Stages of Labor
- 4 Adjustments of the Infant at Birth
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Birth is the act or process of bearing or bringing forth offspring, also referred to in technical contexts as parturition. In mammals, the process is initiated by hormones which cause the muscular walls of the uterus to contract, expelling the fetus at a developmental stage when it is ready to feed and breathe.
In some species the offspring is precocial and can move around almost immediately after birth but in others it is altricial and completely dependent on parenting.
In marsupials, the fetus is born at a very immature stage after a short gestational period and develops further in its mother’s womb’s pouch.
It is not only mammals that give birth. Some reptiles, amphibians, fish and invertebrates carry their developing young inside them. Some of these are ovoviviparous, with the eggs being hatched inside the mother’s body, and others are viviparous, with the embryo developing inside her body, as in the case of mammals.
Onset of Labor
Prodromal labor, which includes the latent phase of labor, marks the initial stages of parturition.
Prodromal labor, the body’s preparation for real labor, may include an increase in blood volume, Braxton Hicks contractions, the presence of colostrum in the breasts, and the dislodging of the mucus plug.
In contrast to Braxton Hicks contractions, the latent phase is marked by the point at which the woman perceives regular uterine contractions; cervical effacement will reach completion at the end of this stage.
The latent phase ends with the onset of active first stage, which is marked by advanced dilation.
The duration of labor varies widely, but its active phase averages some 20 hours for women giving birth to their first child (primiparae) and 8 hours for women who have already given birth (multiparae).
In a caesarean section, the removal of the neonate is through a surgical incision in the abdomen, rather than through vaginal birth.
- caesarean section: A caesarean section, also known as C-section, is a surgical procedure in which one or more incisions are made through a mother’s abdomen (laparotomy) and uterus (hysterotomy) to deliver one or more babies, or, rarely, to remove a dead fetus.
- Braxton Hicks contractions: Known as false labor or practice contractions. They are sporadic uterine contractions that sometimes start around six weeks but are not usually felt until the second or third trimester of pregnancy.
- cervical effacement: The thinning and stretching of the cervix.
- parturition: The act or process of bearing or bringing forth offspring from the uterus; giving birth.
Pre-labor (First Stage of Labor)
Pre-labor, also called prodromal labor, consists of the early contractions and labor signs before actual labor starts. It is the body’s preparation for real labor.
Prodromal labor, often misnamed false labor, begins much as traditional labor but does not progress to the birth of the baby. Not everyone feels this stage of labor although it does always occur. However, this does not mean that every woman will experience every symptom.
The term is used to describe a cluster of physical changes that may take place in a pregnant woman before she goes into actual labor. These changes can include:
- An increase in blood volume (sometimes resulting in edema).
- Braxton Hicks contractions.
- The presence of colostrum in the breasts.
- The dislodging of the mucous plug that sealed the cervix during the pregnancy.
The term false labor is sometimes used to describe a cluster of Braxton Hicks contractions that are mistaken for real labor. The terms false labor and false pains are sometimes considered equivalent.
The latent phase is generally defined as beginning at the point at which the woman perceives regular uterine contractions. In contrast, Braxton Hicks contractions should be infrequent, irregular, and involve only mild cramping.
Cervical effacement (the thinning and stretching of the cervix and cervical dilation) occurs during the closing weeks of pregnancy and is usually complete, or near complete, by the end of the latent phase.
When the contractions of labor begin, the walls of the uterus start to contract through stimulation by the release of the pituitary hormone oxytocin. The contractions cause the cervix to widen and begin to open.
As labor progresses the amniotic sac can rupture and cause a slow or a fast gush of fluids. Labor usually begins within a 24-hour period after the amniotic sac ruptures. As contractions become closer and stronger the cervix will gradually start to dilate.
The degree of cervical effacement may be felt during a vaginal examination. A long cervix implies that effacement has not yet occurred. At this stage, the cervix may dilate from 1 to 4 cm. The latent phase ends with the onset of the active phase, which is marked by an accelerated cervical dilation.
Stages of Labor
Vaginal delivery childbirth has three distinct phases: dilation of the cervix, delivery of the infant, and delivery of the placenta.
The first stage of labor begins when the effaced (thinned) cervix is 3 cm dilated. During effacement, the cervix is pulled upward during contractions. Full dilation occurs when the cervix is wide enough to allow passage of the baby’s head, about 10 cm for a full-term infant.
The duration of labor varies widely, but the active phase averages some 20 hours for women giving birth to their first child (primiparae) and 8 hours for women who have previously given birth (multiparae).
The second stage begins when the cervix is fully dilated, ending when the baby is born. Fetal movement through the birth canal is assisted by the additional maternal efforts of bearing down or pushing. The fetal head is seen to crown as the labia part, and the baby is born.
The period from just after the fetus is expelled until just after the placenta is expelled is called the third stage of labor, which lasts, on average, 10–12 minutes.
A fourth stage of labor may refer to the hour immediately after delivery of the placenta, or may refer to the weeks following delivery. It is the time in which the mother’s body returns to a nonpregnant state.
- multiparae: Women who have given birth to more than one viable fetus.
- Ferguson reflex: An example of positive feedback and the female body’s response to pressure application in the cervix or vaginal walls. Upon application of pressure, oxytocin is released and uterine contractions are stimulated (which will in turn increase oxytocin production, and hence, increase contractions even more), until the baby is delivered.
- primiparae: Women who have given birth to only one child, or who are giving birth for the first time.
The infant’s head and shoulders must go through a specific sequence of maneuvers in order to pass through the ring of the mother’s pelvis. The six phases of a typical vertex (head-first presentation) delivery are:
- Engagement of the fetal head in the transverse position.
- Descent and flexion of the fetal head.
- Internal rotation of the fetal head so that the baby’s face is towards the mother’s rectum.
- Delivery by extension (the fetal head passes out of the birth canal).
- Restitution: The fetal head turns through 45 degrees to restore its normal relationship with the shoulders, which are still at an angle.
- External rotation: The shoulders repeat the corkscrew movements of the head, which can be seen in the final movements of the fetal head.
The Bishop score defines several factors that midwives and physicians use to assess the laboring mother’s progress.
The score is used to predict whether the mother is likely to spontaneously progress into the second stage of labor and whether induction of labor will be required. It has also been used to assess the odds of spontaneous preterm delivery.
Stages of Labor
First Stage (Active Phase)
The first stage of labor classically starts when the effaced (thinned) cervix is 3 cm dilated, although there is variation as some women may or may not have active contractions prior to reaching this point. The onset of actual labor is defined when the cervix begins to progressively dilate. Rupture of the membranes or a bloody discharge may or may not occur at or around this stage.
Tterine muscles form opposing spirals from the top of the upper segment of the uterus to its junction with the lower segment. During effacement, the cervix becomes incorporated into the lower segment of the uterus. During a contraction, these muscles shorten the upper segment, drawing upwards the lower segment in a gradual expulsive motion.
The presenting fetal part is then permitted to descend. Full dilation is reached when the cervix has widened enough to allow passage of the baby’s head, around 10 cm dilation for a term baby. The duration of labor varies widely, but the active phase averages some 20 hours for women giving birth to their first child (primiparae), and 8 hours for women who have already given birth (multiparae).
Active phase arrest is defined, in a primigravid woman, as the failure of the cervix to dilate at a rate of 1.2 cm/hr over a period of at least two hours. This definition is based on Friedman’s Curve, which plots the typical rate of cervical dilation and fetal descent during active labor. Some practitioners may diagnose failure to progress, and consequently, perform a caesarean.
Second Stage (Fetal Expulsion)
The second stage begins when the cervix is fully dilated and ends when the baby is born. As pressure on the cervix increases, the Ferguson reflex increases uterine contractions.
At the beginning of the normal second stage, the head is fully engaged in the pelvis: the widest diameter of the head has passed below the level of the pelvic inlet. The fetal head then continues descending into the pelvis, below the pubic arch, and out through the vagina.
This is assisted by the additional maternal efforts of bearing down or pushing. The fetal head is seen to crown as the labia part. At this point, the woman may feel a burning or stinging sensation. The complete expulsion of the baby signals the successful completion of the second stage of labor.
The second stage of birth will vary by factors including parity, fetal size, anesthesia, or the presence of infection. Longer labors are associated with declining rates of spontaneous vaginal delivery and increasing rates of infection, perineal laceration, obstetric hemorrhage, as well as need for intensive care of the neonate.
Third Stage (Placental Delivery)
The third stage of labor is the period from just after the fetus is expelled until just after the placenta is expelled. The average time from delivery of the baby until complete expulsion of the placenta is estimated to be 10–12 minutes.
Placental expulsion can be managed actively, by giving a uterotonic, such as oxytocin, along with appropriate cord traction and fundal massage to assist in delivering the placenta by a skilled birth attendant. Alternatively, it can be managed expectantly, allowing the placenta to be expelled without medical assistance. The umbilical cord is routinely clamped and cut in this stage, but it would normally close naturally even if not clamped and cut.
When the amniotic sac has not ruptured during labour or pushing, the infant can be born with the membranes intact. This is referred to as being born in the caul. The caul is harmless and its membranes are easily broken and wiped away. With the advent of modern interventive obstetrics, the artificial rupture of the membranes has become common, so babies are rarely born in the caul (en-caul birth).
Fourth Stage (Postpartum Period)
The fourth stage of labor is a term used in two different senses:
- It can refer to the immediate puerperium, or the hours immediately after delivery of the placenta.
- It can be used in a more metaphorical sense to describe the weeks following delivery.
Adjustments of the Infant at Birth
Post birth, an infant’s physiology must adapt to breathing independently, changes in blood flow and energy access, and a cold environment.
Following birth, the newborn’s lungs inflate and respiration begins to occur through the pulmonary system. The subsequent changes to oxygenated blood flow that occur follow the adult pattern of circulation.
The energy metabolism of the infant is also quickly converted from a continuous placental supply to intermittent feeding. Glycogen stores created during the second and third trimesters of pregnancy are relied upon for energy during the first few days following birth.
Physiological changes occur via thermoregulation to maintain core temperature including: vasoconstriction (a decrease of blood flow to skin), maintaining the fetal position (a decrease of the surface area of exposure), jittery large muscle activity (to generate muscular heat), and non-shivering thermogenesis.
- respiratory distress syndrome: Infant respiratory distress syndrome (IRDS), also called neonatal respiratory distress syndrome or respiratory distress syndrome (previously called hyaline membrane disease), is a syndrome in premature infants caused by the developmental insufficiency of surfactant production and structural immaturity in the lungs. It can also result from a genetic problem with the production of surfactant-associated proteins.
- surfactant: A lipoprotein in the tissues of the lung that reduces surface tension and permits more efficient gas transport.
- non-shivering thermogenesis: The process of heat production in organisms. Non-shivering thermogenesis usually occurs in brown adipose tissue (brown fat) that is present in human infants and hibernating mammals.
The first challenge of a newborn is to perfuse its body by breathing independently instead of utilizing placental oxygen. At birth, the baby’s lungs are filled with fetal lung fluid (which is not amniotic fluid) and are not inflated.
As the newborn is expelled from the birth canal, its central nervous system reacts to the sudden change in temperature and environment. This triggers it to take the first breath within about 10 seconds of delivery.
With the first breaths, there is a fall in pulmonary vascular resistance and an increase in the surface area available for gas exchange. Over the next 30 seconds, the pulmonary blood flow increases and is oxygenated as it flows through the alveoli of the lungs. Oxygenated blood now reaches the left atrium and ventricle and, through the descending aorta, reaches the umbilical arteries.
Respiration and Circulation
Oxygenated blood now stimulates constriction of the umbilical arteries resulting in a reduction in placental blood flow. As the pulmonary circulation increases, there is an equivalent reduction in the placental blood flow that normally ceases completely after about three minutes. These two changes result in a rapid redirection of blood flow into the pulmonary vascular bed, from approximately 4% to 100% of cardiac output.
The increase in pulmonary venous return results in left atrial pressure being slightly higher than right atrial pressure, which closes the foramen ovale. This change in the pattern of flow results in a drop in blood flow across the ductus arteriosus.
The higher blood oxygen content of blood within the aorta stimulates the constriction and ultimately the closure of this fetal circulatory shunt. All of these cardiovascular system changes result in the adaptation from fetal circulation patterns to an adult circulation pattern.
During this transition, some types of congenital heart diseases that were not symptomatic in utero during fetal circulation will present with cyanosis or respiratory signs.
Following birth, the expression and re-uptake of surfactant, which begins production at 20 weeks gestation, is accelerated. Expression of surfactant into the alveoli is necessary to prevent alveolar closure.
At this point, rhythmic breathing movements also commence. If there are any problems with breathing, management can include stimulation, bag and mask ventilation, intubation, and ventilation. Cardiorespiratory monitoring is essential to track potential problems.
Pharmacological therapy, such as caffeine, can also be given to treat apnea in premature newborns. A positive airway pressure should be maintained and neonatal sepsis must be ruled out. Potential neonatal respiratory problems include apnea, transient tachypnea of the newborn (TTNB), respiratory distress syndrome (RDS), meconium aspiration syndrome (MAS), airway obstruction, and pneumonia.
The energy metabolism of the fetus must be converted from a continuous placental supply of glucose to intermittent feeding. While the fetus is dependent on maternal glucose as the main source of energy, it can use lactate, free fatty acids, and ketone bodies under some conditions. Plasma glucose is maintained by glycogenolysis.
Glycogen synthesis in the liver and muscle begins in the late second trimester of pregnancy, and storage is completed in the third trimester. Glycogen stores are maximal at term, but even then, the fetus only has enough glycogen available to meet its energy needs for 8–10 hours, which can be depleted even more quickly if the demand is high.
Newborns will then rely on gluconeogenesis for energy. Fat stores are the largest storage source of energy. At 27 weeks gestation, only 1% of a fetus’ body weight is fat. At 40 weeks, that number increases to 16%.
Inadequate glucose substrate can lead to hypoglycemia, fetal growth restriction, preterm delivery, or other problems. Similarly, an excess substrate can also lead to problems, such as infant of a diabetic mother (IDM), hypothermia, or neonatal sepsis.
Anticipating potential problems is the key to managing most neonatal problems of energy metabolism. For example, early feeding in the delivery room or as soon as possible may prevent hypoglycemia. If the blood glucose is still low, then an intravenous (IV) bolus of glucose may be delivered, with continuous infusion if necessary. Rarely, steroids or glucagon may have to be employed.
Newborns come from a warm environment to the cold and fluctuating temperatures of this world. They are naked, wet, and have a large surface area to mass ratio, with variable amounts of insulation, limited metabolic reserves, and a decreased ability to shiver.
Physiologic mechanisms for preserving core temperature include vasoconstriction (a decrease of blood flow to the skin), maintaining the fetal position (a decrease of the surface area exposed to the environment), jittery large muscle activity (to generate muscular heat), and non-shivering thermogenesis.
Non-shivering thermogenesis occurs in brown fat. Brown fat is specialized adipose tissue with a high concentration of mitochondria designed to rapidly oxidize fatty acids in order to generate metabolic heat. The newborn’s capacity to maintain these mechanisms is limited, especially in premature infants.