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1. MECHANISM OF NORMAL LABOUR BY: DR. SANA ALAM MODERATOR: DR. SUBHI

2. MECHANISM OF NORMAL LABOR DEFINITION:The series of movements that occur on the head in the process of adaptation during its journey through the pelvis is called mechanism of labor . The cardinal movements of labor are : E ngagement, D escent, F lexion, I nternal rotation, E xtension, E xternal rotation, E xpulsion . During labor, these movements not only are sequential but also show great temporal overlap. For example, as part of engagement, there is both flexion and descent of the head.

3. CARDINAL MOVEMENTS OF LABOR

4. ENGAGEMENT: The mechanism by which the biparietal diameter —the greatest transverse diameter in an occiput presentation—passes through the pelvic inlet is designated engagemen t. The fetal head may engage during the last few weeks of pregnancy or not until after labor commencement. In many multiparous and some nulliparous women, the fetal head is freely movable above the pelvic inlet at labor onset. In this circumstance, the head is sometimes referred to as “floating.” A normal-sized head usually does not engage with its sagittal suture directed anteroposteriorly. Instead, the fetal head usually enters the pelvic inlet either transversely or obliquely.

5. A synclitism : Head brim relation prior to the engagement , due to lateral inclination of the head, the sagittal suture does not strictly correspond with the available transverse diameter of the inlet. Instead, it is either deflected anteriorly toward the symphysis pubis or posteriorly toward the sacral promontory . Such deflection of the head in relation to the pelvis is called asynclitism. When the sagittal suture lies anteriorly, the posterior parietal bone becomes the leading presenting part and is called posterior asynclitism or posterior parietal presentation. This is more frequently found in primigravidae because of good uterine tone and a tight abdominal wall. In others, the sagittal suture lies more posteriorly with the result that the anterior parietal bone becomes the leading presenting part and is then called anterior parietal presentation or anterior asynclitism. It is more commonly found in multiparae. Mild degrees of asynclitism are common but severe degrees indicate cephalopelvic disproportion

6. Advantages of Asynclitism : 1. Engagement of head with asynclitism, the two parietal eminences cross the brim one at a time. This helps lesser diameter (super subparietal: 8.5 cm), to cross the pelvic brim instead of larger biparietal diameter (9.5 cm) for engagement in synclitism. 2. Asynclitism is benefcial in the mechanism of engagement of head. 3. Marked and persistent asynclitism is abnormal and indicates cephalopelvic disproportion.

7. SYNCLITISM

8. DESCENT: This movement is the first requisite for birth of the newborn. . Descent is brought about by one or more of four forces: (1) pressure of the amnionic fluid, (2) direct pressure of the fundus upon the breech with contractions, (3) bearing-down efforts of maternal abdominal muscles, and (4) extension and straightening of the fetal body. In nulliparas, engagement may take place before the onset of labor, and further descent may not follow until the onset of the second stage. In multiparas, descent usually begins with engagement.

9. FLEXION: As soon as the descending head meets resistance, whether from the cervix, pelvic walls, or pelvic floor, it normally flexes. With this movement, the chin is brought into more intimate contact with the fetal thorax, and the appreciably shorter suboccipitobregmatic diameter is substituted for the longer occipitofrontal diameter.

10. FLEXION OF HEAD

11. INTERNAL ROTATION: This movement consists of a turning of the head in such a manner that the occiput gradually moves toward the symphysis pubis anteriorly from its original position or, less commonly, posteriorly toward the hollow of the sacrum . The theories which explain the anterior rotation of the occiput are: 1. Slope of pelvic foor : Two halves of levator ani form a gutter and viewed from above, the direction of the fibers is backward and toward the midline. Thus, during each contraction, the occiput, in well-fexed position, stretches the levator ani. After the contraction passes off, elastic recoil of the levator ani occurs bringing the occiput forward toward the midline. The process is repeated until the occiput is placed anteriorly. This is called rotation by law of pelvic foor ( Hart’s rule ). 2. Pelvic shape : Forward inclination of the side walls of the cavity, narrow bispinous diameter and long anteroposterior diameter of the outlet result in putting the long axis of the head to accommodate in the maximum available diameter, i.e. anteroposterior diameter of the outlet leaving behind the smallest bispinous diameter. 3. Law of unequal fexibility (Sellheim and Moir) : The internal rotation is primarily due to inequalities in the fexibility of the component parts of the fetus

12. CROWNING: After internal rotation of the head, further descent occurs until the subocciput lies underneath the pubic arch. At this stage, the maximum diameter of the head ( biparietal diameter ) stretches the vulval outlet without any recession of the head even after the contraction is over—called “crowning of the head” EXTENSION: After internal rotation, the sharply flexed head reaches the vulva and undergoes extension. This brings the base of the occiput into direct contact with the inferior margin of the symphysis pubis

13. Torsion of the neck: Torsion of the neck is an inevitable phenomenon during internal rotation of the head. If the shoulders remain in the antero posterior diameter, the neck has to sustain a torsion of two-eighths of a circle corresponding with the same degree of anterior rotation of the occiput. But the neck fails to withstand such major degree of torsion and as such there will be some amount of simultaneous rotation of the shoulders in the same direction to the extent of one-eighth of a circle placing the shoulders to lie in the oblique diameter with one-eighth of torsion still left behind. Thus, the shoulders move to occupy the left oblique diameter in left occipitolateral position and right oblique diameter in right occipitolateral position. In oblique occipitoanterior position, there is no movement of the shoulders from the o blique diameter as the neck sustains a torsion of only one-eighth of a circle.

14. MECHANISM OF LABOUR IN LOT

15. RESTITUTION : The delivered head next undergoes restitution . If the occiput was originally directed toward the left, it rotates toward the left ischial tuberosity . If it was originally directed toward the right, the occiput rotates to the right. EXTERNAL ROTATION : Restitution of the head to the oblique position is followed by external rotation completion to the transverse position . This movement corresponds to rotation of the fetal body and serves to bring its bisacromial diameter into relation with the anteroposterior diameter of the pelvic outlet. Thus, one shoulder is anterior behind the symphysis and the other is posterior. EXPULSION : Almost immediately after external rotation, the anterior shoulder appears under the symphysis pubis, and the perineum soon becomes distended by the posterior shoulder. After delivery of the shoulders, the rest of the body quickly passes

16. CARDINAL MOVEMENTS OF LABOR IN LOA

17. Fetal Head Shape Changes Caput Succedaneum : In prolonged labors before complete cervical dilatation, the portion of the fetal scalp immediately over the cervical os becomes edematous. This swelling, known as the caput succedaneum, i t usually attains a thickness of only a few millimeters,but in prolonged labors it may be sufficiently extensive to prevent differentiation of the various sutures and fontanels. More commonly, the caput is formed when the head is in the lower portion of the birth canal and frequently only after the resistance of a rigid vaginal outlet is encountered.

18. Molding : T he bony fetal head shape is also altered by external compressive forces and is referred to as molding . Most studies indicate that there is seldom overlapping of the parietal bones. A “locking” mechanism at the coronal and lambdoidal connections actually prevents such overlapping . Molding results in a shortened suboccipitobregmatic diameter and a lengthened mentovertical diameter. These changes are of greatest importance in women with contracted pelves or asynclitic presentations. In these circumstances, the degree to which the head is capable of molding may make the difference between spontaneous vaginal delivery and an operative delivery. Most cases of molding resolve within the week following delivery .

19. MOLDING, CAPUT SUCCEDANEUM, CEPHALHEMATOMA

20. ANATOMY OF LABOR : As labor advances, the body of uterus , cervix and vagina together form a uniformly curved canal called the birth canal. As the head descends down with progressive dilatation of the vagina, it displaces the anterior structures upward and forward, and the posterior structures downward and backwards. The bladder which remains a pelvic organ throughout the first stage becomes an abdominal organ in the second stage of labor. However, there is no stretching of the urethra as was previously thought. Rather, the urethra is pushed anteriorly with the neck of the bladder still lying in the vulnerable position behind the symphysis pubis.

21. The changes in the posterior structures due to downward and backward displacement are marked when the head is sufficiently low down and in the stage of “crowning”. The perineum which is a triangular area of about 4 cm thickness becomes a thinned out, membranous structure of less than 1 cm thickness. The anus , from being a closed opening, becomes dilated to the extent of 2–3 cm. The anococcygeal raphe is also thinned and stretched

22. ANATOMY OF LABOR

23. STAGES OF LABOR

24. First Stage of Labor : SHOW : expulsion of blood stained mucus (show) per vaginam. Only few drops of blood mixed with mucus is expelled and any excess should be considered abnormal. PAIN : with simultaneous hardening of the uterus. Initially at varying intervals of 15–30 minutes with duration of about 30 seconds. But in late first stage the contraction comes at intervals of 3–5 minutes and lasts for about 45 seconds. Clinically pains are said to be good if they come at intervals of 3–5 minutes and at the height of contraction the uterine wall cannot be indented by the fingers.

25. DILATATION AND EFFACEMENT OF THE CERVIX : Cervical dilatation relates with dilatation of the external os and effacement is determined by the length of the cervical canal in the vagina. The anterior lip of the cervix is the last to be effaced. The first stage is said to be completed only when the cervix is completely retracted over the presenting part during contractions

26. Latent labour: The onset of latent labor is the point at which the mother perceives regular contractions. The latent phase for most women ends once dilatation of 3 to 5 cm is achieved. This threshold may be clinically useful, for it defines dilatation limits beyond which active labor can be expected . Normal duration of latent phase of labor in a primigravida is about 20 hours (average 8.6 hours , cervical dilatation averaging only 0.35 cm/h ) and 14 hours (average 5.3 hours) in a multipara.

27. Active labour: The active phase has got three components. (i) Acceleration phase with cervical dilatation of 3–4 cm. (ii) Phase of maximum slope of 4–9 cm dilatation. (iii) Phase of deceleration of 9–10 cm dilatation. Dilatation of the cervix at the rate of 1 cm/h in primigravidae and 1.5 cm in multigravidae beyond 4 cm dilatation (active phase of labor) is considered satisfactory

28. STATUS OF THE MEMBRANES : remain intact until full dilatation of the cervix / even beyond in the second stage. Early rupture - after the onset of labor but before full dilatation of cervix Premature rupture- before the onset of labor M ATERNAL SYSTEM : transient fatigue appears following a strong contraction. Pulse rate increased by 10–15 beats per minute during contraction, which settles down in between contractions. Systolic blood pressure is raised by about 10 mm Hg during contraction. Temperature remains u nchanged FETAL EFFECT : As long as the membranes are intact, there is hardly any adverse effect on the fetus . S lowing of fetal heart rate by 10–20 beats per minute ; returns to its normal rate of about 140 per minute as the intensity of contraction diminishes provided the fetus is not compromised .

29. During the preparatory division (LATENT PHASE): Sedation and conduction analgesia are capable of arresting this labor division. The dilatational division (ACTIVE PHASE): is unaffected by sedation. Last, the pelvic division commences with the deceleration phase of cervical dilatation.

30. Second Stage of Labor : This stage begins with complete cervical dilatation and ends with fetal delivery . P AIN :The intensity of the pain increases. The pain comes at intervals of 2–3 minutes and lasts for about 1–1½ minutes . It becomes successive with increasing intensity in the second stage. BEARING-DOWN EFFORTS : It is the additional voluntary expulsive efforts that appear during the second stage of labor (expulsive phase) - initiated by nerve reflex ( Ferguson Reflex) set up due to stretching of the vagina by the presenting part. In majority, this expulsive effort start spontaneously with full dilatation of the cervix. Along with uterine contraction, the woman is instructed to exert downward pressure as done during straining at stool. Sustained pushing beyond the uterine contraction is discouraged.

31. DESCENT OF THE FETUS : Features of descent of the fetus are evident from abdominal and vaginal examinations. Abdominal findings are—progressive descent of the head, assessed in relation to the brim , rotation of the anterior shoulder to the midline and change in position of the fetal heart rate—shifted downward and medially. Abdominal assessment of progressive descent of the head (using fifth formula ) : Progressive descent of the head can be usefully assessed abdominally The amount of head felt suprapubically in finger breadth is assessed by placing the radial margin of the index finger above the symphysis pubis successively until the groove of the neck is reached.

32. Internal examination reveals descent of the head in relation to ischial spines and gradual rotation of the head evidenced by position of the sagittal suture , and the occiput in relation to the quadrants of the pelvis. Advantages over “station of the head” in relation to ischial spines : 1. It excludes the variability due to caput and molding or by a diferent depth of the pelvis. 2. T h e assessment is quantitative and can be easily reproduced. 3. Repeated vaginal examinations are avoided.

33. Clinical pelvimetry: This is commonly done. Time :any time beyond 37th week but better at the beginning of labor. Procedures: The patient has to empty the bladder. The pelvic examination is done with the patient in dorsal position taking aseptic preparations. The following features are to be noted simultaneously: (1) State of the cervix; (2) To note the station of the presenting part in relation to ischial spines; (3) To t est for cephalopelvic disproportion in nonengaged head (described later); (4) To note the resilience and elasticity of the perineal muscles. Steps: The internal examination should be gentle, thorough, methodical and purposeful. It should be emphasized that the sterilized gloved fingers once taken out should not be reintroduced.

34. Sacrococcygeal joint — Its mobility and presence of hooked coccyx, if any, are noted. Pubic arch — Normally, the pubic arch is rounded and should accommodate the palmar aspect of two fingers. Configuration of the arch is more important than pubic angle. Diagonal conjugate — Procedure is described before. After the procedure, the fingers are now taken out. Subpubic angle: The inferior pubic rami are defined and in female, the angle roughly corresponds to the fully abducted thumb and index fingers. In narrow angle, it roughly corresponds to the fully abducted middle and index fingers . Transverse diameter of the outlet (TDO) — It is measured by placing the knuckles of the first interphalangeal joints or knuckles of the clinched fist between the two ischial tuberosities . Normally, it accomodates four knuckles. Anteroposterior diameter of the outlet—The distance between the inferior margin of the symphysis pubis and the skin over the sacrococ

35. Sacrum : The sacrum may be smooth, short and well curved, and the sacral promontory usually cannot be reached or the sacrum may be long or straight Sacrosciatic notch— The notch is sufficiently wide so that two fingers can be easily placed over the sacrospinous ligament covering the notch. The configuration of the notch denotes the capacity of the posterior segment of the pelvis and the sidewalls of the lower pelvis. Ischial spines — Spines are usually smooth (everted) and difficult to palpate. They may be prominent and encroach to the cavity thereby diminishing the available space in the midpelvis. Iliopectineal lines — To note for any beaking suggestive of narrow fore pelvis (android feature). Sidewalls — Normally they are parallel or divergent. They may be convergent. Posterior surface of the symphysis pubis — It normally forms a smooth rounded curve. Presence of angulation or beaking suggests abnormality.

36. THIRD STAGE OF LABOR: I ncludes separation, descent and expulsion of the placenta with its membranes. PAIN : For a short time, the patient experiences no pain intermittent discomfort in the lower abdomen reappears, corresponding with the uterine contractions. BEFORE SEPARATION : Per abdomen—Uterus : discoid in shape, firm and non - ballottable. Fundal height - s lightly below the umbilicus. Per vaginam: trickling of blood. Length of the umbilical cord - remains static.

37. AFTER SEPARATION: Per abdomen : 1. Uterus - globular, f i rm, and ballottable. 2. fundal height - slightly raised as the separated placenta comes down in the lower segment and the contracted uterus rests on top of it. 3. Slight bulging in the suprapubic region Per vaginum: 4. Slight gush of vaginal bleeding. 5. Permanent lengthening of the cord is established.

38. EXPULSION OF PLACENTA AND MEMBRANES : voluntary bearing down efforts / more commonly aided by manipulative procedure. followed by slight to moderate bleeding amounting to 100–250 mL. MATERNAL SIGNS : There may be chills and occasional shivering. Slight transient hypotension is not unusual