The Placenta and Fetal Circulation

Before birth, the fetus does not obtain oxygen by breathing air. Oxygen and nutrients reach the fetus through the placenta and umbilical cord. Waste products move back towards the pregnant person for removal.

The placenta is a temporary organ The placenta develops within the uterus during pregnancy. It attaches to the uterine wall and connects to the fetus through the umbilical cord. The placenta is not simply a bag of maternal blood. It contains branching fetal blood vessels surrounded by spaces supplied with maternal blood. This arrangement brings the two circulations close enough for exchange while normally keeping the blood itself separate. The placenta grows and changes throughout pregnancy. Its ability to exchange substances depends on its surface area, blood flow and the condition of its tissues. After the baby is born, the placenta separates from the uterine wall and is delivered during the third stage of labour. Maternal and fetal blood normally remain separate Maternal blood enters spaces around microscopic placental structures called chorionic villi. Each villus contains small fetal blood vessels. The walls separating maternal blood from fetal blood become thin enough for many substances to cross. However, maternal blood does not normally flow directly into the fetal circulation. This separation matters because the two people may have different blood groups, blood pressures and immune characteristics. Small amounts of fetal cells or genetic material can still enter the maternal circulation. Small amounts of maternal material may also cross in the other direction. The placental boundary is therefore a selective exchange surface, not an impenetrable wall. How substances cross the placenta Oxygen moves from maternal blood towards fetal blood because of differences in concentration and blood flow. Carbon dioxide moves in the opposite direction. Glucose, amino acids, fatty acids, vitamins, minerals and water also cross through several transport mechanisms. Some substances move by diffusion. Others use carrier proteins or energy dependent transport. Waste products from fetal metabolism return through the placenta. The pregnant person's lungs, liver and kidneys then help remove them. The placenta does not digest food or breathe on behalf of the fetus. It transfers substances between two circulations. The placenta is not a perfect filter The placenta reduces exposure to some substances, but it does not block everything harmful. Many medicines cross the placenta. Alcohol, nicotine, carbon monoxide and some recreational drugs can also reach the fetus. Some viruses, parasites and other infections can cross or affect placental function. Maternal antibodies of the IgG type cross the placenta and provide temporary immune protection after birth. Whether a substance crosses depends on features such as its size, chemistry, concentration and the stage of pregnancy. Calling the placenta a filter can therefore be misleading. It is better understood as an active exchange and hormone producing organ. Hormones produced by the placenta The placenta produces hormones that help maintain pregnancy and alter the pregnant body. Early in pregnancy, it produces human chorionic gonadotrophin, usually shortened to hCG. hCG supports hormone production from the ovary until the placenta can produce enough progesterone itself. Progesterone helps maintain the uterine lining and reduces inappropriate uterine contractions. Placental oestrogens contribute to uterine growth, blood flow changes and breast development. The placenta also produces human placental lactogen, which helps alter maternal metabolism so nutrients remain available to the fetus. These hormone effects are coordinated. One hormone result rarely explains how well the whole pregnancy is progressing. The umbilical cord The umbilical cord links the fetal circulation to the placenta. Most cords contain three vessels: one umbilical vein and two umbilical arteries. The names follow the direction of blood flow relative to the fetal heart. The umbilical vein carries relatively oxygen rich blood from the placenta towards the fetus. The umbilical arteries carry blood lower in oxygen from the fetus back towards the placenta. This is an important exception to the common misconception that every vein carries deoxygenated blood and every artery carries oxygenated blood. The vessels are surrounded by a soft protective substance called Wharton's jelly. Occasionally, a cord contains one artery rather than two. This can occur in an otherwise healthy pregnancy but may lead to additional assessment or growth monitoring. Why fetal circulation is different Before birth, the placenta performs the main gas exchange role. The fetal lungs are developing, but they are filled with fluid rather than air. Blood vessels within the lungs remain relatively constricted. This creates high resistance to blood flow through the lungs. Fetal circulation uses temporary pathways called shunts to direct much of the blood away from the lungs and towards the body. The three main temporary pathways are the ductus venosus, the foramen ovale and the ductus arteriosus. These pathways are normal before birth. From the placenta to the fetal heart Oxygen rich blood travels from the placenta through the umbilical vein. Some blood passes through the liver. A substantial proportion uses the ductus venosus, which directs blood towards the inferior vena cava. The inferior vena cava carries this blood into the right atrium of the fetal heart. The fetal circulation then directs the most oxygenated stream preferentially towards the left side of the heart. The foramen ovale The foramen ovale is an opening between the right and left atria. It allows blood entering the right atrium to pass directly into the left atrium. From the left atrium, blood moves into the left ventricle and then the aorta. This route supplies the coronary circulation, brain and upper body with blood containing relatively more oxygen. Not all blood follows this path. Fetal blood streams mix, and oxygen levels are lower than those seen after normal breathing begins. The ductus arteriosus Blood returning from the upper body enters the right atrium through the superior vena cava. Much of this blood passes into the right ventricle and then the pulmonary artery. Because resistance in the fetal lungs is high, only a limited proportion enters the lung circulation. Most passes through the ductus arteriosus, a vessel connecting the pulmonary artery to the descending aorta. The descending aorta supplies the lower body before blood returns through the umbilical arteries to the placenta. Fetal arteries and veins still follow the direction rule An artery carries blood away from the heart. A vein carries blood towards the heart. This definition remains true in fetal circulation. Oxygen content varies according to where the vessel lies within the circuit. The umbilical vein carries the most oxygenated blood in fetal circulation towards the heart. The umbilical arteries carry blood away from the fetal heart towards the placenta. The placenta also transfers heat and hormones The placenta helps transfer heat between the two circulations. It also transfers hormones, medicines and metabolic substances in both directions. Some maternal conditions affect the placental environment. High blood pressure, diabetes, infection, smoking and autoimmune disease can alter placental function or fetal growth. These factors change risk rather than predicting one outcome with certainty. Placental position The placenta can attach to the front, back, side or upper part of the uterus. These positions are usually normal. A placenta close to or covering the cervix is called placenta praevia. Many placentas described as low at the mid pregnancy scan are no longer low later because the uterus enlarges. A persistently low placenta can affect the safest method and timing of birth. It can also cause painless vaginal bleeding, which requires urgent maternity assessment. Placental abruption A placental abruption occurs when part or all of the placenta separates from the uterine wall before birth. It may cause vaginal bleeding, abdominal pain, a tense uterus or changes in fetal movement. Bleeding may be concealed inside the uterus, so visible blood loss does not always reflect severity. Placental abruption requires urgent assessment because it can affect both maternal circulation and fetal oxygen delivery. Placental function and fetal growth The placenta must receive adequate maternal blood flow and maintain effective exchange. When placental function is reduced, fetal growth may slow. This is called fetal growth restriction when the fetus does not reach its growth potential because of a pathological process. A fetus can be small without being growth restricted. Family size, gestational age and measurement variation also matter. Clinicians use repeated growth measurements, amniotic fluid assessment and Doppler ultrasound when indicated. Doppler studies assess patterns of blood flow. They do not directly measure every aspect of placental health. Fetal movements provide important information Fetal movement patterns usually become more recognisable as pregnancy progresses. There is no single number of movements that is normal for every pregnancy. A significant reduction or change from the usual pattern requires immediate contact with maternity services. Reduced movement does not prove placental failure. It is an important signal that needs assessment. What happens at birth When the baby begins breathing, resistance within the lungs falls. More blood travels through the pulmonary circulation. Clamping the umbilical cord removes the placenta from the circulation. Pressure changes then encourage the foramen ovale, ductus arteriosus and ductus venosus to close. These changes begin around birth but are not always completed instantly.

The placenta supports exchange between two normally separate circulations, while fetal shunts direct blood through the body before the lungs begin working after birth.