PHYSIOLOGY OF LABOR Flashcards
What characterizes labor?
Forceful and painful uterine contractions that effect cervical dilation and cause the fetus to descend through the birth canal.
What happens to the myometrium during the first 36 to 38 weeks of normal gestation?
It is in a preparatory yet unresponsive state.
What happens to the cervix during early gestation?
It begins an early stage of remodeling yet maintains structural integrity.
What occurs during the transitional phase following uterine quiescence?
Myometrial unresponsiveness is suspended, and the cervix undergoes ripening, effacement, and loss of structural cohesion.
What are the three theories describing labor initiation?
- Functional loss of pregnancy maintenance factors.
- Synthesis of factors that induce parturition.
- The mature fetus provides the initial signal for parturition.
What model does research support regarding labor onset?
Labor onset represents the culmination of a series of biochemical changes in the uterus and cervix, resulting from endocrine and paracrine signals from both mother and fetus.
Why is understanding labor onset complicated between species?
The relative contributions of maternal and fetal signals vary between species.
What is the primary function of the myometrial layer of the uterus?
To facilitate uterine contractions by adapting to environmental changes via phenotypic transitions of smooth muscle cells.
What characteristics make uterine smooth muscle cells efficient for contractions?
High degree of shortening, multidirectional force exertion, random filament arrangement, and augmented force generation.
What is the decidua, and what is its role during pregnancy?
The decidua is the endometrium transformed by pregnancy hormones. It maintains pregnancy via immunoregulatory functions, suppresses inflammatory signals, and later transitions to promote parturition.
What functions does the cervix perform during pregnancy?
Maintains an epithelial barrier, sustains cervical competence, and orchestrates extracellular matrix changes for greater tissue compliance.
What is the difference in smooth muscle cell composition at the internal vs. external os of the cervix?
Smooth muscle cells comprise ~50% of stromal cells at the internal os but only ~10% at the external os.
What is the role of the placenta during pregnancy?
It exchanges nutrients and waste between mother and fetus, produces hormones and growth factors, and aids in parturition transition.
What is the primary function of the amnion?
Provides tensile strength to resist tearing, acts as a selective filter, and protects maternal tissues from amniotic fluid constituents.
What role do prostaglandins play in parturition?
Prostaglandins regulate myometrial contractility, relaxation, and inflammation by interacting with specific G-protein-coupled receptors.
What is the significance of prostaglandin metabolism in pregnancy?
Prostaglandin metabolism via 15-hydroxyprostaglandin dehydrogenase (PGDH) inactivates prostaglandins, balancing synthesis and activity.
What is phase 1 of parturition characterized by?
Uterine quiescence, cervical structural integrity, and extensive uterine changes for fetal growth.
What are Braxton Hicks contractions?
Low-intensity uterine contractions felt during the quiescent phase that do not cause cervical dilation.
What mechanisms promote myometrial quiescence during phase 1 of parturition?
Actions of estrogen and progesterone, increased cAMP and cyclic GMP, and ion channel modifications.
What conditions favor uterine relaxation?
Low intracellular calcium levels, reduced crosstalk, hyperpolarized membrane potential, and activation of uterotonin degradation.
What is the function of the BKCa channel in uterine contractility?
Regulates potassium efflux to maintain interior electronegativity, preventing calcium influx and contraction.
How does progesterone withdrawal influence parturition?
It promotes cervical ripening, greater distensibility, and augmented uterine sensitivity to uterotonins.
What role do estrogen and progesterone play in maintaining uterine quiescence?
Estrogen advances progesterone responsiveness, and both regulate genes that inhibit contractility and promote cervical competency.
How does the amnion contribute to parturition?
Synthesizes prostaglandins like PGE2 and PGF2α, promoting cascades for membrane rupture and labor activation.
What is the role of actin-myosin interactions in myometrial contractions?
Myosin phosphorylation activates actin-myosin interactions, which drive uterine contractions.
How do ligand-activated calcium channels affect labor?
They increase intracellular calcium, promoting myosin phosphorylation and sustained uterine contractions.
What are the clinical stages of labor in phase 3 of parturition?
The preparatory (latent and active phases), dilatational (maximum slope of dilation), and pelvic (deceleration and fetal descent) divisions.
What is the function of myometrial gap junctions?
They allow the transfer of cellular signals between myocytes through connexin-43-based channels, promoting electrical and metabolic coupling essential for myometrial synchrony.
Which protein forms myometrial gap junctions, and what happens to its levels near labor onset?
Connexin-43; its concentrations rise near labor onset.
How does progesterone contribute to uterine quiescence?
It lowers the expression of contraction-associated proteins (CAPs) like connexin-43, oxytocin receptor, and prostaglandin F receptor.
What roles do G protein–coupled receptors play in myometrial contractility?
They regulate contractility by mediating signaling pathways that maintain uterine quiescence or promote contractions, depending on the receptor and ligand.
What is the shared receptor for LH and hCG, and what is its role during pregnancy?
The LH-hCG receptor; it activates adenylyl cyclase, reducing contraction frequency and force to maintain uterine quiescence.
How do β-adrenergic receptors affect the myometrium?
They mediate Gαs-stimulated relaxation of myometrial cells and are used for tocolysis in preterm labor.
What is the function of CRH in pregnancy and labor?
CRH promotes uterine quiescence via the Gs-adenylate cyclase-cAMP pathway during pregnancy but enhances contractility via Gq and Gi signaling during labor.
What role does cGMP play in the pregnant myometrium?
It promotes smooth muscle relaxation and is increased by factors like nitric oxide and natriuretic peptides.
What happens to uterotonin-degrading enzymes during pregnancy?
Their activity increases during phase 1 of parturition to maintain uterine quiescence, but levels decline late in gestation.
What changes occur in cervical collagen during softening?
Collagen cross-linking decreases, resulting in reduced stiffness and increased compliance.
What is progesterone withdrawal, and how does it influence parturition?
It involves functional changes in progesterone activity, allowing increased expression of contraction-associated proteins to prepare the uterus for labor.
What changes occur in the lower uterine segment during phase 2 of parturition?
It forms from the isthmus, allowing the fetal head to descend into the pelvic inlet, and expresses contraction-associated proteins.
How does progesterone inhibit oxytocin receptor expression?
Progesterone interacts with PR-B to regulate gene expression and enhance receptor degradation, maintaining uterine quiescence.
What molecular changes are associated with cervical ripening?
Increased production of glycosaminoglycans like hyaluronan, which enhances hydration, viscoelasticity, and matrix disorganization.
What inflammatory changes occur in the cervix during labor?
Neutrophils, M1 macrophages, and M2 macrophages activate, increasing proinflammatory and tissue repair responses.
What is one potential signal the fetus provides to initiate parturition?
The fetus may give signals through blood-borne agents that act on the placenta or through secretion into the amniotic fluid.
How does uterine stretch contribute to parturition?
Uterine stretch increases the expression of connexin-43 and oxytocin receptors, as well as levels of gastrin-releasing peptide.
What clinical observation suggests a role for uterine stretch in preterm labor?
Multifetal pregnancies and pregnancies with hydramnios have a greater risk for preterm labor.
What is mechanotransduction in the context of parturition?
Mechanotransduction involves stretch-activated cell signaling, including activation of cell-surface receptors, ECM signaling, or autocrine molecule release.
What role does the fetal hypothalamic-pituitary-adrenal-placental axis play in parturition?
It is a critical component of normal parturition and premature activation can prompt preterm labor.
What is the role of placental CRH in parturition?
Placental CRH stimulates fetal adrenal production of DHEA-S and cortisol, creating a feed-forward endocrine cascade that continues until delivery.
When do maternal plasma CRH levels peak?
They rise exponentially during the last 12 weeks of pregnancy, peak during labor, and fall after delivery.
What happens to CRH-binding protein levels during late pregnancy?
CRH-BP levels decline, leading to greater levels of bioavailable CRH.
What role does fetal lung surfactant protein A (SP-A) play in parturition?
SP-A prompts signaling pathways in human myometrial cells and may affect prostaglandin levels.
What is the senescent-associated secretory phenotype (SASP) in fetal membranes?
SASP is a form of sterile inflammation induced by stretch and oxidative stress, which propagates inflammatory signals to weaken fetal membranes.
What are the three stages of labor?
The first stage is cervical effacement and dilation, the second stage is fetal expulsion, and the third stage is placental separation and expulsion.
What marks the beginning of the second stage of labor?
The second stage begins when cervical dilation is complete and ends with delivery of the fetus.
What is the Ferguson reflex?
It is the enhancement of uterine activity due to mechanical stretching of the cervix.
How does uterine contraction intensity vary during active labor?
Contractions last 30-90 seconds, average 1 minute, and amniotic fluid pressures range from 20-60 mm Hg.
What is the function of the upper uterine segment during labor?
It contracts, retracts, and expels the fetus while thickening progressively.
What clinical condition is associated with the pathological retraction ring (Bandl ring)?
The Bandl ring occurs in obstructed labor due to extreme thinning of the lower uterine segment.
How does the uterus change shape during contractions?
The uterus elongates, narrows in diameter, and pulls the cervix upward around the lower pole of the fetus.
What is the primary force in fetal expulsion after full cervical dilation?
Maternal intraabdominal pressure, generated by contraction of abdominal muscles during pushing.
What cervical changes occur during labor?
Cervical effacement and dilation occur due to contraction forces in the ripened cervix.
What marks the beginning of the third stage of labor?
The delivery of the fetus.
What happens to the uterine cavity after fetal delivery?
It becomes nearly obliterated and forms a solid mass of muscle.
Where does the uterine fundus lie immediately after fetal delivery?
Just below the level of the umbilicus.
Why does the placenta separate during the third stage of labor?
Because the uterine size diminishes, causing the placental implantation site to shrink and pull away.
What layer separates the placenta during delivery?
The decidua spongiosa.
What helps with placental cleavage?
The loose structure of the spongy decidua.
What forms between the placenta and the decidua during separation?
A hematoma.
What is the Schultze mechanism of placental expulsion?
Blood collects in the membrane sac and does not escape until the placenta is expelled.
What is the Duncan mechanism of placental expulsion?
The placenta separates at the periphery, causing blood to escape vaginally as it descends sideways.
Where is oxytocin synthesized?
In the magnocellular neurons of the supraoptic and paraventricular neurons.
How is oxytocin stored and transported to the posterior pituitary?
As a prohormone bound to neurophysin in membrane-bound vesicles.
What increases oxytocin receptor numbers in myometrial and decidual tissues?
The end of gestation.
What does oxytocin promote in decidual tissue?
Prostaglandin release.
What phases of parturition are influenced by oxytocin?
Phase 3 (active labor) and phase 4 (puerperium).
What is the role of prostaglandins in phase 3 of parturition?
They promote uterine contractions and cervical dilation.
What happens to prostaglandin levels in amniotic fluid during labor?
They increase significantly.
What effect do prostaglandins have when administered to pregnant women?
They can induce labor or abortion at any gestational age.
What is the function of endothelin 1 in labor?
It induces myometrial contractions and stimulates prostaglandin synthesis.
What does angiotensin II regulate in pregnancy?
Uteroplacental blood flow through vasodilation and vascular resistance modulation.
What receptors mediate angiotensin II effects in pregnancy?
AT1R and AT2R receptors.
What happens during the puerperium to prevent hemorrhage?
The myometrium contracts persistently, compressing uterine vessels.
When does ovulation generally occur postpartum?
4 to 6 weeks after birth, depending on breastfeeding duration.
What hormone mediates lactation-induced anovulation?
Prolactin.
