Normal Labor and Delivery Notes PDF

August 31, 2023 Normal Labor and Delivery Page 1 of 12 Normal Labor and Delivery Instructor: Danielle Cullen, MD ([email protected]) Block 5: Urology/Endocrine/Reproduction Thread: Physiology Conflict of Interest: Dr. Cullen has no conflicts of interest to disclose Learning Objective...

August 31, 2023 Normal Labor and Delivery Page 1 of 12 Normal Labor and Delivery Instructor: Danielle Cullen, MD ([email protected]) Block 5: Urology/Endocrine/Reproduction Thread: Physiology Conflict of Interest: Dr. Cullen has no conflicts of interest to disclose Learning Objectives By exam time, you should be able to do the following: • Define labor • Recognize the stages of labor • Identify how the “three Ps” affect labor and birth • Explain the physiologic mechanisms of normal labor Study Questions What occurs during the first stage of labor? What is important about the fetus in the process of normal labor? What are the most important hormones in initiation of labor? Define Labor The initiation of regular uterine contractions causing cervical change • Can happen before or after water breaks • Generally painful • Can occur preterm (20 0/7-36 6/7 weeks), term (37 0/7-41 6/7) or post-term (>42 weeks) • Components of cervical assessment • Dilation: from 0 cm (closed cervix) to 10 cm (fully dilated) August 31, 2023 Normal Labor and Delivery Page 2 of 12 • Effacement: how thin the cervix is, from 0% to 100% • Station: location of fetal vertex compared to ischial spines, in cm (-3 is 3cm higher in the pelvis than the ischial spines, +3 is cm lower/past the ischial spines) • Consistency: firm, medium, soft Stages of Labor 1. First stage: onset of labor to complete dilation (10 cm): Composed of latent and active stages o Latent: “early labor”  Contractions are irregular and cause gradual cervical change  Preferably takes place at home  Defined as PROLONGED if > 20 hours in first vaginal birth (primiparous patients), or > 14 hours in patients who have had prior vaginal births (multiparous patients)  Historically lasted until 3 cm, now we use 6 cm as the start of active labor  If you google this topic, much of the information you find will say 3cm is the start of active labor- THIS IS NOT ACCURATE! o Active:  Contractions are regular and strong and lead to more rapid cervical change  For patient guidance, we use the 5-1-1 rule- we typically admit patients to the hospital once in “active labor”  Contractions 5 minutes apart (measure from beginning of one to beginning of next  Contractions last about 1 minute each  This pattern should have been going on for at least 1 hour  Cervical dilation is typically 1-2 cm/hour August 31, 2023 Normal Labor and Delivery  Page 3 of 12 PROTRACTED: defined as < 0.5 cm cervical dilation per hour o Example: if I check someone’s cervix at 12pm and they are 6 cm dilated, and then recheck their cervix 4 hours later at they are 7cm, this is PROTRACTED (0.25cm/h)  ARREST OF DILATION: must be >6cm dilated with ruptured membranes (“water broken”) and EITHER o No cervical change for 4+ hours with adequate contractions o No cervical change for 6+ hours with inadequate contractions o Adequacy of contractions is only calculated/measured with an intrauterine pressure catheter (IUPC) 2. Second stage: complete dilation (10 cm) to birth of baby o Normal is up to 3 hours for primiparous patients and 2 hours for multiparous patients o No absolute maximum identified o May take longer for patients with dense epidurals or fetal malposition (occiput posterior, occiput transverse)- more on this shortly! 3. Third stage: Birth of baby to delivery of placenta o 5-10 minutes on average, up to 30 minutes is considered normal o Placenta separates from wall of the uterus which is notable clinically for: o  Gush of blood  Lengthening of the cord  Rise of the fundus During this time we are also assessing for bleeding/hemorrhage and examining the vagina, cervix and perineum for lacerations that need repair o The placenta is inspected to ensure it appears intact The Three Ps: Pelvis, Passenger, Power August 31, 2023 Normal Labor and Delivery Page 4 of 12 1. Pelvis: the bony structure a. Four shapes, percentages below are how often these shapes are found in XX folks i. Gynecoid – 50% * thought to be best for vaginal birth ii. Android – 20% iii. Anthropoid – 25% iv. Platypelloid – 5% b. Important measurements: review from your first anatomy lecture of the block i. Obstetric conjugate ii. Transverse diameter iii. Anterior-posterior diameter (A-P) iv. Curve and length of sacrum 2. Passenger: the fetus a. Head size: Biparietal diameter (BPD), anteroposterior diameter (APD) b. Attitude: flexion of the fetal next i. Vertex: fetal neck fully flexed, smaller AP diameter, *BEST FOR VAGINAL BIRTH* ii. Military: fetal neck moderately flexed, larger AP diameter iii. Brow: fetal neck moderately extended, larger AP diameter iv. Face: fetal neck fully extended, larger AP diameter, small AP diameter BUT fetal head unable to adjust well c. Lie: longitudinal vs transverse/oblique i. Longitudinal includes both cephalic (head down) and breech (butt or legs down) ii. Transverse is lying side to side August 31, 2023 Normal Labor and Delivery Page 5 of 12 iii. Oblique is lying diagonally in uterus d. Presentation: i. cephalic (head presenting) ii. complete breech (butt presenting, knees flexed) iii. frank breech (but presenting, knees extended) iv. footling breech (single or double feet presenting) e. Position: direction the fetal occiput is facing compared to bony pelvis of birthing person 3. Power: uterine contractions a. Combination of frequency and strength of contractions b. Frequency can be monitored by external monitoring c. Strength can only be measured by internal monitoring with intra-uterine pressure catheter (IUPC) 4. THE CARDINAL MOVEMENTS OF LABOR: how the “three Ps” come together a. Engagement b. Descent c. Flexion d. Internal Rotation e. Extension f. Restitution (external rotation) g. Expulsion (delivery) August 31, 2023 Normal Labor and Delivery Page 6 of 12 The Physiologic Mechanisms of Normal Labor In this section we are going to take a wider view of what happens during the pregnancy that is preparing the body for labor and the physiologic mechanisms that first maintain uterine quiescence, and later initiate and maintain labor  Phase 1: Quiescence: characterized by uterine smooth muscle tranquility (unresponsiveness) with softening of cervix but maintenance of cervical structural integrity  Phase 2: Following quiescence, transition to responsive myometrium and cervical ripening, effacement, and loss of structural integrity. This should happen late in the third trimester.  Phase 3: * INCLUDES ALL THE STAGES OF LABOR THAT WE ALREADY DISCUSSED* Transition into labor– organized uterine contractions leading to cervical dilation and expulsion of the fetus and placenta August 31, 2023  Normal Labor and Delivery Page 7 of 12 Phase 4: Recovery from labor following childbirth- uterus goes back to normal size, cervix remodels back to non-pregnant state, hormonal changes that allow full milk production and restoration of fertility Regulation of labor initiation  Loss/decrease/downregulation of pregnancy maintenance factors (progesterone)  Synthesis of factors that induce labor: uterotonics (prostaglandins and oxytocin) as well as their receptors, influenced by increased estrogen  Initiation is likely influenced by changes in the fetus, the placenta and the pregnant person  There are many changes that occur (see chart below) - do NOT memorize the entire chart, I will go through the ones that I’d like you to know about August 31, 2023 Normal Labor and Delivery Page 8 of 12 Regulation of the Phases of Pregnancy:  Phase 1: Uterine quiescence and cervical softening o Progesterone: Dominates phase 1; made by the corpus luteum and then the placenta  suppresses genes encoding contraction-associated proteins (CAPs) such as oxytocin receptors, connexins (gap junction proteins) and prostaglandin receptors  the evidence for increased P:E ratio in maintenance of pregnancy and decline in P:E ratio for labor/parturition is overwhelming  Anti-progestins (such as mifepristone) play a role in cervical ripening and labor, as well as abortion pharmacology  Progesterone is prescribed clinically to help maintain cervical structural integrity and prevent preterm birth o Relaxin: peptide made in the placenta (trophoblasts), decidua (modified endometrium of pregnancy- located between the myometrium and fetal membranes) and corpus luteum August 31, 2023 Normal Labor and Delivery  Page 9 of 12 Mediates lengthening of the pubic ligament, cervical softening, vaginal relaxation and inhibition of myometrial contractions o o HCG: made by the placenta  Decreases contraction frequency and force  Decreases gap junction number via G-protein-coupled receptor  Levels decrease just before labor and remain low during labor Corticotropin-Releasing Hormone (CRH): made in the placenta and hypothalamus  Low in the first trimester, rises slowly in second trimester, then increases exponentially in the last 6-8 weeks of pregnancy.  This increase is thought to be controlling timing of labor.  CRH appears to promote myometrial quiescence during most of pregnancy and aids myometrial contractions with onset of labor. These opposing actions are achieved by differential actions of CRH via its receptor CRHR1 in NON-LABORING vs LABORING myometrium.  Phase 2: Activation and cervical ripening o Cervical Ripening and remodeling: the cervix is primarily connective tissue  Collagen breakdown and rearrangement  Increase in vascularity, stromal and glandular hypertrophy  Alterations in cervical glycosaminoglycans and proteoglycans within the cervical matrix leads to softening of the cervix  Inflammatory cell infiltration and cytokine production leading to collagen degradation  Collagen becomes more loosely packed and disorganized during this time leading to increased tissue compliance; this is due to interaction of collagen fibrils with proteoglycans o Formation of the lower uterine segment (LUS): occurs during the third trimester  The uterine isthmus becomes the LUS  The fetal head descends into the LUS (“drops” into the pelvis) August 31, 2023 Normal Labor and Delivery  Page 10 of 12 Mechanical stretch from the fetal head increases local oxytocin and prostaglandin release  As the LUS forms, the fundal muscle retracts and thickens because there is less fetal volume in the upper uterus  The upper segment retracts only to the extent that the LUS distends and the cervix dilates  The thickening of the fundus allows for stronger/more effective contractions in labor  The LUS expresses prostaglandin and oxytocin receptors differently than the fundus o Fetal activation:  The fetal hypothalamus secretes CRH, leading to increased ACTH secretion from fetal anterior pituitary. ACTH acts on the fetal adrenal gland which increases production of DHEAS and cortisol. These are released into the fetal circulation and make their way to the placenta.  Fetal DHEAS leads to increased placental estrogen production (estriol, estrone and estradiol are included in estrogens), which causes upregulation of oxytocin receptors, prostaglandin receptors and connexins (to make gap junctions) o  ESTROGEN DOMINATES PHASE 2 Cortisol increases placental oxytocin, prostaglandin and CRH production. The CRH from the placenta positively feeds back to the fetal pituitary. o Progesterone: in phase 2 progesterone is downregulated. This is likely through a combination of production decease and receptor desensitization.  Progesterone has been working to decrease CAPs, so this downregulation is necessary to prepare the uterus for contractions  Phase 3: Uterine Stimulation (LABOR + BIRTH) o Dominated by prostaglandins and oxytocin  CONTRACTIONS! August 31, 2023 Normal Labor and Delivery Page 11 of 12  These bind the receptors that were increased in phase 2  Through binding of these receptors, ligand activated calcium channels open allowing for the organized contractions that we have been preparing for in phase 2!  Coordinated myometrial contractions strong and frequent enough to dilate the prepared cervix and push the fetus through the birth canal  The uterus contracts and myometrium continues to retract throughout normal labor, thus each contraction should pick up where its previous contraction left off  Progressive shortening of the muscle fibers, not relaxing back to its original length after each contraction.  Fundus after delivery is tremendously thickened, allowing for regained uterine tone  The lower segment, on the other hand, is stretched and thin and does not immediately contract well (can be a source of postpartum hemorrhage)  Phase 4: Involution and cervical repair o During this phase, oxytocin dominates and causes persistent contraction of the myometrium, leading to compression of vessels o Removal of glycosaminoglycans, proteoglycans, and structurally compromised collagen o Synthesis of matrix and cellular components to complete uterine involution o Onset of lactogenesis and milk let down o Reinstitution of ovulatory signals (depending on lactation) Answers to Study Questions Q: What occurs during the first stage of labor? August 31, 2023 Normal Labor and Delivery Page 12 of 12 A: The first stage of labor is made up of latent (early) labor and active labor. During latent labor, there are irregular contractions leading to slow cervical change. Latent labor lasts until about 6 cm of cervical dilation. During active labor there are regular, strong contractions that cause more rapid cervical change. The cervix typically dilates between 1-2 cm per hour during active labor. Q: What is important about the fetus in the process of normal labor? A: The fetal factors that are important in normal labor include head (skull) size including BPD and APD, attitude (flexion) of the head, fetal lie (longitudinal, transverse or oblique), fetal presentation (cephalic, breech), and fetal head position (direction of the fetal occiput compared to the pelvis of the birthing person) Q: What are the most important hormones in initiation of labor? A: Upregulation of estrogen and downregulation of progesterone are the most dominant changes that occur to allow initiation of labor. These are influenced by the fetal pituitary secretion of CRH, leading to increased ACTH and ultimately increased DHEAs and cortisol. Estrogen is made from DHEAs, leading to upregulation of contraction associated proteins. Cortisol itself increases production of oxytocin and prostaglandins.