5.1 Onset of Labor, True Versus False Labor & Nutrition in Labor

Quick Summary: Physiological labor begins with a neuroendocrine transition that converts a quiescent uterus into a coordinated, contracting one. The certified nurse-midwife must be able to describe that cascade, distinguish true labor from benign false labor by the one criterion that actually separates them — progressive cervical change — and plan oral intake and hydration for a labor that may last many hours.

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Neuroendocrine Mechanics & Initiation of Labor

Human parturition transitions through four distinct physiological phases:

1. Phase 0 (Quiescence): Characterizes roughly 95% of pregnancy. The myometrium is rendered refractory to contractile stimuli through the inhibitory actions of progesterone, prostacyclin (PGI2), relaxin, and nitric oxide (NO), which maintain high levels of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) to inhibit intracellular calcium release.
2. Phase 1 (Activation): Occurs during late third trimester. The uterus is prepared for labor through the synthesis of contraction-associated proteins (CAPs). This process is driven by functional progesterone withdrawal, wherein systemic progesterone levels remain elevated, but the ratio of myometrial progesterone receptor isoforms shifts from the transcriptionally active PR-B to the repressive PR-A and PR-C forms. Simultaneously, placental corticotropin-releasing hormone (CRH) rises exponentially (the "placental clock"), stimulating fetal pituitary adrenocorticotropic hormone (ACTH), which drives fetal adrenal production of cortisol and dehydroepiandrosterone sulfate (DHEA-S). Cortisol enhances placental estrogen production (estriol and estradiol), tilting the estrogen-to-progesterone ratio heavily toward uterine activation. Estrogen markedly upregulates oxytocin receptors (increasing by more than 100- to 200-fold in the fundus) and stimulates the assembly of inter-myocyte gap junctions composed of connexin 43.
3. Phase 2 (Stimulation): The clinical onset of active labor. Coordinated myometrial contractions are stimulated by oxytocin and intrauterine prostaglandins (PGE2 and PGF2α). Prostaglandin E2 facilitates cervical collagen breakdown, glycosaminoglycan restructuring, and hyaluronic acid dispersion (ripening), while prostaglandin F2α acts directly on myometrial smooth muscle to mobilize intracellular calcium (Ca2+), activating myosin light-chain kinase (MLCK) to produce forceful contractions. Cervical dilation triggers the Ferguson reflex: mechanical distension of the cervix and lower uterine segment stimulates sensory pathways via spinal nerves to the supraoptic and paraventricular nuclei of the hypothalamus, prompting neurohypophyseal pulsatile release of oxytocin.
4. Phase 3 (Involution): Postpartum uterine contraction, placental site hemostasis, and restoration of the non-pregnant reproductive state.

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True Labor vs. False Labor (Braxton Hicks Contractions)

Accurate intrapartum assessment requires distinguishing physiological pre-labor contractions from progressive true labor to prevent unnecessary admissions, unindicated labor inductions, and premature interventions.

Clinical Pearl: The single definitive hallmark separating true labor from false labor is progressive cervical effacement and dilation on serial digital examinations over time. A patient with painful contractions every 3 minutes who remains at 2 cm dilation and 50% effacement over a 4-hour observation period is experiencing false or prodromal labor, not active labor arrest.

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Nutrition and Hydration in Labor

Planning oral intake is an explicit AMCB task ("initiates a plan to meet the nutritional needs of the laboring woman") and one of the clearest places where midwifery evidence diverges from inherited hospital routine.

Where the NPO Rule Came From

Blanket nil-per-os policies trace to Mendelson's 1946 description of aspiration pneumonitis during obstetric general anesthesia at a time when cricoid pressure, rapid-sequence induction, and modern airway management did not exist. Aspiration is now vanishingly rare, and the residual risk is concentrated in patients who actually receive general anesthesia.

Current Evidence and Recommendations

• A Cochrane review of oral intake in labor found no evidence that restricting food and fluids in low-risk laboring women improves any maternal or neonatal outcome — and no evidence of harm from unrestricted intake.
• ACOG and the American Society of Anesthesiologists support modest amounts of clear liquids throughout labor for uncomplicated patients, while advising against solid food during active labor.
• ACNM and WHO support responsive eating and drinking for low-risk women in spontaneous labor.

Acceptable clear liquids: water, clear fruit juice without pulp, carbonated beverages, clear tea, black coffee, clear broth, sports drinks, and ice pops — nothing with particulate matter, milk, or fat.

Why It Matters Physiologically

Labor imposes an energy demand comparable to sustained moderate exercise. Caloric restriction drives lipolysis and maternal ketosis, which is associated with maternal exhaustion, longer labor, and the perception of "failure to progress." Adequate hydration supports maternal cardiac output and therefore uteroplacental perfusion.

Building the Plan

1. Stratify risk. Patients with a high likelihood of general anesthesia (morbid obesity with a difficult airway, uncontrolled diabetes, coagulopathy contraindicating neuraxial block, an unstable fetal status, or planned operative birth) warrant more conservative restriction. Everyone else should be offered clear liquids freely.
2. Encourage responsive intake of small amounts frequently rather than a rigid schedule; nausea and vomiting in transition are normal and do not require restriction thereafter.
3. Intravenous fluids are not routine. Continuous IV infusion in a low-risk spontaneous labor restricts mobility without benefit. When IV fluid is needed, lactated Ringer's is preferred.
4. Avoid high-volume dextrose-containing infusions. Large glucose loads can produce maternal hyponatremia and fetal hyperinsulinemia with rebound neonatal hypoglycemia.
5. Document the plan and the patient's preference, and re-evaluate it if risk status changes — for example, when oxytocin augmentation, neuraxial analgesia, or a non-reassuring tracing is introduced.