Maternal & Fetal Physiological Adaptations During Pregnancy

Fetal Circulation and In-Utero Gas Exchange

  • Fast-flow, low-vascular-resistance systemic circuit; high pulmonary vascular resistance.
  • Specialized shunts divert blood away from fetal lungs toward placenta:
    • Umbilical vein → ductus venosus → inferior vena cava.
    • Foramen ovale shunts right-atrial blood to left atrium.
    • Ductus arteriosus carries right-ventricular output to descending aorta.
    • Umbilical arteries (branches of internal iliac) return mixed blood to placenta.
  • Oxygen carriage optimized by fetal haemoglobin (HbF):
    • Left-shifted Hb–O2\text{Hb–O}_2 dissociation curve increases affinity vs. maternal HbA.
    • High cardiac output compensates for lower SaO2\text{SaO}_2 of mixed blood.
  • Timeline of pulmonary readiness:
    • 2525 weeks: primitive alveoli present; epithelium thick; sparse capillaries; little surfactant → minimal post-natal gas-exchange potential.
    • 2828 weeks: terminal sacs, capillaries & surfactant production mature → viable gas exchange possible.
  • Fetal breathing movements promote lung growth; surfactant lowers surface tension, aids post-natal fluid re-absorption.
  • Stressors (preeclampsia, IUGR, PROM) ↑ fetal glucocorticoids/catecholamines ⇒ accelerated lung maturation.
  • Antenatal maternal glucocorticoids ↑ surfactant synthesis, ↓ incidence of IRDS; exogenous surfactant can be instilled via endotracheal tube after birth.

Comparison: In-Utero vs. Post-Natal Gas Exchange

  • In utero: placenta performs gas exchange; high PvO2\text{PvO}_2 gradient maternal→fetal; shunts bypass lungs.
  • At birth: first breaths ↓ PVR, ↑ pulmonary blood flow; shunts close (foramen ovale, ductus arteriosus); ventilation–perfusion replaces placental exchange.

Hormonal Regulation of Pregnancy

Key Placental & Ovarian Hormones

  • hCG (glycoprotein; detectable 99 days post-fertilisation in blood, 10$–$12 days in urine):

    • Maintains corpus luteum → continued oestrogen/progesterone output until ~77 weeks when placenta assumes role.
    • Linked to first-trimester nausea, altered taste/smell; possible immunosuppression (↓ maternal rejection).

  • hPL (polypeptide; begins 5$–$10 days post-implantation, peaks pre-term):

    • Insulin antagonist → maternal cells ↓ glucose uptake, ensuring fetal glucose supply.
    • Mobilises maternal lipids for maternal energy; spares glucose.
    • Accelerates amino-acid transfer to fetus.

  • Oestrogens (oestriol ≫ oestradiol > oestrone):

    • Synthesised via fetoplacental cooperation (requires fetal DHEAS & liver enzymes).
    • Promote uterine/breast growth, vascularisation, connective-tissue softening, fluid retention (↑ angiotensin/aldosterone).
    • ↑ respiratory-centre CO2_2 sensitivity, ↑ uterine sensitivity to progesterone.

  • Progesterone (corpus luteum → placenta):

    • ↓ smooth-muscle excitability (uterus, GI, ureters, vessels).
    • ↑ chemoreceptor sensitivity to CO<em>2<em>2 (hyperventilation at lower PaCO</em>2</em>2).
    • Thermogenic (+0.5$–$1.0°C), immunomodulatory, essential precursor for fetal adrenal steroids.
    • Side-effects: reflux, constipation, urinary stasis, varicosities, postural hypotension.

Additional Placental Neurohormones

  • CRH, TRH, ACTH, β-endorphin: influence maternal pituitary, labour onset, pain threshold.

Maternal Endocrine Gland Adaptations

  • Anterior pituitary enlarges; ↑ ACTH, prolactin, TSH, MSH; late-pregnancy ↑ β-endorphins.
  • Posterior pituitary: ↑ oxytocin (labour, lactation); ADH unchanged but osmostat reset (lower threshold).
  • Adrenals: cortisol ×2$–$3; aldosterone ↑ (fluid retention, striae formation).
  • Thyroid: gland hypertrophy (↑ renal iodine loss), ↑ T<em>4<em>4-binding globulin → total T</em>4</em>4↑ but free hormone stable; high T4_4 may exacerbate nausea.
  • Parathyroids: ↑ PTH → enhanced Ca2+^{2+} absorption/re-uptake for fetal skeleton.

Reproductive Tract Changes

Uterus

  • Mass 5050 g → 1100$–$1200 g; capacity 1010 mL → 55 L.
  • Muscle fiber hypertrophy/hyperplasia to 2020 weeks; thereafter mechanical stretch.
  • Myometrial layers:
    1. Inner circular (cornua/lower segment/cervix) stretches.
    2. Middle oblique “living ligatures” constrict vessels postpartum.
    3. Two outer longitudinal layers shorten & thicken upper segment during labour.
  • Sparse adrenergic innervation wanes toward term.
  • Braxton Hicks ↑ ~5%5\% per week; nocturnally dominant.
  • Development milestones (Table 18.4):
    • 1212 wks: fundus palpable above symphysis.
    • 2020 wks: thicker, rounded; tubes vertical.
    • 3030 wks: lower uterine segment defined.
    • 3636 wks: reaches xiphisternum; lightening.

Cervix

  • High connective-tissue content; softens/swells mid-pregnancy (oestrogen, prostaglandins, relaxin, NO).
  • Forms mucus plug (operculum); protects against ascending pathogens.

Vagina

  • Oestrogen → hyperaemia, bluish coloration (Jacquemier’s sign); pulsatile uterine arteries palpable (Osiander’s).
  • ↑ epithelial shedding → leucorrhoea; glycogen-rich environment acidic (protective) yet predisposes to Candida/Trichomonas.

Haematological Adaptations

  • Blood-volume ↑ 30$–$40\% (≈15001500 mL) between 77–3434 wks; plasma ↑ 40$–$50\% by 28$–$32 wks.
  • RBC mass ↑ only 2$–$30\% ⇒ haemodilution (↓ viscosity 20%20\%) producing “physiological anaemia.”
  • Typical Hb ranges: 1st tri 116$–$139 g/L; 2nd 97$–$148 g/L; 3rd 95$–$150 g/L.
  • Iron demand ↑ (erythropoiesis & fetal transfer); ferritin ↓; supplementation debated.
  • WBC (neutrophils) ↑; immunoglobulins concentration ↓ (dilution + immune modulation).
  • Coagulation: ↑ fibrinogen + factors VII, VIII, IX, X, XII → hypercoagulable yet balanced by ↑ plasminogen (fibrinolysis); platelets diluted.

Cardiovascular System

  • Plasma-volume expansion via renin–angiotensin activation + oestrogen-mediated renal Na+^+/water retention.
  • Cardiac output ↑ 30$–$50\% (from 55 L/min at 1010 wks to 6.56.5 L/min at 2525 wks):
    • Stroke volume ↑ 30%30\%.
    • Heart rate ↑ 15%15\%.
  • TPR ↓ (progesterone-induced vasodilation) ⇒ BP maintained/slightly↓.
  • Veins dilate → ↑ capacitance; predisposition to varicosities.
  • Blood-flow redistribution: uterine flow 5050 mL/min → 500500 mL/min at term (≈10%10\% CO; 80%80\% of that to placenta). Skin/mucosa flow ↑ 70%70\% by 3636 wks.
  • Supine vena-caval compression in 3rd tri ↓ CO 25$–$30\%; left-lateral positioning alleviates.

Respiratory Adjustments

  • BMR ↑ → O<em>2<em>2 demand ↑ 20%20\%; progesterone ↑ central CO</em>2</em>2 sensitivity.
  • Minute ventilation ↑ 40%40\% (tidal volume ↑ 25$–$40\%; rate near constant) ⇒ mild chronic respiratory alkalosis.
  • Diaphragm elevates 44 cm; rib-flare widens subcostal angle.
  • Functional residual capacity & residual volume ↓; potential for sleep-disordered breathing.
  • Research links habitual snoring/OSA with preeclampsia; mechanism may involve intermittent hypoxia → endothelial dysfunction.

Renal System

  • GFR ↑ 40$–$50\% early; declines slightly pre-term.
  • ↑ renin–angiotensin–aldosterone → Na+^+/H2_2O retention balancing high filtration.
  • Glycosuria common (tubular carrier saturation at high GFR).
  • Proteinuria ≤0.30.3 g/day physiologic.
  • Dilated ureters/kink at pelvic brim + bladder hypotonia (progesterone) → urinary stasis & UTI risk.
  • Frequency: early (high GFR), late (mechanical pressure).

Musculoskeletal Changes

  • Relaxin + oestrogen remodel pelvic ligaments; symphysis pubis widens (<1010 mm); SI & sacrococcygeal joints soften → birth canal enlargement.
  • Joint laxity may contribute to back/symphysial pain; evidence mixed.
  • Bone remodelling ↑; overall bone mass unchanged.

Integumentary System

  • ↑ MSH → chloasma, linea nigra, areolar/perineal darkening.
  • Striae gravidarum (red→silver stretch marks) linked to cortisol-mediated collagen changes & skin stretching.

Gastrointestinal & Dental Effects

  • Progesterone-induced smooth-muscle relaxation:
    • LES incompetence → reflux/heartburn, worsened by increased intra-abdominal pressure.
    • Gastric emptying time ↑ (up to 4848 h during labour); oral drug absorption ↓.
    • Intestinal transit ↓ → enhanced nutrient absorption but constipation.
    • Nausea/vomiting multifactorial: hCG/hormonal stimulation of medullary centres + gut stasis.
  • Gum oedema, vascularity ↑ ⇒ bleeding; hormonal milieu predisposes to periodontal disease → importance of dental hygiene.

Integrated Summary of Maternal Adaptations

  • Steroid hormones (oestrogen, progesterone) plus hCG, hPL, cortisol & aldosterone orchestrate systemic changes supporting fetal growth while preserving maternal homeostasis.
  • Fluid retention & haemodilution expand blood volume; vasodilation maintains BP.
  • Respiratory, renal, musculoskeletal, integumentary & GI systems each undergo targeted modifications preparatory for birth & lactation.
  • Many discomforts (oedema, reflux, dyspnoea, urinary frequency) stem directly from physiological adaptations.

Critical / Reflective Considerations

  • hPL creates a maternal "diabetogenic" state ensuring glucose availability for fetal growth; midwives should monitor maternal glucose tolerance.
  • Explaining physiological bases (e.g., ligament laxity causing pelvic pain; respiratory drive causing breathlessness) empowers pregnant women, normalises symptoms, and flags when deviations (e.g., hypertension, proteinuria) warrant investigation.