Pregnancy

Physiological changes during pregnancy

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Respiratory system

• ↑ Oxygen consumption (by approx. 20%)
• ↑ Intraabdominal pressure through uterine growth → dyspnea (the diaphragm is displaced upwards → ↓ total lung capacity, residual volume, functional residual capacity, and expiratory reserve volume)
• Progesterone stimulates the respiratory centers in the brain → hyperventilation (to eliminate fetal CO2 more efficiently) → physiological, chronic compensated respiratory alkalosis
  • Ventilation is increased primarily ↑ Tidal volume → ↑ minute ventilation
    • Elevated progesterone levels trigger a sensation of shortness of breath and stimulate the hypothalamus to increase respiratory drive.
    • Facilitated by increased diaphragmatic excursion and hormone-induced laxity of the intercostal muscles that allows for enlargement of the thoracic cavity (although the expanding uterus displaces the resting position of the diaphragm upward, diaphragmatic excursion is not impaired).
  • ↓ PCO2 (∼ 30 mm Hg)
  • Slight increase in respiratory rate

Renal system

• ↑ Renal plasma flow → ↑ GFR → ↓ BUN and creatinine
  • Increased cardiac output leads to increased renal perfusion.
  • Therefore, serum creatinine levels considered normal in nonpregnant patients (eg, 1.0 mg/dL) represent significant renal dysfunction in this population.
• ↑ Glucose levels in urine: Increased glomerular filtration results in overload of the glucose carrier responsible for its resorption.
• Mild proteinuria: Increased GFR and glomerular permeability to albumin increases protein excretion.

Hematologic system

• ↑ Plasma volume → ↓ hematocrit, especially towards the end of pregnancy (30–34th week of gestation) → dilutional anemia (hemoglobin value rarely drops below 11 g/dL)
• Hypercoagulability is due to an increase in fibrinogen, factor VII, and factor VIII and a decrease in protein S (reduces the risk of intrapartum blood loss).
  • Increased estrogen and progesterone levels
• ↓ Platelet count → gestational thrombocytopenia
• ↑ RBC mass (increases from 8–10th week of gestation until the end of pregnancy)
• ↓ Iron and folate levels due to increased vitamin and mineral requirements

Endocrine system

• Human placental lactogen (hPL): a hormone synthesized by syncytiotrophoblasts of the placenta, which promotes the production of insulin.
  • Increased maternal glucose levels: allows glucose to freely cross the placenta for consumption by the fetus for energy
    • Causes insulin resistance
  • Increased maternal proteolysis: provides a readily available supply of amino acids for the fetus
  • Increased maternal lipolysis: leads to increased free fatty acids and ketones to provide energy to the mother, preserving glucose and amino acids for the fetus

Gastrointestinal system

• ↑ Salivation
• ↓ Lower esophageal sphincter tone → gastroesophageal reflux
• ↓ Motility → constipation and bloating
  • Progesterone decreases colonic smooth muscle activity and decreases fasting MMCs (migrating myoelectric complex) in the small intestine
  • Gravid uterus mechanically impedes small intestine transit
• Gallbladder stasis → gallstones
  • Estrogen increases cholesterol secretion into bile
  • Progesterone reduces gallbladder motility
• Hemorrhoids
  • Gravid uterus increases venous pressure

Musculoskeletal system

• ↑ Body weight → forward shift in center of gravity → ↑ lumbar lordosis
• ↑ Intraabdominal pressure → diastasis recti; meralgia paresthetica
• Relaxation of the pelvic girdle ligaments and symphysis pubis → pelvic girdle pain, coccygeal pain
  • Due to progesterone and relaxin
    • Relaxin: a vasodilating hormone secreted by the corpus luteum, and later, by the placental and decidual membranes. Increases endothelial nitric oxide secretion, which causes vasodilation and decreases vascular resistance and mean arterial pressure, which increases the glomerular filtration rate during pregnancy.
• Fluid retention in tissue → carpal tunnel syndrome

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Signs of Pregnancy by Stage

First Trimester (Weeks 1-12)

This trimester is characterized by presumptive (subjective), probable (objective, but not definitive), and positive (diagnostic) signs.

• Presumptive Signs (Patient Reported)
  • Amenorrhea: Missed menstrual period.
  • Nausea/Vomiting: “Morning sickness.”
  • Breast Tenderness & Enlargement: Occurs as early as 6 weeks.
  • Fatigue: Common, starts around 12 weeks.
  • Urinary Frequency: Due to uterine pressure on the bladder and increased blood volume.
• Probable Signs (Clinician Observed)
  • Positive Pregnancy Test (hCG): Detectable in serum or urine; considered probable because other conditions (e.g., molar pregnancy, certain tumors) can also elevate hCG.
  • Goodell’s Sign: Softening of the cervix, occurs around 6-8 weeks.
  • Chadwick’s Sign: Bluish discoloration of the cervix and vagina due to increased vascularity, seen around 6-8 weeks.
  • Hegar’s Sign: Softening of the lower uterine segment (isthmus), felt on bimanual exam around 6-12 weeks.
    • Reason: Hormonally induced softening and compressibility of the uterine isthmus (the segment between the cervix and the uterine body) allows it to be compressed on bimanual exam.
• Positive Signs (Confirmatory)
  • Fetal Heart Tones: Detected by Doppler ultrasound around 10–12 weeks.
  • Fetus Visualized on Ultrasound: A gestational sac can be seen as early as 5 weeks.
  • Fetal Movement Palpated by Examiner: Typically not until the second trimester.

Second Trimester (Weeks 13-28)

• Maternal Symptoms
  • Quickening: First perception of fetal movement.
    • Primigravida: ~18-20 weeks.
    • Multigravida: ~16-18 weeks due to prior experience.
  • Braxton Hicks Contractions: Irregular, painless uterine contractions that do not cause cervical change.
  • Skin Changes:
    • Striae Gravidarum: Stretch marks on the abdomen, breasts, and thighs.
    • Linea Nigra: Dark vertical line on the abdomen.
    • Chloasma (Melasma): “Mask of pregnancy,” hyperpigmentation on the face.
• Physical Exam Findings
  • Fundal Height: A key marker for fetal growth.
    • 12 weeks: Fundus is at the level of the pubic symphysis.
    • 20-22 weeks: Fundus is at the umbilicus.
    • After 20 weeks: Fundal height in centimeters (cm) should roughly equal gestational age in weeks (e.g., 25 cm at 25 weeks).
  • Ballottement: Palpable fetal rebound when the uterus is pushed during a vaginal exam, around 16-18 weeks.

Third Trimester (Weeks 29-40)

• Maternal Symptoms & Signs of Approaching Labor
  • Increased Braxton Hicks Contractions: May become more frequent and noticeable.
  • Lightening: The fetal head descends into the pelvis, which can relieve shortness of breath but increase pelvic pressure and urinary frequency.
  • Cervical Changes: Softening, effacement (thinning), and dilation begins.
  • Loss of Mucous Plug (“Bloody Show”): Expulsion of the cervical mucus plug signals that the cervix is changing.
    • Reason: The mucous plug acts as a protective barrier in the cervical canal throughout pregnancy. As the cervix begins to efface and dilate, the plug is dislodged. The associated “bloody show” is from the tearing of small cervical blood vessels as the cervix changes.
  • Spontaneous Rupture of Membranes (SROM): Gush or trickle of amniotic fluid.
  • True Labor Contractions: Regular, painful contractions that increase in frequency, duration, and intensity, causing progressive cervical change.

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Placenta

Microscopic Anatomy (Chorionic Villi)

• Functional Unit: The chorionic villus is the core functional unit for exchange.
• Villus Structure: The bulk of the villus consists of a mesenchymal connective tissue core containing fetal capillaries, venules, and macrophages (Hofbauer cells).
• Trophoblast Layers: The villi are covered by two layers of trophoblast cells, especially in early pregnancy:
  1. Syncytiotrophoblast (Outer Layer): A multinucleated, continuous cell layer that covers the villi surface. It is in direct contact with maternal blood in the intervillous space. It is responsible for hormone production (e.g., hCG, estrogen, progesterone) and nutrient/gas exchange.
  2. Cytotrophoblast (Inner Layer / Langhans’ Layer): A single layer of mitotically active, mononuclear cells located beneath the syncytiotrophoblast. They are the stem cells for the syncytiotrophoblast. This layer becomes discontinuous and less prominent in late pregnancy.