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OBSTETRICS > PHYSIOLOGY OF LABOR > Flashcards

PHYSIOLOGY OF LABOR Flashcards

1
Q

What characterizes labor?

A

Forceful and painful uterine contractions that effect cervical dilation and cause the fetus to descend through the birth canal.

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2
Q

What happens to the myometrium during the first 36 to 38 weeks of normal gestation?

A

It is in a preparatory yet unresponsive state.

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3
Q

What happens to the cervix during early gestation?

A

It begins an early stage of remodeling yet maintains structural integrity.

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4
Q

What occurs during the transitional phase following uterine quiescence?

A

Myometrial unresponsiveness is suspended, and the cervix undergoes ripening, effacement, and loss of structural cohesion.

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5
Q

What are the three theories describing labor initiation?

A
  1. Functional loss of pregnancy maintenance factors.
  2. Synthesis of factors that induce parturition.
  3. The mature fetus provides the initial signal for parturition.
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6
Q

What model does research support regarding labor onset?

A

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.

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7
Q

Why is understanding labor onset complicated between species?

A

The relative contributions of maternal and fetal signals vary between species.

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8
Q

What is the primary function of the myometrial layer of the uterus?

A

To facilitate uterine contractions by adapting to environmental changes via phenotypic transitions of smooth muscle cells.

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9
Q

What characteristics make uterine smooth muscle cells efficient for contractions?

A

High degree of shortening, multidirectional force exertion, random filament arrangement, and augmented force generation.

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10
Q

What is the decidua, and what is its role during pregnancy?

A

The decidua is the endometrium transformed by pregnancy hormones. It maintains pregnancy via immunoregulatory functions, suppresses inflammatory signals, and later transitions to promote parturition.

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11
Q

What functions does the cervix perform during pregnancy?

A

Maintains an epithelial barrier, sustains cervical competence, and orchestrates extracellular matrix changes for greater tissue compliance.

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12
Q

What is the difference in smooth muscle cell composition at the internal vs. external os of the cervix?

A

Smooth muscle cells comprise ~50% of stromal cells at the internal os but only ~10% at the external os.

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13
Q

What is the role of the placenta during pregnancy?

A

It exchanges nutrients and waste between mother and fetus, produces hormones and growth factors, and aids in parturition transition.

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14
Q

What is the primary function of the amnion?

A

Provides tensile strength to resist tearing, acts as a selective filter, and protects maternal tissues from amniotic fluid constituents.

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15
Q

What role do prostaglandins play in parturition?

A

Prostaglandins regulate myometrial contractility, relaxation, and inflammation by interacting with specific G-protein-coupled receptors.

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16
Q

What is the significance of prostaglandin metabolism in pregnancy?

A

Prostaglandin metabolism via 15-hydroxyprostaglandin dehydrogenase (PGDH) inactivates prostaglandins, balancing synthesis and activity.

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17
Q

What is phase 1 of parturition characterized by?

A

Uterine quiescence, cervical structural integrity, and extensive uterine changes for fetal growth.

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18
Q

What are Braxton Hicks contractions?

A

Low-intensity uterine contractions felt during the quiescent phase that do not cause cervical dilation.

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19
Q

What mechanisms promote myometrial quiescence during phase 1 of parturition?

A

Actions of estrogen and progesterone, increased cAMP and cyclic GMP, and ion channel modifications.

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20
Q

What conditions favor uterine relaxation?

A

Low intracellular calcium levels, reduced crosstalk, hyperpolarized membrane potential, and activation of uterotonin degradation.

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21
Q

What is the function of the BKCa channel in uterine contractility?

A

Regulates potassium efflux to maintain interior electronegativity, preventing calcium influx and contraction.

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22
Q

How does progesterone withdrawal influence parturition?

A

It promotes cervical ripening, greater distensibility, and augmented uterine sensitivity to uterotonins.

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23
Q

What role do estrogen and progesterone play in maintaining uterine quiescence?

A

Estrogen advances progesterone responsiveness, and both regulate genes that inhibit contractility and promote cervical competency.

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24
Q

How does the amnion contribute to parturition?

A

Synthesizes prostaglandins like PGE2 and PGF2α, promoting cascades for membrane rupture and labor activation.

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25
Q

What is the role of actin-myosin interactions in myometrial contractions?

A

Myosin phosphorylation activates actin-myosin interactions, which drive uterine contractions.

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26
Q

How do ligand-activated calcium channels affect labor?

A

They increase intracellular calcium, promoting myosin phosphorylation and sustained uterine contractions.

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27
Q

What are the clinical stages of labor in phase 3 of parturition?

A

The preparatory (latent and active phases), dilatational (maximum slope of dilation), and pelvic (deceleration and fetal descent) divisions.

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28
Q

What is the function of myometrial gap junctions?

A

They allow the transfer of cellular signals between myocytes through connexin-43-based channels, promoting electrical and metabolic coupling essential for myometrial synchrony.

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29
Q

Which protein forms myometrial gap junctions, and what happens to its levels near labor onset?

A

Connexin-43; its concentrations rise near labor onset.

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30
Q

How does progesterone contribute to uterine quiescence?

A

It lowers the expression of contraction-associated proteins (CAPs) like connexin-43, oxytocin receptor, and prostaglandin F receptor.

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31
Q

What roles do G protein–coupled receptors play in myometrial contractility?

A

They regulate contractility by mediating signaling pathways that maintain uterine quiescence or promote contractions, depending on the receptor and ligand.

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32
Q

What is the shared receptor for LH and hCG, and what is its role during pregnancy?

A

The LH-hCG receptor; it activates adenylyl cyclase, reducing contraction frequency and force to maintain uterine quiescence.

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33
Q

How do β-adrenergic receptors affect the myometrium?

A

They mediate Gαs-stimulated relaxation of myometrial cells and are used for tocolysis in preterm labor.

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34
Q

What is the function of CRH in pregnancy and labor?

A

CRH promotes uterine quiescence via the Gs-adenylate cyclase-cAMP pathway during pregnancy but enhances contractility via Gq and Gi signaling during labor.

35
Q

What role does cGMP play in the pregnant myometrium?

A

It promotes smooth muscle relaxation and is increased by factors like nitric oxide and natriuretic peptides.

36
Q

What happens to uterotonin-degrading enzymes during pregnancy?

A

Their activity increases during phase 1 of parturition to maintain uterine quiescence, but levels decline late in gestation.

37
Q

What changes occur in cervical collagen during softening?

A

Collagen cross-linking decreases, resulting in reduced stiffness and increased compliance.

38
Q

What is progesterone withdrawal, and how does it influence parturition?

A

It involves functional changes in progesterone activity, allowing increased expression of contraction-associated proteins to prepare the uterus for labor.

39
Q

What changes occur in the lower uterine segment during phase 2 of parturition?

A

It forms from the isthmus, allowing the fetal head to descend into the pelvic inlet, and expresses contraction-associated proteins.

40
Q

How does progesterone inhibit oxytocin receptor expression?

A

Progesterone interacts with PR-B to regulate gene expression and enhance receptor degradation, maintaining uterine quiescence.

41
Q

What molecular changes are associated with cervical ripening?

A

Increased production of glycosaminoglycans like hyaluronan, which enhances hydration, viscoelasticity, and matrix disorganization.

42
Q

What inflammatory changes occur in the cervix during labor?

A

Neutrophils, M1 macrophages, and M2 macrophages activate, increasing proinflammatory and tissue repair responses.

43
Q

What is one potential signal the fetus provides to initiate parturition?

A

The fetus may give signals through blood-borne agents that act on the placenta or through secretion into the amniotic fluid.

44
Q

How does uterine stretch contribute to parturition?

A

Uterine stretch increases the expression of connexin-43 and oxytocin receptors, as well as levels of gastrin-releasing peptide.

45
Q

What clinical observation suggests a role for uterine stretch in preterm labor?

A

Multifetal pregnancies and pregnancies with hydramnios have a greater risk for preterm labor.

46
Q

What is mechanotransduction in the context of parturition?

A

Mechanotransduction involves stretch-activated cell signaling, including activation of cell-surface receptors, ECM signaling, or autocrine molecule release.

47
Q

What role does the fetal hypothalamic-pituitary-adrenal-placental axis play in parturition?

A

It is a critical component of normal parturition and premature activation can prompt preterm labor.

48
Q

What is the role of placental CRH in parturition?

A

Placental CRH stimulates fetal adrenal production of DHEA-S and cortisol, creating a feed-forward endocrine cascade that continues until delivery.

49
Q

When do maternal plasma CRH levels peak?

A

They rise exponentially during the last 12 weeks of pregnancy, peak during labor, and fall after delivery.

50
Q

What happens to CRH-binding protein levels during late pregnancy?

A

CRH-BP levels decline, leading to greater levels of bioavailable CRH.

51
Q

What role does fetal lung surfactant protein A (SP-A) play in parturition?

A

SP-A prompts signaling pathways in human myometrial cells and may affect prostaglandin levels.

52
Q

What is the senescent-associated secretory phenotype (SASP) in fetal membranes?

A

SASP is a form of sterile inflammation induced by stretch and oxidative stress, which propagates inflammatory signals to weaken fetal membranes.

53
Q

What are the three stages of labor?

A

The first stage is cervical effacement and dilation, the second stage is fetal expulsion, and the third stage is placental separation and expulsion.

54
Q

What marks the beginning of the second stage of labor?

A

The second stage begins when cervical dilation is complete and ends with delivery of the fetus.

55
Q

What is the Ferguson reflex?

A

It is the enhancement of uterine activity due to mechanical stretching of the cervix.

56
Q

How does uterine contraction intensity vary during active labor?

A

Contractions last 30-90 seconds, average 1 minute, and amniotic fluid pressures range from 20-60 mm Hg.

57
Q

What is the function of the upper uterine segment during labor?

A

It contracts, retracts, and expels the fetus while thickening progressively.

58
Q

What clinical condition is associated with the pathological retraction ring (Bandl ring)?

A

The Bandl ring occurs in obstructed labor due to extreme thinning of the lower uterine segment.

59
Q

How does the uterus change shape during contractions?

A

The uterus elongates, narrows in diameter, and pulls the cervix upward around the lower pole of the fetus.

60
Q

What is the primary force in fetal expulsion after full cervical dilation?

A

Maternal intraabdominal pressure, generated by contraction of abdominal muscles during pushing.

61
Q

What cervical changes occur during labor?

A

Cervical effacement and dilation occur due to contraction forces in the ripened cervix.

62
Q

What marks the beginning of the third stage of labor?

A

The delivery of the fetus.

63
Q

What happens to the uterine cavity after fetal delivery?

A

It becomes nearly obliterated and forms a solid mass of muscle.

64
Q

Where does the uterine fundus lie immediately after fetal delivery?

A

Just below the level of the umbilicus.

65
Q

Why does the placenta separate during the third stage of labor?

A

Because the uterine size diminishes, causing the placental implantation site to shrink and pull away.

66
Q

What layer separates the placenta during delivery?

A

The decidua spongiosa.

67
Q

What helps with placental cleavage?

A

The loose structure of the spongy decidua.

68
Q

What forms between the placenta and the decidua during separation?

A

A hematoma.

69
Q

What is the Schultze mechanism of placental expulsion?

A

Blood collects in the membrane sac and does not escape until the placenta is expelled.

70
Q

What is the Duncan mechanism of placental expulsion?

A

The placenta separates at the periphery, causing blood to escape vaginally as it descends sideways.

71
Q

Where is oxytocin synthesized?

A

In the magnocellular neurons of the supraoptic and paraventricular neurons.

72
Q

How is oxytocin stored and transported to the posterior pituitary?

A

As a prohormone bound to neurophysin in membrane-bound vesicles.

73
Q

What increases oxytocin receptor numbers in myometrial and decidual tissues?

A

The end of gestation.

74
Q

What does oxytocin promote in decidual tissue?

A

Prostaglandin release.

75
Q

What phases of parturition are influenced by oxytocin?

A

Phase 3 (active labor) and phase 4 (puerperium).

76
Q

What is the role of prostaglandins in phase 3 of parturition?

A

They promote uterine contractions and cervical dilation.

77
Q

What happens to prostaglandin levels in amniotic fluid during labor?

A

They increase significantly.

78
Q

What effect do prostaglandins have when administered to pregnant women?

A

They can induce labor or abortion at any gestational age.

79
Q

What is the function of endothelin 1 in labor?

A

It induces myometrial contractions and stimulates prostaglandin synthesis.

80
Q

What does angiotensin II regulate in pregnancy?

A

Uteroplacental blood flow through vasodilation and vascular resistance modulation.

81
Q

What receptors mediate angiotensin II effects in pregnancy?

A

AT1R and AT2R receptors.

82
Q

What happens during the puerperium to prevent hemorrhage?

A

The myometrium contracts persistently, compressing uterine vessels.

83
Q

When does ovulation generally occur postpartum?

A

4 to 6 weeks after birth, depending on breastfeeding duration.

84
Q

What hormone mediates lactation-induced anovulation?

A

Prolactin.

OBSTETRICS (13 decks)

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