Normal Labor and Delivery The Obstetrics and Gynecology Hospital of Fudan University Jing-Xin Ding
Definition • Labor is the period from the onset of regular uterine contractions until expulsion of the fetus and the placenta, and it is defined as that occurring after 28 completed weeks of gestation.
Preterm delivery occurring after 28 weeks and before 37 completed weeks of gestation. In some developing countries, this time point has been advanced to 20 gestational weeks. • Term delivery occurring after 37 weeks and before 42 completed weeks of gestation. • Postterm delivery occurring after 42 completed weeks of gestation.
CHAPTER 1 THE HYPOTHESIS OF PARTURITION INITIATION 1. Mechanic theory UTERINE QUIESCENCE During the early stage of pregnancy, a remarkably period of myometrial quiescence is imposed. CERVICAL SOFTENINGBy the end of pregnancy, easily distensible, increase in tissue compliance
Uterine awakening or activation • During the end stage of pregnancy, the fetus compressed the lower segment and cervix, and mechanic effect induced the initiation of labor.
2. Endocrine theory • Some key endocrine proteins that control contractility. • These proteins include the oxytocin and its receptor, prostaglandin and its receptor, progesterone, and endothelin.
Fetal Contributions to Initiation of Parturition • The ability of the fetus to provide endocrine signals that initiate parturition has been demonstrated in several species. • This signal was shown to come from the fetal hypothalamic-pituitary-adrenal axis .
3. Neuromediator theory • The uterine contraction is controlled by the autonomic nerve. • It is still uncertain the role of autonomic nerve in the initiation of labor.
Mechanic theory Endocrine theory • Neuromediator theory Summary • Culmination of a series of biochemical changes in the uterus and cervix. • Many hypothesis: • Not fully defined.
CHAPTER 2 THE FACTORS DECIDING LABOR AND DELIVERY Mental and psychological factors Force of the labor Fetus Birth canal
I Force of the labor • Uterine Contractions — Main force • Maternal intra-abdominal pressure and the contranction of levatorani—Ancillary forces
Characteristics of the uterine contractions • Rhythmicity • Symmetry • Polarity • Retraction effect
1. Rhythmicity Each contraction increase progressively in intensity and maintains the maxium intensity and then diminishes gradually.
the uterine baseline tone -- from 8 to 12 mm Hg • 25 mm Hg at commencement of labor to 50 mm Hg at the end of first stage • During second-stage labor, aided by maternal pushing, contractions of 100 to 150 mm Hg are typical.
At the beginning, the contracts occurs every 5-6 minutes, and last 30 s. With the progression of labor, frequency increases to every 1-2 min and the duration increases to 60 s when the cervix is fully dilated.
2. Symmetry The normal contractile wave of labor originates near the uterine end of the fallopian tubes. Thus, these areas act as "pacemakers". Contractions spread from the pacemaker area throughout the uterus at 2 cm/sec, depolarizing the whole uterus within 15 seconds.
3. Polarity • Intensity is greatest in the fundus • Diminishes in the lower uterus. • Presumably, this descending gradient of pressure serves to direct fetal descent toward the cervix.
4. Retraction effect • The muscle fiber retracts after contractions, and the cavity of the uterus becomes small, and the fetus is forced to descend.
Maternal intra-abdominal pressure -- pushing • Contraction of the abdominal muscles simultaneously with forced respiratory efforts with the glottis closed is referred to as pushing. • After the cervix is dilated fully, the most important force. • Accomplishes little in the first stage. It exhausts the mother, and its associated increased intrauterine pressures may be harmful to the fetus.
The contraction of levator ani contributes to: • the internal rotation, extention and expulsion of the fetal head in the 2nd stage of labor • the delivery of placenta in the 3rd stage of labor.
II Birth canal • Bony Pelvis • The soft birthing canal
Bony Pelvis The false pelvis above and the true pelvis below the lineaterminalis. The true pelvis is formed by the sacrum and coccyx posteriorly and by the ischium and pubis laterally and anteriorly.
Pelvic Planes 1.The pelvic inlet plane 2.The mid plane of pelvis--the plane of least diameter 3.The pelvic outlet plane
The pelvic inlet plane • bordered by the pubic crest anteriorly, the iliopectineal line of the innominate bones laterally, and the promontory of the sacrum posteriorly.
Four diameters: anteroposterior, transverse, and two oblique diameters. • The obstetric conjugate of the inlet -- distance between the promontory of the sacrum and the symphysis pubis. -- 11 cm.
The transverse diameter is constructed at right angles to the obstetrical conjugate and represents the greatest distance between the linea terminalis on either side. • 2 oblique diameters extends from one of the sacroiliac synchondroses to the iliopectineal eminence on the opposite side.
The mid plane of pelvis--the plane of least diameter • The most important -- most arrest of descent occur at this level. It is bordered by the lower edge of the pubis anteriorly, the ischial spines and sacrospinous ligaments laterally, and the lower sacrum posteriorly.
The interspinous diameter, 10 cm or slightly greater-- the smallest pelvic diameter.
The plane of the pelvic outlet • 2 approximately triangular areas with a common base • The apex of the posterior triangle is at the tip of the sacrum, and the lateral boundaries are the sacrosciatic ligaments and the ischial tuberosities. • The anterior triangle is formed by the area under the pubic arch.
Important Diameters: • The transverse (bituberous) diameter extends between the inner surfaces of the ischialtuberosities—an average of 9 cm • The posterior sagittal diameter — from the middle of the transverse diameter to the sacrococcygeal joint —an average of 8.5 cm • If bituberous diameter < 9 cm , but the bituberous diameter + the posterior sagittal diameter >15 cm — delivered through the posterior triangle.
Pelvic axis — an imaginary curved line that passes through the centers of the various diameters of the pelvis. The pelvic axis first goes inferior and posterior, and then inferior, and then inferior and anterior.
Inclination of pelvis — The angle which the plane of the pelvic inlet makes with the horizontal plane when the patient is standing. The degree is usually 60 °, if it is too much, the engagement and delivery is difficult.
The soft birthing canal • the lower uterine segments • the cervix • the vagina • the pelvic floor
Formation of the Lower Uterine Segments — derived from the isthmus which is about 1 cm in nonpregnant uterus, and when the labor is started, with regular contractions of the upper uterine segment, it distended to 7 to 10cm.
the Physiological Retraction Ring • As a result of the lower segment thinning and concomitant upper segment thickening, a boundary between the two is marked by a ridge on the inner uterine surface—the physiological retraction ring.
Cervical Changes • two fundamental changes—effacement and dilatation • For an average-sized fetal head to pass through the cervix, its canal must dilate to a diameter of approximately 10 cm.
Effacement of cervix • Cervical effacement is "obliteration" or "taking up" of the cervix. • A length of about 2-3 cm canal -- a circular orifice with almost paper-thin edges.
Dilatation of cervix • Forebag of amnionic fluid is the leading portion of the amnionic sac and fluid located in front of the presenting part. • As uterine contractions cause pressure on the membranes, the hydrostatic action of the amnionic sac in turn dilates the cervical canal.
A. Before labor, the primigravid cervix is long and undilated in contrast to that of the multipara, which has dilatation of the internal and external os. B. As effacement begins, the multiparous cervix shows dilatation and funneling of the internal os. This is less apparent in the primigravid cervix. C. As complete effacement is achieved in the primigravid cervix, dilation is minimal. The reverse is true in the multipara.
Pelvic Floor Changes during Labor • Stretching of levatorani muscle fibers. • Thinning of the central portion of the perineum, which becomes transformed from a wedge-shaped, 5-cm-thick mass of tissue to a thin, almost transparent membranous structure less than 1 cm thick.
III Fetus • Size of fetus • Fetal lie, presentation and position • Fetal abnormalities
FETAL HEADImportant sutures and fontanelles • two frontal, two parietal, and two temporal bones, along with the occipital bone.
Sutures The membrane-occupied spaces between the cranial bones are known as sutures.
Fontanelles The membrane-filled spaces located at the point where the sutures intersect are known as fontanelles. • The anterior fontanelle (bregma) is at the intersection of the sagittal, frontal, and coronal sutures. It is diamond shaped and measures approximately 2×3cm, and it is much larger than the posterior fontanelle. • The posterior fontanelleis Y- or T-shaped and is found at the junction of the sagittal and lambdoid sutures.
Diameters • OccipitofrontalDiameter (11.3cm), extends from the external occipital protuberance to the glabella. The fetus usually engage by this diameter. • SuboccipitobregmaticDiameter (9.5cm),-- extends from the undersurface of the occipital bone at the junction with the neck to the center of the anterior fontanelle. -- the presenting anteroposterior diameter when the head is well flexed, and it is the shortest anteroposterior diameter .
OccipitomentalDiameter (13.3cm), the presenting anteroposterior diameter in a brow presentation and the longest anteroposterior diameter of the head; it extends from the vertex to the chin.
BiparietalDiameter (9.3cm), the largest transverse diameter; it extends between the parietal bones. • This diameter detected by antenatal ultrasonic examination was used to estimate the size of the fetus.