The placenta is a temporary organ that develops during pregnancy, connecting the developing fetus to the uterine wall. This structure serves as the interface between the maternal and fetal circulatory systems, supporting the fetus until birth. The fetal circulatory system differs from the adult system, as the fetus depends entirely on this organ for gas exchange and life support. The placenta facilitates all nutrient and waste exchange, but it does not possess the capacity to pump blood itself.
The Placenta’s Role in Blood Exchange
The placenta functions solely as an exchange organ, structurally lacking the muscular tissue necessary to generate a heartbeat. Its main purpose is to facilitate the transfer of materials between the mother’s blood and the fetus’s blood without the two supplies ever mixing.
Nutrients (oxygen, glucose, vitamins) from the mother’s circulation diffuse across the placental membrane into the fetal blood. Waste products like carbon dioxide and metabolic waste move from the fetal blood across the membrane back into the mother’s bloodstream for elimination.
This exchange occurs over a vast surface area within the placenta’s structures, which contain fetal blood vessels bathed by the mother’s blood. The placenta’s role is passive, relying on pressure and flow gradients to move substances, not on an internal pump.
The Source of Circulatory Power
All blood flow traveling through the placenta is driven entirely by the rhythmic contractions of the developing fetal heart. The fetal heart begins beating early in pregnancy, establishing the pressure needed to propel blood through the umbilical cord and into the placental network.
The umbilical cord contains three vessels: two umbilical arteries and one umbilical vein. The arteries carry deoxygenated blood away from the fetus to the placenta for gas and waste exchange. The vein carries the enriched, oxygenated blood back to the fetal circulation, completing the circuit powered by the fetal pump.
The blood pressure within the fetal aorta, which drives this circulation, is relatively low, starting at approximately 30 mmHg at twenty weeks of gestation and rising to around 45 mmHg by term. The fetal heart must work rapidly to maintain this low-resistance system, sustaining a rate between 110 and 160 beats per minute. This swift, steady pumping action ensures the continuous exchange of life-sustaining materials across the placental barrier.
Understanding Prenatal Heartbeat Sounds
The confusion about a placental heartbeat often arises from the sounds heard during prenatal checkups using a Doppler ultrasound device. The primary sound medical professionals listen for is the fetal heart rate, which is a fast, distinct, and rhythmic “galloping” sound. This sound is the amplified noise of the fetal heart valves opening and closing.
However, the Doppler device, which uses high-frequency sound waves to detect movement, also picks up the sound of blood moving rapidly through vessels. A common sound often mistaken for a separate beat is the whooshing or swishing noise caused by the turbulent blood flow through the umbilical cord vessels. This “whooshing” is a passive sound created by the blood being pushed through the placenta by the fetal heart, not a beat generated by the placental tissue itself.
Sometimes, the Doppler may also pick up the mother’s own pulse from nearby arteries, which is much slower than the fetal heart rate and is distinguishable. When a healthcare provider hears a sound and identifies it as the “placental flow,” they are referring to the sound of the blood coursing through the umbilical vessels at the placenta, all of which is ultimately propelled by the fetal heart.