Physiological Changes in Late Pregnancy

Cardiovascular and Hemodynamic Changes

Inferior vena caval compression by the gravid uterus in the supine position causes an abrupt decrease in cardiac preload, leading to hypotension with weakness and lightheadedness in pregnant women at 36 weeks gestation, according to the American Heart Association 1
The left lateral position significantly increases cardiac output compared to the supine position, which explains why pregnant women require multiple pillows for sleep, as recommended by the American College of Cardiology 1
The fundal height of 35 cm and uterine fundus at the xiphoid process level at 36 weeks creates maximal diaphragmatic elevation, reducing lung volumes and contributing to dyspnea in pregnant women, as reported by the American Heart Association 1
Vital signs, such as blood pressure 118/72 mmHg and heart rate of 88 bpm, are within normal pregnancy parameters, reflecting the expected decrease in diastolic pressure and normal 10-20 bpm increase, according to the American College of Cardiology 1

Distinguishing normal pregnancy-associated changes from true cardiomyopathy requires clinical judgment, as symptoms of dyspnea, edema, and orthopnea can mimic cardiac disease, but normal vital signs and absence of concerning cardiac findings make peripartum cardiomyopathy unlikely, as stated by the American Heart Association 2
The American Heart Association recommends careful evaluation to avoid confusing normal physiological changes with pathological conditions, such as peripartum cardiomyopathy, in pregnant women at 36 weeks gestation 3

Maternal heart rate increases by 15-20 beats per minute during pregnancy, particularly in the third trimester, as part of normal cardiovascular adaptation, according to the American College of Cardiology 4
Cardiac output rises 30-50% during pregnancy due to increased stroke volume and elevated maternal heart rate, as stated by the American Heart Association 4, 5
Systemic vascular resistance decreases due to endogenous vasodilators, leading to decreased mean arterial pressure that reaches its lowest point in the second trimester, as reported by the American College of Obstetricians and Gynecologists 4, 6

Sleep cycles may cause normal decreases in fetal heart rate variability, and medications such as analgesics and anesthetics can reduce variability, according to the American Academy of Family Physicians 7, 8
Maternal position significantly affects cardiac output, with the left lateral position increasing cardiac output compared to the supine position, as noted by the American College of Cardiology 4

Loss of variability accompanied by late or variable decelerations may indicate potential fetal acidosis if uncorrected, as warned by the American Academy of Family Physicians 7
Position changes can significantly affect measurements, and avoiding the supine position after 20 weeks is recommended, as advised by the American College of Cardiology 4

The American Heart Association recommends that during cardiac arrest resuscitation in pregnancy, continuous leftward manual displacement of the uterus is mandatory, as the left lateral position itself reduces external cardiac massage efficacy to only 10% of normal cardiac output, in patients after 20 weeks gestation 9, 10
The left lateral position increases cardiac output by 24%, ejection fraction by 11%, and stroke volume by 35% at 32 weeks compared to supine positioning, in pregnant women 9
Supine positioning can overwhelm compensatory mechanisms that maintain uterine blood flow, potentially compromising fetal oxygenation, in pregnant women after 20 weeks gestation 9

Persistent occipito-posterior (OP) or occipito-transverse (OT) fetal positions occur in 1.8-12.9% of pregnancies and are associated with significantly increased maternal and neonatal morbidity, according to the American College of Obstetricians and Gynecologists 11

All pregnant women after 20 weeks gestation must avoid prolonged supine positioning during sleep, procedures, examinations, or imaging studies, as recommended by the American College of Cardiology 9
The left lateral position or left pelvic tilt should be maintained during endoscopic procedures, surgical interventions, and any prolonged recumbent positioning, in pregnant women, as suggested by the Society of Obstetric Anesthesia and Perinatology 12
In maternal cardiac arrest, perimortem cesarean delivery should be initiated at 4 minutes if no effective cardio-circulatory activity is observed, with the goal of fetal extraction within 5 minutes of resuscitation onset, according to the American Heart Association 10
Manual leftward uterine displacement by a designated team member using two-handed traction is required during resuscitation, as the left lateral position compromises chest compression efficacy, in pregnant women 10