Overview
▶ Age of the Fetus
▶ Growth of the Fetus
▶ Fetal Physiology
- Nutrition
- Fetal Blood
- Leukocytes and fetal defense
- Urinary system
- Skin
- Gastrointestinal Tract
- Respiratory System
- Fetal Endocrinology
- Fetus CNS
Three periods are distinguished in the prenatal development of the fetus.
- Ovular period or germinal period- which lasts for the 2 weeks following ovulation. In spite of the fact that the ovum is fertilized, it is still designated as ovum.
- Embryonic period- begins at 3rd week following ovulation and extends up to 10 weeks of gestation (8 weeks post-conception). The crown-rump length (CRL) of the embryo is 4 mm.
- The fetal period begins after the 8thweek following conception and ends in delivery. The chronology in the fetal periods is henceforth expressed in terms of menstrual age and not in embryonic age.
How to determine the length of the Fetus?
To determine the length of the fetus, the measurement is commonly taken from the vertex to the coccyx (crown-rump length)in earlier weeks. While from the end of the 20th week onwards, the measurements are taken from the vertex to the hell (crown-heel length)
How to measure the first 5 months of Fetus CH?
The crown-heel (CH) measurement of the first 5months is calculated by squaring the number of the lunar months to which the pregnancy belongs. In the second half, the same is calculated by multiplying the lunar months by 5. The length is expressed in centimeters.
How to calculate the age of the Fetus?
Gestational age is the duration of pregnancy calculated from the first day of the last menstrual period (LMP). It is greater than the post conception (fertilization) age by 2 weeks. The length is a more reliable criterion than the weight to calculate the age of the fetus. In the first trimester, CRL (mm)+6.5 =gestational age in weeks. Assessment of gestational age by sonography has been discussed.
How is normal Fetus growth is Characterized?
Normal fetal growth is characterized by cellular hyperplasia followed by hyperplasia and hypertrophy and lastly by hypertrophy alone. The fetal growth increases linearly until the 37th week. It is controlled by genetic factors in the first half and by environmental factors in the second half of pregnancy. Imprinted Genes (maternal or paternally acquired alleles)primarily control fetal growth. The important physiological factors are Race (European babies are heavier than Indians); sex (male baby weighs>female); parental height and weight (tall and heavier mothers have heavier babies); birth order (weight rises from to second pregnancy)and socioeconomic factors (heavier babies in social class I and II). Fetal growth is predominantly controlled by IGF-1, insulin, and other growth factors. Growth hormone is essential for postnatal growth. At term, the average fetal weight in india varies from 2.5 to 3.5 kg. Pathological factors affect it adversely.
Types of Fetal nutrition
There are three stages of fetal nutrition following fertilization:
- Absorption: In the early post-fertilization period, nutrition is stored in deutoplasm within the cytoplasm and very little extra nutrition needed is supplied from the tubal and uterine secretion.
- Histotrophic transfer: The following nidation and before the establishment of uteroplacental circulation, nutrition is derived from eroded decidua by diffusion and later on from the stagnant maternal blood in the trophoblastic lacunae.
- Amyotrophic: With the establishment of fetal circulation, nutrition is obtained by active and passive transfer from the 3rd week onwards.
The fetus is a separated physiological entity and it takes what it needs from the mother even at the cost of reducing her resources. While all the nutrients are reaching the fetus throughout the intrauterine period, the demand is not squarely distributed. Two-thirds of the total calcium,three-fifths of the total proteins, and four-fifths of the total iron are drained from the mother during the last 3months. Thus, in preterm births, the store of the essential nutrients to the fetus is much low. The excess iron reserve is to compensate for the low supply of iron in breast milk which is the source of nutrients following birth.
From which phase Hematopoiesis is demonstrated?
Hematopoiesis is demonstrated in the embryonic phase first in the yolk sac by the 14th day. By the 10th week, the liver becomes the major site. The great enlargement of the early fetal liver is due to its erythropoietic function. Gradually, the red cell production sites extend to the spleen and bone marrow and near term, the bone marrow becomes the major site of red cell production. In the early period, the erythropoiesis is megaloblastic but near term it becomes neuroblastic.The fetal blood picture at term shows RBC 5-6 million/mm3;Hb=16.5-18.5 g%,reticulocytes -5% and erythroblast -10%.During the first half, the hemoglobin is of fetal type (α-2,y-2) but from 24 weeks onwards, the adult type of hemoglobin (α-2, β-2) appears and at term, about 75-80% of the total hemoglobin is of fetal type (HbF). Between 5 and 8 weeks, the embryo manufactures some additional hemoglobin: Hb Gower 1 Hb Gower 2 (a-and chains), and Hb portland. Between 6 and 12 months after birth, the fetal hemoglobin is completely replaced by adult hemoglobin. The fetal hemoglobin has got a greater affinity to oxygen due to the lower binding of 2,3-diphosphoglycerate compared to adult hemoglobin. It is also resistant to alkali in the formation of alkaline hematin. Fetal metabolism is aerobic with arterial blood PO2 of 25-35 mm Hg. There is no metabolic acidosis. Total fetoplacental blood volume at term is estimated to be 125 mL/kg body weight of the fetus. The red cells develop their group antigen quite early and the presence of Rh factor has been demonstrated in the fetal blood from as early as 38 days after conception. The lifespan of the fetal RBC is about 80 days. The activities of all glycolytic enzymes in fetal erythrocytes except phosphofructokinase and 6- phosphogluconate dehydrogenase are higher than those of adults or term or premature infants.
The cord blood level of iron, ferritin, vitamin B12, and folic acid is consistently higher than maternal blood.
When do leukocytes appear?
Leukocytes appear after 2 months of gestation. The white cell count rises to about 15-20 thousand/mm3 at term. The thymus and spleen soon develop and produce lymphocytes, a major source of antibody formation. The fetus, however, rarely forms antibodies because of the relatively sterile environment. Maternal immunoglobulin G (igG) crosses the placenta from the 12th week onwards to give the fetus a passive immunity which increases with the increase in the gestation period. At-term fetal IgG level is 10% higher than the mother. IgM is predominantly of fetal origin and its detection by cordocentesis may be helpful in the diagnosis of intrauterine infection. IgA is produced only after birth in response to antigens of enteric infection.
When do the nephrons become active and secrete urine?
By the end of the first trimester, the nephrons become active and secrete urine. Near term, the urine production rises to 650mL/day. However, kidneys are not essential for the survival of the fetus in utero but are important in the regulation of the composition and volume of the liquor amnii. Oligohydramnios may be associated with renal hypoplasia or obstructive uropathy.
Which week lanugo appears?
AT 16th week, lanugo(downy thin colorless hairs)appears but near term almost completely disappears. Sebaceous glands appear at the 20th week and the sweat glands somewhat later. Vernix caseosa-the secretion of the sebaceous glands mixed with the exfoliated epidermal cells is abundantly present smearing the skin. The horny layer of the epidermis from the fetal capillaries into the liquor amnii.
At which week fetus swallows amniotic fluid?
As early as 10-12 weeks, the fetus swallows amniotic fluid. The meconium appears from 20th a week and at term, it is distributed uniformly through the gut up to the rectum indicating the presence of intestinal peristalsis. In intrauterine hypoxia the anal sphincter is relaxed and the meconium may be voided into the liquor amnii.
Composition of the meconium
It is chiefly composed of the waste products of the hepatic secretion. It contains lanugo, hairs, and epithelial cells from the fetal skin which are swallowed with the liquor amnii. Mucus exfoliated intestinal epithelium, and intestinal juices are added to the content. The greenish-black color is due to the bile pigments, especially biliverdin.
When do the alveoli expand in the fetus?
In the early months,the lungs are solid. At the 28th week, alveoli expand and are lined by cuboidal
epithelium. There is intimate contact with the endothelium of the capillaries. At 24th
week, lung surfactants related to phospholipids-phosphatidylcholine(lecithin)and
phosphatidylglycerol appear. Surfactant is secreted by type-II alveolar cells. These
substances lower the surface tension of the lung fluid so that the alveoli can be opened up
easily when breathing starts the following delivery. Lecithin: sphingomyelin (L:S) ratio of 2:1 in the
liquor amnii signifies full maturity of the fetal lung. Fetal cortisol is the natural trigger for
augmented surfactant synthesis. Fetal growth restriction and prolonged rupture of
membranes also accentuates surfactant synthesis.
When breathing movements are identified?
Breathing movements are identified by 11 weeks but are irregular until 20thweek. Their frequency varies from 30 to 70 per minute and is dependent on the maternal blood sugar concentration. Hypoxia and maternal cigarette smoking reduce FBM while hyperglycemia increases it. The tracheobronchial tree is filled up with liquor amnii.
What is Fetal endocrinology?
Growth hormone, ACTH, prolactin, TSH, and gonadotropic hormones are produced by the fetal pituitary as early as the 10th week. Vasopressor and oxytocic activity of the posterior pituitary has also been demonstrated as early as 12 weeks. Fetal adrenals show hypertrophy of the reticular zone (fetal zone)which is the side of synthesis of estriol precursor, cortisol, and dehydroepiandrosterone. The adrenal medulla produces a small amount of catecholamines. Fetal thyroid starts synthesizing a small amount of thyroxine by the 11th week. While the fetal ovaries remain inactive, the fetal testicles mediate the development of the male reproductive structures. The fetal pancreas secretes insulin as early as the 12th week and glucagon by 8 weeks.
Fetal central nervous system
Fetal body movements, heart rate accelerations, and breathing movements reflect functions of the fetal CNS. In the later months of pregnancy, fetal activity periods are noted as
a) reactive, and
b) quiet (non-reactive)period.The fetus remains in the active state for about 70%of the time and the quiet period ranges from 15 to 25 minutes. Hypoxemia decreased fetal breathing activity.
Fetal circulation - Umbilical vein?
The umbilical vein carrying the oxygenated blood (80%saturated)from the placenta,enters the fetus at the umbilicus and runs along the free margin of the falciform ligament of the liver. In the liver, it gives off branches to the left lobe of the liver and receives the deoxygenated blood, mixed with some portal venous blood, short circuits the liver through the ductus venosus to enter the heart. The O2 content of this mixed blood is thus reduced. Although both the ductus venosus and hepatic portal/fetal trunk blood enter the right atrium through the IVC receives blood from the right hepatic vein.
In the right atrium, most of the well-oxygenated (75%) ductus venosus blood is preferentially directed into the foramen ovale by the value of the IVC and crista dividends and passes into the left atrium. Here it is mixed with a small amount of venous blood returning from the lungs through the pulmonary veins. This left atrial blood is passed on through the mitral opening into the left ventricle.
The remaining lesser amount of blood (25%), after reaching the right atrium via the superior and inferior vena cava (carrying the venous blood from the cephalic and caudal parts of the fetus respectively)passes through the tricuspid opening into the right ventricle.
During ventricular systole, the left ventricular blood is pumped into the ascending and arch of the aorta and distributed by their branches to the heart, head, neck brain, and arms. The right ventricular blood with low oxygen content is discharged into the pulmonary trunk. Since the resistance in the pulmonary arteries during fetal life is very high, the main portion of the blood passes directly through the ductus arteriosus into the descending aorta bypassing the lungs where it mixes with the blood from the proximal aorta. About 70% of the cardiac output (60% from the right and 10% from the left ventricle) is carried by the ductus arteriosus to the descending aorta. About 40% of the combined output goes to the placenta through the umbilical arteries. The deoxygenated blood leaves the body by way of two umbilical arteries to reach the placenta where it is oxygenated and gets ready for recirculation. The mean cardiac output is comparatively high in the fetus and is estimated to be 350mL/kg/min.
Changing in the fetal - Circulation at birth
The hemodynamics of the fetal circulation undergoes profound changes soon after birth due to
(1) cessation of the placental blood flow and
(2) initiation of respiration.
The following changes occur in the vascular system.
Closure of the umbilical arteries: Functional closure is almost instantaneous, preventing even a slight amount of fetal blood from draining out. Actual obliteration takes about 2-3 months. The distal parts form the lateral umbilical ligaments the proximal parts remain open as superior vesical arteries.
Closure of the umbilical vein: The obliteration occurs a little later than the arteries, allowing a few extra volumes of blood (80-100mL)to be received by the fetus from the placenta.The ductus venosus collapses and the venous pressure of the IVC falls and so also the right atrial pressure. After obliteration, the umbilical vein forms the ligamentum teres and the ductus venosus becomes ligamentum teres and the ductus venosus becomes ligamentum venosum.
Closure of the ductus arteriosus: Within few hours of respiration, the muscle wall of the ductus arteriosus contracts probably in response to the rising oxygen tension of the blood flowing through the duct. The effects of variation of the O2 tension on ductus arteriosus are thought to be mediated through the action of prostaglandins. Prostaglandin antagonists given to the mother may lead to the premature closure of the ductus arteriosus. Whereas functional closure of the ductus may occur soon after the establishment of pulmonary circulation, the anatomical obliteration takes about 1-3 months and becomes ligamentum arteriosum.
Closure of the foramen ovale: This is caused by increased pressure of the left atrium combined with a decreased pressure on the right atrium. Functional closure occurs soon after birth but anatomical closure occurs in about 1 year's time. During the first few days, the closure may be reversible.
Post a Comment