How many organs are functioning in the last trimester of pregnancy?

Fetal Organ Function in the Third Trimester of Pregnancy

By the third trimester of pregnancy, all major fetal organs are functioning, with continued maturation occurring to prepare for extrauterine life. 1

Cardiovascular System Development

• The fetal heart is fully formed and functioning by the third trimester, with cardiac output increasing to meet the metabolic demands of the growing fetus 1
• Uteroplacental blood flow increases from 50 mL/min early in pregnancy to approximately 1000 mL/min during the third trimester, receiving up to 20% of maternal cardiac output 1
• Fetal circulation has unique adaptations including the foramen ovale and ductus arteriosus that allow blood to bypass the lungs 1
• The oxyhemoglobin dissociation curve is shifted to the left in the fetus (P50 is 19 mmHg), conferring relative resilience to hypoxic conditions 1

Respiratory System Development

• By the third trimester, the lungs have developed sufficiently to allow gas exchange, though they continue to mature until term 1
• Functional residual capacity in the mother decreases by 10-25% during pregnancy as the uterus enlarges and elevates the diaphragm 1
• Surfactant production increases significantly during the third trimester, which is crucial for preventing alveolar collapse after birth 1

Renal System Development

• The fetal kidneys are fully formed and functioning by the third trimester, producing urine that contributes to amniotic fluid 1
• Maternal renal blood flow increases by 40% during pregnancy to accommodate the maternal role of fetal detoxification 1
• Glomerular filtration begins early in pregnancy but continues to mature throughout the third trimester 1

Gastrointestinal System Development

• The fetal digestive system is developed by the third trimester, with the fetus swallowing amniotic fluid 1
• Digestive enzymes are being produced, though full digestive capacity isn't reached until after birth 1
• Maternal glucose homeostasis changes and cholesterol levels increase in adaptation to fetal-maternal nutritional needs 1

Neurological System Development

• Brain development continues rapidly during the third trimester with significant neuronal growth and myelination 1
• Fetal movements become more coordinated as the nervous system matures 1
• Sleep-wake cycles become established during the third trimester 2

Endocrine System Development

• The fetal endocrine system is functioning by the third trimester, including the thyroid, adrenal glands, and pancreas 1
• These glands produce hormones essential for growth, development, and preparation for extrauterine life 1

Hepatic System Development

• The liver is functioning by the third trimester, though some metabolic functions remain immature until after birth 1
• Bilirubin metabolism and glycogen storage capabilities continue to develop throughout the third trimester 1

Immune System Development

• The fetal immune system is developing but remains immature in the third trimester 1
• Maternal antibodies cross the placenta, providing passive immunity to the fetus 1

Clinical Implications

• Understanding fetal organ development is crucial for managing preterm deliveries, as organ immaturity significantly impacts neonatal outcomes 1
• The rapid development of all organ systems during the third trimester explains why each additional week of gestation improves outcomes for premature infants 1
• Maternal physiological adaptations, including increased cardiac output (30-50% above baseline) and expanded blood volume (40% above baseline), support these developing fetal organ systems 1