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Mechanisms and Management of Labor

Mechanisms and Management of Labor

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Mechanisms and Management of Labor

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  1. Mechanisms and Management of Labor Nancy Goodwine Wozniak, MD

  2. Definition of Labor • Labor is the physiologic process by which the fetus is expelled from the uterus to the outside world • Could also be described as the transition from “contractures” to “contractions” • Bottom line defination: Contractions with cervical change. The diagnosis is a clinical one. Gabbe: Obstetrics Normal and Problem pregnancies 4th edition

  3. Full term pregnancy is 280 days (40 weeks) or 36 completed weeks. Post term pregnancy is beyond 42 weeks SROM is seen in about 8% of patients

  4. Labor Physiology Labor is contractions with cervical change The fetus is in control of the timing of labor The factors responsible for initiating labor are not well-defined…likely an autocrine and/or paracrine event. We do know there is some endocrine maternal/fetal cross talk (eg horses and donkeys indicate that fetal genotype is a factor—365 vs 340 days) Gabbe: Obstetrics Normal and Problem pregnancies 4th edition

  5. Labor Physiology No matter what seems to initiate labor it involves regular uterine contractions, mediated through ATP-dependent binding of myosin to actin. Unlike vascular smooth muscle, myometrium has sparse innervation, thus regulation of contractions is hormonal. There is thought to be a parturition cascade. Ultimately, human labor is a multifactorial physiologic process involving an integrated set of changes that occur gradually over days to weeks. Changes include prostaglandin synthesis and release within the uterus, an increase in myometrial gap junction formation, and up-regulation of myometrial oxytocin receptors. At some point labor begins with the activation of the fetal-hypothalamic-pituitary adrenal axis in a way likely common to all species. Gabbe: Obstetrics Normal and Problem pregnancies 4th edition

  6. Labor Physiology The regulation of uterine activity can be divided into 4 physiologic phases Phase 0: uterus is quiet due to progesterone, relaxin, prostacyclin I2 (PGI2), parathyroid hormone Phase 1: before term “activation” phase- uterus is more responsive to estrogen and more receptors for oxytocin and prostaglandins Phase 2: uterus more stimulated because of increase in gap junctions so that it can be stimulated by oxytocins and prostaglandins (PGE2 and PGF2 alpha) Phase 3: involution of the uterus (mediated by oxytocin) Gabbe: Obstetrics Normal and Problem pregnancies 4th edition

  7. Labor Mechanics For a successful vaginal delivery, the fetus must negotiate the maternal pelvis. Three factors: the power, the passage, and the passenger.

  8. Labor Mechanics • The passenger Estimating fetal size: ultrasound, leopolds, what does mom think? How big is too big? Definition of macrosomia is diabetics: 4500g non-diabetics: 5000g

  9. Labor Mechanics • Power Assessing amplitude, duration, and intensity of ctx internal IUPC vs external toco What’s adequate contractions? (ultimately it is a clinical dx) 3-5 ctx in 10 min 7 ctx in 15 min 250 MVU’s – the average strength of ctx in mm Hg multiplied by the number of contractions in 10 minutes. No real data support an absolute number of ctx or MVU’s to be adequate…adequacy is still a clinical determination. If ctx are adequate either the cervix will dilate or the caput will become worse.

  10. Labor Mechanics • The most precise way of determining uterine contractions are adequate is with internal monitoring by IUPC • External monitoring measures the change in shape of the abdominal wall relative to contractions thus is qualitative rather than quantitative. Does allow for accurate correlation between fetal heart rate and contraction pattern .

  11. Labor Mechanics The Passenger The passenger is the fetus. Fetal size can influence labor Can be assessed by Leopold’s, US or both. ( Mom’s opinion counts, too!) ACOG definition of Macrosomia is defined as >4500 g

  12. Labor Mechanics • The passenger Fetal lie: Fetal position relative to the maternal spine. longitudinal, oblique, transverse Presentation: refers to the fetal part that is above the pelvic inlet. (eg a fetus can have a logitudinal lie but be breech or cephalic) Attitude: refers to position of fetal head relative to the fetal spine Position: referes to the relationship of a nominated site of the presenting part to a denomintating location in the internal pelvis. Eg. Occiput/sacrum ROA, RSA Station: a measure of descent of the presenting part. Abnormalilty of any of these variables can influence whether or not to proceed with a vaginal delivery.

  13. Fetal presentation: Fetal part directly over the pelvic inlet; eg breech, cephalic, compound, funic

  14. Labor Mechanics • The passenger Malpresentation is any presentation that is not cephalic with occiput leading. (about 5%) Multifetal pregnancies increase the risk of malpresetnation

  15. The cephalic presentation can be classified by boney landmarks of the skull; eg occiput , mentum, brow passenger pasenger

  16. A: Right occiput anterior (ROA); B: Left occiput anterior (LOA); C: Occiput anterior (OA).* Posterior fontanel. This is the smaller of the two fontanels and is at the intersection of the three sutures: the sagittal suture and two lambdoid sutures.** Anterior fontanel. This large fontanel is at the intersection of four sutures: the sagittal, frontal, and two coronal sutures.From

  17. Occiput posterior From

  18. Occiput transverse From

  19. Labor Mechanics • The passenger Station: measure of descent of the presenting part through the birth canal relative to ischial spines this is the relationship between the leading bony part of fetal presenting part ( skull bone NOT scalp) and the maternal ischial spines. Must take into account molding and caput succedaneum (not doing so is a common error) Often described as -3 to + 3 Newer scale is -5 to +5

  20. Nucleus medical

  21. Labor Mechanics • The Passage The passage consists of the bony pelvis (sacrum, ilium, ischium, pubis) and the resistance provided by the soft tissues. Bony pelvis is divided into the greater (false) and lesser(true) pelvis by the pelvic brim which is demarcated by the sacral promontory. The diagonal conjugate is the distance from the sacral promontory to the inferior margin of the symphysis pubis as assessed on examination ( see next slide ) Clinical pelvimetry is the only way to assess the dimensions of the pelvis in labor.

  22. To figure out the true conjugate, measure the diagonal conjugate and subtract 1.5 – 2cm. The limiting factor is the interspinous diameter.

  23. Bony pelvis—most favorable is gynecoid and antropoid

  24. From

  25. A little bit about cervical ripening… • When induction is attempted against an unripe cervix the likelihood of succcess is reduced. Bishops score: dilatation, effacement, position, consistency, station. Total score is up to 13. Bishop’s = 8 chances of successful induction are the same as spontaneous labor Bishop’s = 6 “favorable cervix”

  26. (A) Cervix is uneffaced and minimally dilated. (B) Cervix is almost completely effaced and dilated. From

  27. Methods of cervical ripening Non Pharmacologic methods: membrane stripping – digital separation of chorionic and amniotic membranes from the cervix. Releases endogenous prostaglandins from the decidua and adjacent membranes. May also cause “Ferguson reflex” stimulating release of oxytocin from the pituitary. Foley bulb Amniotomy – needs favorable cervix, but if cervix is favorable amniotomy by itself can get labor started (better still when combined with Pitocin) risks: cord prolapse, prolonged ROM, fetal injury, rupture of vasa previa with fetal hemorrhage, fetal malposition and asynclitism Benefits: FSE placement, can determine if MSF or blood, high success in inducing labor

  28. Methods of cervical ripening • Pharmacologic methods Dinoprostone (Prepadil and Cervadil) PGE2 , oxytocin, misoprostyl (cytotec) PGE1 The uterus has precursors of the prostaglandin of the 2 series. PGE2 : important for cervical maturation PGF 2 alpha. : causes myometrial contractions

  29. Stages of Labor • First stage: Onset of labor to full dilatation latent phase- onset of labor until cervix starts to make change. active phase-greater rate of cervical change 1.2 cm/h for nulliparous 1.5 cm/h for multiparous Second stage: full dilation to delivery Length of Pushing: nullip: 2h without epidural, 3 h with epidural multip: 1 h without epidural, 2 h with epidural Third stage: delivery of placenta-can take up to 30 minutes

  30. Cardinal movements of labor Engagement: passage of the widest diameter of the presenting part to a level below the plane of the pelvic inlet. In cephalic fetus, the largest diameter is the biparietal diameter (9.5 cm); in a breech fetus the widest diameter is the bitrochanteric diameter. The presenting part is engaged if you can feel presenting part both abdominally and vaginally. Descent: downward passage of the presenting part Flexion: occurs passively d/t boney maternal pelvis Internal rotation: refers to rotation of presenting part from its original position (usually transverse) to the AP position Extension: Occurs once the fetus has descended to the introitus External rotation (aka restitution) return of the fetal head to the correct anatomic position in relation to the fetal torso. Expulsion: delivery of the rest of the fetus.

  31. Management of Normal Labor and Delivery • All women need adequate surveillance throughout labor and delivery. • Okay to let women walk (doesn’t shorten course of labor, the need for augmentation, the use of analgesia, or the rate of C/S) • Record FHT’s q 30 minutes (minimum) • During second stage, FHT’s should be recorded q 15 and after each contraction

  32. Augmentation of Labor at Term Abnormalities of the first stage of labor may be either protraction or arrest disorders and can occur during active or latent phases of labor. --Administer Pitocin as long as no malpresentation. Goal is ctx q 2-3 min lasting 60-90 seconds. Resting tone should be 10-15 mm Hg if IUPC is used. --Takes 30 to 40 minutes to see full effect of Pitocin dose --A slow rate of pitocin increase is as effective as a fast rate. --Whether to add pitocin to a patient who is already adequately contracting is controversial, but 80% of patients will respond to pitocin

  33. Augmentation of Labor at Term • Advantages: Oxytocin is cheap, and well known to us Short t1/2 Complications: uterine hyperstimulation (tachysystole) increased uterine tone (hypertonia) water intoxication (at doses of 30-40 miu since it’s a vasopressin analogue) hypotension (usually if pitocin is given as a bolus) uterine rupture (associated with “excessive oxytocin”)

  34. Abnormal patterns of labor • “Latent phase arrest”—means labor never began • “prolonged latent phase”—greater than 20h in nullip ---greater than 14h in multip • Prolongation of latent phase is variable; doesn’t mean the fetus will have a bad outcome or that the patient needs a c/s. Can be managed expectantly (presuming mom and baby other wise look good) Can administer analgesics (eg morphine 15-20 mg for therapeutic rest) Augmentation (Pitocin) Defer amniotomy!

  35. Pitocin regimen

  36. Abnormal patterns of labor • Abnormalities of second stage “Failure to Progress”“Arrest of dilatation” generally patient is falling off Friedman’s curve, or no cervical change in 2 hours Consider augmentation, placement of IUPC

  37. Abnormal patterns of labor “Protraction of descent” Descent of < 1 cm/h in nullips Descent of < 2 cm/h in multips Deliveries complicated by prolonged second stage put the fetus at risk of acidosis, thus, ACOG recommends intervention after 2 h without epidural, 3 h with epidural. In reality, can consider expectant management if mother and fetus are otherwise reassuring, descent is progressive, and delivery is imminent.

  38. Abnormal patterns of labor • “Arrest of Descent” This requires an assessment of contractions, maternal fetal well being, and CPD Re-evaluate clinical pelvimetry, fetal station, caput. The decision to proceed with assisted vaginal delivery or C/S should be individualized

  39. Do you really want to do that episiotomy?? • Episiotomy– the incision in the perineal body during the second stage of labor. Indicated in 1) cases of arrested or protracted descent 2) expedite delivery in NRFHT’s Median: performed when the fetal head is on the perineum. Associated with occasional extensions to 3rd or 4th degree Mediolateral: 45 degree angle from the hymenal ring. Does not increase risk of 3rd or 4th degree extension. Procedure of choice in patients with inflammatory bowel disease. More pain post partum.


  41. Episiotomy Fewer episiotomies are being performed…most repairs after a vaginal delivery are a result of tears. Episiotomies (and lacerations) are graded on a scale of 1 to 4

  42. Episiotomy/Lacerations 1st degree lacerations: involve the forchette, perineal skin, and vaginal mucosa 2nd degree lacerations: above plus extend to the fascia and muscles of the perineal body but not to the anal sphincter 3rd degree lacerations: skin, mucosa, perineal body and anal sphincter 4th degree: exposed lumen of the rectum

  43. Vaccuum Deliveries