Placental and Fetal Physiology

Learning objectives

After completing this chapter, you should be able to:

1. Describe the separate maternal and fetal circulations within the placenta.
2. Explain how maternal cardiac output, perfusion pressure, oxygen content, uterine blood flow, and placental function determine fetal oxygen delivery.
3. Trace fetal blood through the umbilical vein, ductus venosus, foramen ovale, ductus arteriosus, and umbilical arteries.
4. Explain why the fetal ventricles function largely in parallel and why pulmonary vascular resistance is high before birth.
5. Describe how fetal hemoglobin, blood-flow streaming, and redistribution support oxygen delivery to vital organs.
6. Recognize the progression from fetal compensation to decompensation during maternal or placental compromise.
7. Apply maternal positioning, oxygenation, perfusion, monitoring, communication, and diversion principles to fetal deterioration during transport.

Opening transport scenario

A 31-year-old patient at 29 weeks’ gestation is being transferred for severe fetal growth restriction and abnormal umbilical artery Doppler findings. At departure, maternal vital signs are stable, fetal heart rate is 145/min with moderate variability, and the patient is positioned with a left lateral tilt. Twenty minutes into transport, vomiting and anxiety are followed by maternal hypoxemia and a blood-pressure decline. The fetal tracing develops recurrent late decelerations and then prolonged bradycardia.

The fetal monitor is displaying the downstream effect of a larger system failure. Fetal oxygen delivery depends on the maternal lungs, maternal hemoglobin, maternal cardiac output, uterine perfusion, placental exchange, umbilical blood flow, fetal hemoglobin, and fetal cardiovascular distribution. A problem at any point can reduce oxygen delivery. The transport team must stabilize the mother, assess the fetal response, and determine whether the original destination remains appropriate.

1. The placenta: a temporary multiorgan support system

The placenta is not simply an attachment between mother and fetus. It functions as the fetal lungs, gastrointestinal interface, kidneys, endocrine organ, and immunologic boundary. It transfers oxygen and nutrients to the fetus, removes carbon dioxide and metabolic waste, produces hormones that alter maternal physiology, and helps regulate fetal growth.

1.1 Chorionic villi and intervillous blood

Chorionic villi create an extensive branching exchange surface. Fetal capillaries run within the villi while maternal blood surrounds them in the intervillous space. Efficient exchange depends on adequate flow on both sides, a large functional surface area, a short diffusion distance, and favorable concentration gradients.

1.2 Placental endocrine and metabolic roles

The placenta produces hormones and signaling molecules that help maintain pregnancy and adapt maternal metabolism. It also transports glucose, amino acids, fatty acids, electrolytes, and micronutrients using diffusion, facilitated diffusion, and active transport. Placental transport is selective, but it is not a perfect barrier; many medications and toxins can cross.

2. Uteroplacental circulation

Normal placentation transforms the maternal spiral arteries into high-capacitance, low-resistance vessels. Near term, the uterus and placenta receive a substantial fraction of maternal cardiac output. Uteroplacental blood flow has limited autoregulatory capacity and depends strongly on maternal perfusion pressure, maternal cardiac output, uterine vascular resistance, and maternal position.

2.1 Factors that reduce uterine blood flow

2.2 Contractions and placental reperfusion

During a contraction, increased uterine pressure compresses maternal vessels and transiently reduces intervillous perfusion. A healthy fetus normally tolerates this because oxygen is stored in fetal blood and tissues, and flow recovers between contractions. Tachysystole, prolonged contractions, maternal hypotension, or placental disease can prevent adequate recovery and produce recurrent fetal heart-rate abnormalities.

Why can left lateral positioning improve fetal status even when no fetal medication is given?

Answer: It can relieve vena-caval compression, improve maternal preload and cardiac output, increase uterine perfusion, and therefore improve placental oxygen transfer.

3. Umbilical cord physiology

3.1 Cord compression

Cord compression can reduce blood flow through the thin-walled umbilical vein and, with greater compression, the arteries. Intermittent compression classically produces variable decelerations. Persistent compression, prolapse, knotting, entanglement, or oligohydramnios can produce sustained fetal compromise.

3.2 Transport implications

• Avoid unnecessary pressure over the gravid abdomen.
• Secure the patient without compressing the uterus or dislodging fetal-monitor transducers.
• Recognize abrupt fetal heart-rate changes after patient movement, stretcher loading, or rupture of membranes.
• When cord prolapse is suspected, prioritize pressure relief, maternal positioning, immediate obstetric communication, and rapid delivery capability according to protocol.

4. Fetal circulation

Fetal circulation is arranged around placental—not pulmonary—gas exchange. The ventricles contribute to a combined cardiac output with substantial parallel flow. Three shunts direct blood around organs that are not yet functioning in their postnatal roles.

4.1 Preferential streaming

Oxygen-rich inferior vena-caval blood is preferentially directed across the foramen ovale toward the left atrium, left ventricle, ascending aorta, coronary arteries, and brain. Blood returning through the superior vena cava is more likely to enter the right ventricle and pulmonary artery, then pass through the ductus arteriosus to the descending aorta.

4.2 Why pulmonary flow is low before birth

The fetal lungs are fluid-filled and unexpanded, and alveolar oxygen tension is low. Pulmonary vascular resistance remains high, so only a minority of right-ventricular output flows through the lungs. At birth, lung expansion and rising oxygen tension reduce pulmonary vascular resistance, initiating the transition covered in Chapter 6.

Which fetal shunt directly routes blood from the pulmonary artery into the aorta?

Answer: The ductus arteriosus.

5. Fetal oxygen transport

Fetal arterial oxygen tension is lower than adult arterial oxygen tension, yet fetal tissues receive adequate oxygen through several adaptations: fetal hemoglobin has higher oxygen affinity, fetal hemoglobin concentration is relatively high, cardiac output per kilogram is high, and blood flow is preferentially distributed toward vital organs.

5.1 Placental gas transfer

Oxygen diffuses from maternal blood to fetal blood, while carbon dioxide moves in the opposite direction. Maternal unloading and fetal loading are supported by differences in hemoglobin affinity and the exchange of carbon dioxide and hydrogen ions across the placenta. Efficient transfer still requires adequate flow and functional placental surface area.

5.2 Oxygen delivery equation applied to the fetus

A problem can therefore result from reduced flow, reduced oxygen content, or both. Maternal hypoxemia reduces the oxygen gradient. Maternal anemia reduces available oxygen content. Maternal hypotension reduces uterine flow. Placental disease increases resistance or reduces exchange area. Cord compression reduces umbilical flow.

6. Fetal response to hypoxemia

The fetus has compensatory mechanisms but limited reserve. The clinical trajectory depends on severity, duration, gestational age, placental reserve, and the ability to restore maternal and placental conditions.

6.1 Brain-sparing is not reassurance

Redistribution toward the brain may temporarily preserve essential organs in chronic placental insufficiency. It is evidence of adaptation to stress—not proof that the underlying condition is safe. Severe or prolonged compromise eventually overwhelms the response.

6.2 Fetal heart-rate changes as physiology

Fetal heart-rate patterns reflect autonomic control, oxygenation, acid-base status, medications, sleep cycles, gestational age, and mechanical influences such as head or cord compression. A tracing should be interpreted as a trend in the context of maternal physiology and contractions, not as an isolated number.

7. Placental resistance and Doppler concepts

Umbilical artery Doppler evaluates resistance within the fetoplacental circulation. Normal placental development produces progressively lower resistance and forward end-diastolic flow. Decreased, absent, or reversed end-diastolic flow indicates increasing placental resistance and is associated with placental insufficiency and fetal growth restriction.

Transport clinicians may receive Doppler results from the sending facility rather than perform the study themselves. These results should influence urgency, destination capability, monitoring, corticosteroid or magnesium planning when ordered, and the threshold for escalation.

8. Maternal events and predicted fetal effects

Maternal or placental event	Primary physiologic effect	Possible fetal consequence
Maternal hypoxemia	Lower oxygen available for placental diffusion	Tachycardia, decelerations, bradycardia, acidosis
Maternal hypotension	Reduced uterine perfusion pressure	Late decelerations, reduced variability, prolonged bradycardia
Severe anemia or hemorrhage	Reduced maternal oxygen content and circulating volume	Fetal hypoxemia before dramatic maternal hypotension
Supine aortocaval compression	Reduced venous return and cardiac output	Rapid fetal deterioration that may improve with position
Uterine tachysystole	Inadequate reperfusion between contractions	Recurrent late or prolonged decelerations
Placental abruption	Loss of functional exchange area plus maternal hemorrhage	Acute hypoxia, bradycardia, fetal death
Maternal fever or sepsis	Higher metabolic demand, vasodilation, possible hypoperfusion	Fetal tachycardia and progressive compromise
Cord compression or prolapse	Reduced umbilical venous and arterial flow	Variable decelerations or sustained bradycardia

9. Transport assessment and interventions

9.1 Verify the signal

External fetal monitors can display artifact during movement. Compare the fetal rate with the maternal pulse or ECG, assess transducer placement, listen with Doppler when appropriate, and treat a persistent concerning rate as real until verified otherwise. Do not allow artifact concerns to delay escalation when the clinical picture is deteriorating.

9.2 Treat the maternal mechanism

There is no universal “fetal resuscitation drug” for transport. Correct the maternal cause: relieve vena-caval compression, restore oxygenation and ventilation, control hemorrhage, support perfusion, treat seizure or sepsis, stop a causative medication when ordered, reduce excessive uterine activity according to protocol, and expedite obstetric capability.

10. Evolving transport case

Phase 1: Known placental disease

The patient has severe fetal growth restriction with absent end-diastolic flow. This indicates high placental resistance and limited reserve. The team confirms continuous fetal monitoring, emergency delivery capability at the destination, antenatal medication history, neonatal readiness, and the closest alternate obstetric facility.

Phase 2: Maternal deterioration

Vomiting is followed by SpO₂ 88%, blood pressure 84/50 mm Hg, and fetal late decelerations. The team positions the patient laterally, administers oxygen, assesses ventilation, verifies lines and medications, evaluates for hemorrhage or medication effect, and communicates the change.

Phase 3: Fetal decompensation

Maternal oxygen saturation and blood pressure improve, but fetal heart rate remains 85–90/min. The team verifies that the signal is fetal, continues maternal support, and compares the original destination 35 minutes away with an obstetric surgical facility 9 minutes away. Persistent bradycardia changes the mission from specialty transfer to time-critical access to delivery capability.

11. High-yield chapter summary

1. Maternal and fetal blood normally remain in separate circulations within the placenta.
2. Maternal blood bathes chorionic villi; fetal blood flows through capillaries within those villi.
3. Uteroplacental flow depends heavily on maternal cardiac output and perfusion pressure.
4. The umbilical cord normally contains two arteries and one vein.
5. The umbilical vein carries oxygenated blood toward the fetus; the arteries return blood to the placenta.
6. The ductus venosus partially bypasses the liver, the foramen ovale bypasses the right-to-left atrial route, and the ductus arteriosus bypasses the lungs.
7. Fetal circulation functions largely in parallel because the placenta is the gas-exchange organ.
8. Fetal hemoglobin has higher oxygen affinity than adult hemoglobin.
9. Maternal hypoxemia, hypotension, anemia, fever, acidosis, position, uterine hyperactivity, and placental disease can impair fetal oxygen delivery.
10. Fetal redistribution or brain-sparing is compensation, not proof of safety.
11. Persistent bradycardia may represent loss of fetal reserve and requires immediate escalation.
12. Correcting maternal physiology is usually the fastest fetal intervention.

12. Embedded knowledge checks

Which vessel carries the most oxygenated blood in fetal circulation?

Answer: The umbilical vein, which carries blood from the placenta toward the fetus.

Why can maternal hypotension produce late fetal decelerations?

Answer: Lower maternal perfusion pressure reduces uterine and intervillous blood flow, limiting placental oxygen transfer during contractions.

What are the three major fetal shunts?

Answer: The ductus venosus, foramen ovale, and ductus arteriosus.

What does reversed end-diastolic umbilical artery flow suggest?

Answer: Markedly increased placental resistance and severe fetoplacental compromise.

References

1. International Board of Specialty Certification. Maternal Fetal Transport Microcredential Candidate Handbook. Updated April 2026.
2. Nye GA, Ingram E, Johnstone ED, et al. Human placental oxygenation in late gestation: experimental and theoretical approaches. Journal of Physiology. 2018. https://pmc.ncbi.nlm.nih.gov/articles/PMC6265570/
3. Jensen OE, Chernyavsky IL. Blood flow and transport in the human placenta. Annual Review of Fluid Mechanics. 2019. https://pmc.ncbi.nlm.nih.gov/articles/PMC7615669/
4. Zhang D, et al. Recasting current knowledge of human fetal circulation. 2023. https://pmc.ncbi.nlm.nih.gov/articles/PMC10299027/
5. Thornburg KL, et al. Uteroplacental circulation and fetal vascular function and development. 2013. https://pmc.ncbi.nlm.nih.gov/articles/PMC7527077/
6. Mielke G, Benda N. Cardiac output and central distribution of blood flow in the human fetus. Circulation. 2001;103:1662–1668. https://doi.org/10.1161/01.CIR.103.12.1662
7. Pearce P, et al. Image-based modeling of blood flow and oxygen transfer in feto-placental capillaries. 2016. https://pmc.ncbi.nlm.nih.gov/articles/PMC5082864/
8. Couper S, et al. The effects of maternal position in late gestation on placental oxygen transfer. 2021. https://pmc.ncbi.nlm.nih.gov/articles/PMC7613407/
9. Society for Maternal-Fetal Medicine. Consult Series #52: Diagnosis and management of fetal growth restriction. https://publications.smfm.org/publications/289-society-for-maternal-fetal-medicine-consult-series-52/
10. American College of Obstetricians and Gynecologists. Delayed umbilical cord clamping after birth. Committee Opinion. 2020. https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2020/12/delayed-umbilical-cord-clamping-after-birth