Labor: Why it Begins

The onset of labor is not a single switch, but a synchronous shift of several programs in the uterus, placenta, cervix, and fetal membranes. Pregnancy is maintained by progesterone-dependent "silence" of the myometrium, and as pregnancy progresses, the contractile phenotype of the uterus gradually develops, local inflammation increases, and cervical remodeling begins. [1]

In humans, the main factor is not the decline in progesterone levels in the blood, but its "functional cancellation" in tissues: the functioning of receptors, enzymes, and the ratio of progesterone receptors changes. Against this background, the relative activity of estrogens and sensitivity to oxytocin and prostaglandins increases. [2]

The placenta and fetus become active participants in the final stage: placental corticotropin-releasing hormone is accelerated by the principle of positive feedback, enhancing fetal maturation and simultaneously “pushing” the system towards labor. [3]

Human labor develops against a background of "sterile" controlled inflammation: leukocytes enter the uterine and cervical tissues, proinflammatory mediators increase, and matrix metalloproteinases are activated. This is not an infection, but a physiological phase of pregnancy completion. [4]

It all comes down to the idea of a multi-nodal trigger: hormonal signals, immune activation, mechanical stretch, and fetal maturity signals converge to a threshold after which coordinated labor begins. [5]

Table 1. Basic components of labor initiation and their role

Component	What changes by the deadline?	Key effect
Progesterone activity	"Functional cancellation" in the myometrium	Loss of contractility inhibition
Estrogen activity	Relative gain	Increased expression of the oxytocin receptor
Prostaglandins	Increasing local production	Increased contractions and ripening of the cervix
Immune signals	Controlled inflammation	Tissue remodeling
Placental and fetal signals	Placental CRH growth, lung maturity	Shift of systems to childbirth

Endocrine signals: progesterone "functional withdrawal", estrogens, oxytocin, prostaglandins, placental CRH

The key event is the "functional withdrawal" of progesterone: the balance of receptor isoforms, local metabolism, and transcriptional networks in the myometrium are altered. As a result, the uterus loses its stable "silence" and becomes susceptible to contractile stimuli. [6]

Estrogens increase the expression of the oxytocin receptor and intercellular communication proteins, preparing the myometrium for synchronous waves of contractions. This shift occurs in parallel with an increase in local prostaglandin activity. [7]

Oxytocin becomes more effective in late pregnancy due to increased receptor density and restructuring of intracellular calcium pathways. It becomes the primary "amplifier" of contractions already initiated. [8]

Prostaglandins simultaneously enhance contractility and remodel the cervix, so any conditions with increased prostaglandinogenesis bring about the spontaneous onset of labor. [9]

Placental corticotropin-releasing hormone increases exponentially with gestational age, forming a positive feedback loop with cortisol and linking fetal maturity to the timing of labor. [10]

Table 2. Endocrine triggers and points of application

Signal	Source	The main target	Clinical meaning
Progesterone: Functional Withdrawal	Myometrium	Receptor network, transcription	Loss of contractile inhibition
Estrogens	Placenta	Oxytocin receptor, cell communication	Increased myometrial conductivity
Oxytocin	Mother's pituitary gland	Oxytocin receptor in the myometrium	Strengthening and coordinating contractions
Prostaglandins	Decidual tissue, membranes	Cervix, myometrium	Cervical ripening and contractions
Placental CRH	Placenta	Fetal axis, placenta	The relationship between fruit maturity and timing

Immune-inflammatory restructuring: physiological "fire on the small"

As pregnancy progresses, the influx of monocytes and neutrophils increases, and interleukin-1, tumor necrosis factor, interleukin-6, and toll-like receptor cascades are activated. This accelerates the degradation of the cervical intercellular matrix and increases myometrial excitability. [11]

Evidence has accumulated that progesterone-dependent pathways in the myometrium are suppressed by local mediators, and changes in the enzyme 20-alpha-hydroxysteroid dehydrogenase enhance the “intra-tissue withdrawal” of progesterone. [12]

Despite the obvious inflammatory signs, some authors emphasize that inflammation is a companion, not the only trigger; the key action remains the functional withdrawal of progesterone and hormonal shifts. [13]

Infection is not necessary for the onset of labor in full-term pregnancies; however, if labor begins prematurely, the infectious-inflammatory pathway becomes a key one. This explains why antibacterial prophylaxis reduces the risk for some groups. [14]

Molecular candidates in recent years include the NLRP3 inflammasome, new cytokine profiles, and placental extracellular vesicles capable of transporting messenger molecules and amplifying the coordinating signal. [15]

Table 3. Immune mediators and their contribution

Target	Mediator	Effect
Myometrium	Interleukin-1, tumor necrosis factor	Increased excitability and contractility
Cervix	Matrix metalloproteinases	Collagen loosening, shortening
Fruit membranes	Toll-like receptors	Prostacyclins and prostaglandins
Placenta	Exosomes with CRH mRNA	Strengthening system readiness

Transition of the myometrium to a "contractile phenotype"

As pregnancy progresses, expression of the protein connexin-43 and the density of intercellular contacts increase, transforming the myometrium into a conductive network and ensuring the synchrony of contractions. This is one of the recognizable molecular markers of uterine "readiness." [16]

Oxytocin receptors become more numerous and intracellular calcium signaling pathways become more sensitive, so the same levels of oxytocin produce more powerful contractions.[17]

Ionic and metabolic mechanisms fine-tune excitability: beta-3-adrenergic receptors are capable of “calming” the myometrium, and their activation is considered a potential target for tocolysis. [18]

At the macro level, this is expressed in a transition from irregular, training contractions to regular, directed waves that are coordinated along the longitudinal axis of the uterus. Increased cell connectivity makes the waves more stable. [19]

In total, the myometrium is restructured from “carrying” to “giving birth”: it is simultaneously more sensitive to hormones, better conducts signals, and generates coordinated contractions sufficient to open the cervix and expel the fetus. [20]

Table 4. Signs of the contractile phenotype of the myometrium

Indicator	Before the deadline	On time
Connexin-43	Low expression	High expression
Oxytocin receptor	Moderate density	Increased density
Intracellular calcium	Low sensitivity	High sensitivity
Response to beta-3 agonists	Weak	More pronounced relaxing

Ripening of the cervix and fetal membranes

The cervix softens and shortens due to collagen remodeling, increased water content, and activation of metalloproteinases. This makes the canal more pliable, reducing resistance to the advancement of the presenting part. [21]

Matrix metalloproteinases, particularly metalloproteinase-9 and metalloproteinase-8, are associated with active remodeling and correlate with clinical signs of maturation.[22]

Nitric oxide produced in the cervix further accelerates remodeling, as supported by clinical trials using nitric oxide donors to pre-induction.[23]

Aquaporin water channels and changes in tissue hydration are also involved in the final stages: water redistribution promotes softness and extensibility of the cervix. [24]

The fetal membranes become more susceptible to rupture under the influence of the same remodeling mediators, which links the timing of the rupture of membranes with the general cascade of labor initiation. [25]

Table 5. Mechanisms of maturation of the cervix and membranes

Component	Key molecules	The final effect
Collagen framework	Metalloproteinase-9, metalloproteinase-8	Loosening and shortening
Vasoactive factors	Nitric oxide	Additional softening
Water balance	Aquaporins	Increased hydration
Shells	Metalloproteinases, cytokines	Decreased tensile strength

Modifiers of onset time: fetal cues, uterine distension, circadian rhythms, and preterm initiation pathways

Fetal “signals of maturity” have been discussed for decades: in experiments with rodents, surfactant protein A activates macrophages and accelerates labor; in humans, associations have been shown, but the role appears more complex and is not the only key. [26]

Mechanical stretching of the uterine wall during multiple pregnancies, polyhydramnios, or a large fetus activates stretch-sensitive pathways and increases myometrial excitability, approaching the trigger threshold. This is one of the explainable non-endocrine factors. [27]

Circadian biology adds to the picture: spontaneous labor onset occurs more frequently in the late evening and early morning, consistent with the synergy of melatonin and oxytocin in the myometrium. This explains the typical nocturnal peak in labor. [28]

With premature onset, "alternative" pathways are triggered: infectious-inflammatory, decidual bleeding, severe uterine overstretching, and premature "functional withdrawal" of progesterone. The concept of "one syndrome - many causes" is well-documented. [29]

It is worth noting the popular myths separately: the influence of the phases of the Moon on the onset of labor has not been confirmed by large studies; data on sharp changes in atmospheric pressure remain contradictory and clinically insignificant. [30]

Table 6. Factors shifting the timing of the onset of labor

Factor	Data	The final effect
Fetal lung maturity	Human associations, causality is not obvious	Possible co-signal
Overstretching of the uterus	Mounting evidence	Lowering the threshold of excitability
Circadian rhythms	Nighttime peak, synergy of melatonin and oxytocin	More likely at night
Infection and inflammation	Central route in preterm labor	Early launch
The Moon and Weather	There is no convincing connection	More of a myth than a factor

Practical conclusions

During full-term pregnancy, labor begins when the following coincide: localized progesterone withdrawal, increased sensitivity to oxytocin and prostaglandins, controlled inflammation, cervical ripening, and fetal readiness. External factors play a lesser role, and circadian mechanisms merely modulate the likelihood of onset during the day. [31]

Approaches to inducing and preventing preterm labor are effective precisely because they target these nodes: prostaglandins and mechanical methods target the cervix, calcium antagonists target the myometrium, and progestogens and cervical cerclage target vulnerable pathways for preterm labor in at-risk groups. The choice of treatment depends on the clinical context and the level of evidence. [32]

If the goal is to understand the individual risk of preterm birth, markers such as cervical length and fetal fibronectin are useful, especially when combined with clinical and demographic models. These are not "crystal balls," but risk stratification tools. [33]

Table 7. Key clinical markers of labor proximity and their purpose

Marker	What does it show?	Main benefit
Bishop scale	Cervical maturity	Prediction of induction success
Cervical length by ultrasound	Risk of premature birth	Stratification and prevention
Fetal fibronectin (quantitative)	Risk of giving birth in the coming weeks	High negative predictive value
Dynamics of contractions and dilation	Myometrial activity and progression	Decisions on tactics

A short checklist

1. Without “functional withdrawal” of progesterone and increased sensitivity to oxytocin, sustained labor will not begin. [34]
2. Cervical maturation is an independent program based on matrix remodeling. [35]
3. Immune mechanisms are normal and necessary, infection is not obligatory during full-term birth. [36]
4. Circadian rhythms shift the probability to night but do not replace the main triggering nodes. [37]
5. The moon has almost no influence on the timing, and weather data is contradictory. [38]

Table 8. What is proven and what should be interpreted with caution

Statement	Level of support
"Functional withdrawal" of progesterone is more important than a simple drop in its blood level	High
Cervical maturation is a matrix-dependent process involving metalloproteinases.	High
Circadian synergy of melatonin and oxytocin enhances the nocturnal peak of labor.	Average
Lunar phases influence the onset of labor	Short
Sudden drops in blood pressure often trigger labor.	Low and ambiguous