Placental Physiology
• The placenta has several functions, including the maternal-fetal transfer of nutrients and oxygen, the clearance of fetal waste, and the synthesis of proteins and hormones.
• The human placenta is classified as hemochorioendothelial because only three cell layers separate the maternal and fetal circulations: fetal trophoblast, fetal villous stroma, and fetal capillary endothelium. Fetal villi are suspended in intervillous spaces bathed with maternal blood
• Placental villi create a high surface area/volume ratio with a total surface area at the term of around 10 m2.
• Transfer across the placenta occurs by passive diffusion (oxygen, CO2, electrolytes, simple sugars), active transport (iron, vitamin C), or carrier-mediated facilitated diffusion (immunoglobulins).
• There is a large placental reserve; 30–40% of placental villi can be lost without evidence of placental insufficiency.
Fetal Physiology
Below we explain the important functions and they work in the different parts of the fetus.
Nutrition
• The embryo consists almost entirely of water. After 10 weeks, however, the fetus is dependent on nutrients from the maternal circulation via the developing placenta.
• The average term fetus weighs 3,400 g. Birthweight is influenced by race, socioeconomic status, parity, genetic factors, diabetes, smoking, and fetal gender. At term, the fetus grows around 30 g/day.
Cardiovascular System
• The fetal heart starts beating at 4–5 weeks’ gestation.
• The fetoplacental blood volume at term is 120 mL/kg (or a total of approximately 420 mL).
• After birth, the fetal circulation undergoes profound hemodynamic changes. The umbilical vessels, ductus arteriosus, foramen ovale, and ductus venosus constrict. This is thought to be due to a change in oxygen tension within minutes of birth. The distal portions of the umbilical arteries atrophy within 3–4 days to become the umbilical ligaments, and the umbilical vein becomes the ligamentum teres. The ductus venosus is functionally closed within 10–90 hours of birth, but anatomic closure and formation of the ligamentum venosum is achieved only by 2–3 weeks of life.
Respiratory System
• Within minutes of birth, the fetal lungs must be able to provide oxygen and eliminate CO2 if the fetus is to survive.
• Movements of the fetal chest can be detected at 11 weeks. The ability of the fetus to “breathe” amniotic fluid into the lungs at 16–22 weeks appears to be important for normal lung development. Pulmonary hypoplasia may result if this does not occur.
• Surfactant is a heterogeneous detergent-like substance that lowers alveoli surface tension and prevents alveoli collapse after birth. It is made in the lungs by type II pneumocytes.
• Functional maturation results in an increase in surfactant in the lungs. Respiratory compromise due to surfactant deficiency is known as hyaline membrane disease (HMD) or respiratory distress syndrome (RDS), and is seen primarily in premature infants. Antenatal corticosteroid therapy promotes surfactant production and decreases the risk of RDS by 50%.
Fetal Blood
• Sites of hemopoiesis change with gestational age.
• The hemoglobin of fetal blood rises to the adult level of 15 g/dL by mid-pregnancy and increases to 18 g/dL at term. The average fetal hematocrit is 50%.
• Hemoglobin F (fetal hemoglobin) has a higher affinity for oxygen than hemoglobin A (adult hemoglobin). Hemoglobin A is present in the fetus from 11 weeks and increases linearly with increasing gestational age. A switch from hemoglobin F to hemoglobin. A begins at around 32–34 weeks. By term, 75% of the total hemoglobin is hemoglobin A.
• The average fetal hematocrit is 50%.
Gastrointestinal System
• The small intestine is capable of peristalsis by 11 weeks. By 16 weeks, the fetus can swallow.
• The fetal liver absorbs drugs rapidly but metabolizes them slowly because the hepatic pathways for drug detoxification and inactivation are poorly developed until late in fetal life.
• During the last trimester, the liver stores large amounts of glycogen and the enzyme pathways responsible for glucose synthesis mature. Genitourinary system
• Fetal urination starts early in pregnancy, and fetal urine is a major component of amniotic fluid, especially after 16 weeks.
• Renal function improves slowly as the pregnancy progresses.
Nervous System
• Neuronal development continues throughout gestation and at least into the second year of extrauterine life. The development of the central nervous system requires normal thyroid activity.
• The fetus can perceive sounds at 24–26 weeks. By 28 weeks, the fetal eye is sensitive to light.
• Gonadal steroids are the major determinant of sexual behavior.
Immune System
• Fetal IgG (immunoglobulin G) is derived almost exclusively from the mother. Receptor-mediated transport of IgG from mother to fetus begins at 16 weeks’ gestation, but the bulk of IgG is acquired in the last 4 weeks of pregnancy. As such, preterm infants have very low circulating IgG levels. IgM (immunoglobulin M) is not actively transported across the placenta. As such, IgM levels in the fetus accurately reflect the response of the fetal immune system to infection.
• B lymphocytes appear in the fetal liver by 9 weeks, and in the blood and spleen by 12 weeks. T cells leave the fetal thymus at around 14 weeks.
• The fetus does not acquire much IgG (passive immunity) from colostrum, although IgA (immunoglobulin A) in breast milk may protect against some enteric infections.
Endocrine system
• Both oxytocin and vasopressin are secreted by the fetal neurohypophysis by 10–12 weeks.
• The fetal thyroid begins functioning at 12 weeks. Very little fetal thyroid hormone is derived from the mother.