Pregnancy is a transformative journey characterized by intricate physiological and anatomical adaptations in the mother to support the developing fetus. Understanding the interplay between maternal and fetal physiology, including the crucial role of the placenta, is paramount for ensuring a healthy pregnancy and positive outcomes.
Maternal Physiological and Anatomic Changes During Pregnancy
Pregnancy triggers a cascade of hormonal, cardiovascular, respiratory, renal, and gastrointestinal changes in the mother, designed to nurture the growing fetus and prepare the body for labor and delivery.
A. Hormonal Changes:
- Human Chorionic Gonadotropin (hCG): Produced by the placenta, hCG maintains the corpus luteum, which is essential for progesterone production in early pregnancy. Levels peak around 8-11 weeks gestation and then decline.
- Progesterone: Primarily produced by the corpus luteum in early pregnancy and later by the placenta, progesterone plays a crucial role in maintaining the endometrium, preventing uterine contractions, and promoting the development of mammary glands. Levels steadily increase throughout pregnancy.
- Estrogen: Similar to progesterone, estrogen is initially produced by the corpus luteum and later by the placenta. Estrogen promotes uterine growth, increases blood flow to the uterus, and stimulates the development of the mammary glands. Different forms of estrogen, such as estriol, are measured to assess fetal well-being.
- Human Placental Lactogen (hPL): Secreted by the placenta, hPL helps to regulate maternal glucose metabolism, ensuring glucose availability for fetal growth. It also has insulin-antagonist properties, contributing to gestational diabetes in some women.
- Relaxin: Produced by the corpus luteum and placenta, relaxin promotes relaxation of ligaments and joints, particularly in the pelvis, preparing the body for delivery. It also contributes to increased renal blood flow.
- Prolactin: Produced by the anterior pituitary gland, prolactin prepares the mammary glands for lactation. Its effects are inhibited during pregnancy by high levels of estrogen and progesterone.
B. Cardiovascular Changes:
- Increased Blood Volume: Blood volume increases by 30-50% during pregnancy, peaking around 32-34 weeks gestation. This increase is crucial for meeting the metabolic demands of the fetus and expanding maternal tissues. The increase is primarily due to an increase in plasma volume, leading to hemodilution and a physiological anemia of pregnancy.
- Increased Cardiac Output: Cardiac output increases by 30-50% due to increases in both heart rate and stroke volume. The increased blood volume contributes to the elevated stroke volume.
- Decreased Systemic Vascular Resistance: Progesterone and other hormones cause vasodilation, leading to a decrease in systemic vascular resistance. This helps to accommodate the increased blood volume and cardiac output.
- Increased Heart Rate: Heart rate increases by 10-20 beats per minute during pregnancy.
- Blood Pressure Changes: Blood pressure typically decreases slightly during the second trimester due to vasodilation, but it returns to pre-pregnancy levels in the third trimester.
- Venous Stasis: The enlarging uterus can compress the inferior vena cava and iliac veins, leading to venous stasis in the lower extremities. This increases the risk of varicose veins, leg edema, and deep vein thrombosis.
C. Respiratory Changes:
- Increased Tidal Volume: Tidal volume (the amount of air inhaled and exhaled with each breath) increases by 30-40% during pregnancy.
- Increased Minute Ventilation: Minute ventilation (the total amount of air breathed per minute) increases due to the increased tidal volume.
- Decreased Functional Residual Capacity: The enlarging uterus pushes upward on the diaphragm, decreasing functional residual capacity (the amount of air remaining in the lungs after a normal exhalation).
- Increased Oxygen Consumption: Oxygen consumption increases by 15-20% to meet the metabolic demands of both the mother and the fetus.
- Nasal Congestion: Increased estrogen levels can cause edema of the nasal mucosa, leading to nasal congestion and nosebleeds.
D. Renal Changes:
- Increased Renal Blood Flow: Renal blood flow increases by 50-80% during pregnancy.
- Increased Glomerular Filtration Rate (GFR): GFR increases by 40-60%, leading to increased excretion of waste products.
- Glycosuria and Proteinuria: Due to the increased GFR, small amounts of glucose and protein may be excreted in the urine. However, significant proteinuria should be investigated further as it could indicate pre-eclampsia.
- Increased Urinary Frequency: The enlarging uterus compresses the bladder, leading to increased urinary frequency, particularly in the first and third trimesters.
E. Gastrointestinal Changes:
- Nausea and Vomiting: Commonly known as “morning sickness,” nausea and vomiting are thought to be caused by hormonal changes, particularly elevated hCG levels.
- Decreased Gastric Motility: Progesterone slows down gastric motility, leading to delayed gastric emptying and increased risk of heartburn and constipation.
- Changes in Appetite and Taste: Many pregnant women experience changes in appetite and taste, including cravings for certain foods and aversions to others.
- Constipation: Decreased gastric motility and increased iron supplementation can contribute to constipation.
Fetal Physiology
Fetal physiology differs significantly from postnatal physiology due to the dependence on the placenta for gas exchange, nutrient delivery, and waste removal.
A. Fetal Circulation:
- Umbilical Vein: Oxygenated blood from the placenta travels to the fetus via the umbilical vein.
- Ductus Venosus: A portion of the oxygenated blood bypasses the liver via the ductus venosus and enters the inferior vena cava.
- Foramen Ovale: Oxygenated blood from the inferior vena cava enters the right atrium and is shunted directly to the left atrium through the foramen ovale, bypassing the pulmonary circulation.
- Ductus Arteriosus: Blood from the right ventricle enters the pulmonary artery, but most of it is shunted to the aorta via the ductus arteriosus, again bypassing the pulmonary circulation.
- Umbilical Arteries: Deoxygenated blood returns to the placenta via the umbilical arteries.
This unique circulatory system ensures that the most oxygenated blood reaches the fetal brain and heart.
B. Fetal Respiration:
- Fetal Lung Development: The fetal lungs are not used for gas exchange in utero. However, they undergo significant development, producing surfactant, a substance that reduces surface tension in the alveoli and prevents lung collapse after birth.
- Fetal Breathing Movements: Fetal breathing movements occur intermittently throughout pregnancy. These movements help to develop the respiratory muscles and prepare the lungs for breathing after birth.
C. Fetal Hematology:
- Fetal Hemoglobin: Fetal hemoglobin (HbF) has a higher affinity for oxygen than adult hemoglobin (HbA), allowing the fetus to extract oxygen efficiently from maternal blood.
- Erythropoiesis: Erythropoiesis (red blood cell production) occurs in the fetal liver, spleen, and bone marrow.
D. Fetal Renal Function:
- Urine Production: The fetal kidneys produce urine, which contributes to the amniotic fluid volume.
- Limited Concentrating Ability: The fetal kidneys have limited concentrating ability, so the fetus relies on the placenta for waste removal and fluid balance.
Placental Physiology
The placenta is a vital organ that serves as the interface between the mother and the fetus, facilitating gas exchange, nutrient transport, waste removal, and hormone production.
A. Placental Structure:
The placenta consists of chorionic villi that project into the intervillous space, which is filled with maternal blood. This arrangement allows for efficient exchange of substances between maternal and fetal circulations.
B. Placental Functions:
- Gas Exchange: Oxygen diffuses from maternal blood into fetal blood, and carbon dioxide diffuses from fetal blood into maternal blood.
- Nutrient Transport: Glucose, amino acids, fatty acids, vitamins, and minerals are transported from maternal blood to fetal blood.
- Waste Removal: Waste products, such as urea and creatinine, are transported from fetal blood to maternal blood for excretion by the mother.
- Hormone Production: The placenta produces hCG, progesterone, estrogen, and hPL, which are essential for maintaining pregnancy and supporting fetal growth.
- Immunological Protection: The placenta acts as a barrier to prevent the passage of some harmful substances from the mother to the fetus, but it also allows the passage of antibodies, providing passive immunity to the fetus.
Diagnostic Studies of Fetal Well-Being
Several diagnostic studies are used to assess fetal well-being during pregnancy. These studies help to identify potential problems and guide management decisions.
A. Non-Stress Test (NST):
- Procedure: The NST monitors the fetal heart rate in response to fetal movement. Electrodes are placed on the mother’s abdomen to detect fetal heart rate and uterine contractions. The mother is asked to note when she feels fetal movement.
- Interpretation: A reactive NST shows at least two accelerations (increase in fetal heart rate) of 15 beats per minute for 15 seconds within a 20-minute period. A non-reactive NST indicates that these accelerations are not present and may warrant further evaluation.
- Value-Based Care: NSTs are non-invasive and relatively inexpensive, making them a valuable screening tool. However, false-positive rates can be high, leading to unnecessary interventions. Clinicians should carefully interpret NST results in conjunction with other clinical information.
B. Biophysical Profile (BPP):
- Procedure: The BPP combines an NST with four ultrasound assessments: fetal breathing movements, fetal body movements, fetal tone (muscle tone), and amniotic fluid volume.
- Interpretation: Each component is scored as either present (2 points) or absent (0 points), for a total possible score of 10. A score of 8-10 is considered normal, while lower scores may indicate fetal compromise.
- Value-Based Care: The BPP provides a more comprehensive assessment of fetal well-being than the NST alone. However, it is more time-consuming and requires specialized equipment and training. The BPP should be used judiciously, considering the potential benefits and risks.
C. Contraction Stress Test (CST):
- Procedure: The CST monitors the fetal heart rate in response to induced uterine contractions. Contractions are typically induced with oxytocin or nipple stimulation.
- Interpretation: A negative CST shows no late decelerations (decrease in fetal heart rate that begins after the start of a contraction) with at least three contractions in a 10-minute period. A positive CST shows late decelerations with more than 50% of contractions, indicating potential fetal hypoxia.
- Value-Based Care: The CST is more invasive than the NST and BPP, and it carries a risk of inducing preterm labor. Therefore, it is typically reserved for cases where the NST or BPP is equivocal or when there is a high risk of fetal compromise.
D. Umbilical Artery Doppler Velocimetry:
- Procedure: Doppler ultrasound is used to measure blood flow velocity in the umbilical artery.
- Interpretation: Increased resistance to blood flow in the umbilical artery (indicated by a high systolic/diastolic ratio or absent/reversed end-diastolic flow) suggests placental insufficiency and potential fetal hypoxia.
- Value-Based Care: Umbilical artery Doppler velocimetry is a valuable tool for assessing placental function and identifying fetuses at risk for growth restriction. It can help guide management decisions, such as timing of delivery.
Conclusion
Understanding the complex interplay of maternal and fetal physiology is essential for providing optimal prenatal care. By recognizing the physiological and anatomical changes in the mother, understanding fetal development and circulation, and appreciating the crucial role of the placenta, healthcare providers can effectively monitor fetal well-being and intervene when necessary to ensure positive pregnancy outcomes. Furthermore, the judicious use of diagnostic studies, guided by the principles of value-based care, can help to minimize unnecessary interventions and optimize resource allocation, ultimately contributing to improved maternal and fetal health.
References
Norwitz, E. R., & Repke, J. T. (2018). Maternal Physiology. In Creasy and Resnik’s Maternal-Fetal Medicine: Principles and Practice (8th ed.). Elsevier.
Cunningham, F. G., Leveno, K. J., Bloom, S. L., Dashe, J. S., Hoffman, B. L., Casey, B. M., & Spong, C. Y. (2018). Williams Obstetrics (25th ed.). McGraw-Hill Education.
Moore, K. L., Persaud, T. V. N., & Torchia, M. G. (2015). The Developing Human: Clinically Oriented Embryology (10th ed.). Elsevier.
ACOG Committee on Practice Bulletins—Obstetrics. (2014). Antepartum fetal surveillance. Obstetrics & Gynecology, 124(4), 757–771.
Manning, F. A., & Platt, L. D. (1993). Antepartum fetal evaluation: Development of a fetal biophysical profile. American Journal of Obstetrics and Gynecology, 169(6), 1295-1301.
Freeman, R. K., Anderson, M., & Dorchester, W. (1982). A prospective multi-institutional study of antepartum fetal heart rate monitoring. II. Contraction stress test versus nonstress test for primary fetal surveillance. American Journal of Obstetrics and Gynecology, 143(7), 778-781.
Trudinger, B. J., Giles, W. B., Cook, C. M., Bombardieri, J., & Collins, L. (1985). Fetal umbilical artery flow velocity waveforms and placental resistance: Clinical significance. The Lancet, 326(8463), 890-892.
