SECTION 2: EMBRYO, THE DEVELOPMENTAL AND PHYSIOLOGICAL CHARACTERISTICS OF THE FETUS
1. Embryo and the characteristics of fetal development
The embryonic period lasts 8 weeks which is also the critical time for organogenesis. From 9 weeks, it is the fetal period. Different organs mature during this time.
By the end of the fourth week, the embryonic disc and the body stalk is present.
By the end of 8 Weeks, fetal outlines developand the fetal heart beat could be observed by ultrasound.
12 Gestational Weeks, The external genitalia are beginning to show definitive signs of male or female gender. The fetus begins to make spontaneous movements.
16 Gestational Weeks, gender can be determined by experienced observers by inspection of the external genitalia, the fetus begins to make spontaneous movements.
20 Gestational Weeks, fetal heart beat could be detected.
24 Gestational Weeks, organogenesis is nearly completed.
28 Gestational Weeks, a fetus born at this time will attempt to breathe. However, morphological or functional immaturity at this stage leads to respiratory distress syndrome and death.
By the end of 32 Gestational Weeks, most fetuses could survive delivery.
By the end of 36 Gestational Weeks, testis migrate into the scrotum. The newborns can cry and suck by themselves. Sur vival ability is good.
40 Gestational Weeks, the fetus is fully mature: the newborn can cry loudly, and suck well, and also could survive easily.
A quick estimate of the due date of a pregnancy is based on:
The crown-rump length of the fetus, the crown heel measurement of the first 5 months 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.
2. Fetal physiology
2.1 Circulation system
2.1.1 Characteristics of anatomy:
This consists of the placenta connected to the fetus by the umbilical cord. The umbilical cord contains two umbilical ar-teries and one vein. The umbilical vein trensports oxygenated blood entering into the liver, proceds as the ductus venosus to join the inferior vena cava.
（1）A single umbilical vein to the fetus, after obliteration, the umbilical vein forms the ligamentum teres and the ductus venosus becomes ligamentum venosum.
（2）Two umbilical arteries, arise from the hypoastric areteries directly from the aorta. They carry the relatively deoxygenated to the placenta for oxygenation.At birth when the cord is tied off the vessels are blocked.Actual obliteration takes about 2-3months, the distal parts form the lateral umbilical ligaments and the proximal parts remain open sa superior vesical arteries.
（3）The ductus arteriosus locates between the pulmonary artery and arch of the aorta, after obliteration it becomes ligamentum arteriosum.
（4）Foramen ovale; is the communication between the ritht and left atria. Functional closure occurs soon after birth but anatomical closure occurs in about 6 months time.
2.1.2 Characteristics of blood circulation
Oxygen and nutrient materials required for fetal growth and maturation are delivered from the placenta. There are three major vascular shunts--the portal sinus carries blood to the hepatic veins primarily on the left side of the liver, then flows back into the inferior vena cava. The ductus venosus is the major branch of the umbilical vein and traverses the liver to enter the inferior vena cava directly.
No pure arterial blood but arteriovenous mixed blood exists in fetus body because of the opening of the foramen ovale, the ductus arteriosus and the ductus venosus
2.2 Blood system
2.2.1 Erythropoiesis. This process is controlled primarily by fetal erythropoietin because maternal erythropoietin does not cross the placenta. Serum levels of erythropoietin increase with fetal maturity, as do the numbers of responsive erythrocytes.
2.2.2 Fetal Hemoglobin
Hemoglobin including original hemoglobin, fetal hemoglobin, adult hemoglobin are produced in proerythroblast, proerythroblast and reticulocyte.
2.2.3 Leucocytes appear after 2 months of gestation. The white cells are a major source of antibody formation. The fetus,however, rarely forms antibody because of relatively sterile environment. After 12 weeks of gestation, thymus gland and spleen can produce lymphocytes. This is the main source of antibody.
2.3 Respiratory system
Oxygen in maternal blood and carbon dioxide in fetus blood are exchanged in the placenta. Respiratory tract, pulmonary circulation and respiratory muscle must be matured before delivery, since they begin to function only then. Their maturity is essential for the fetus to survive.
2.4 Gastrointestinal System
As early as 10-12 weeks, the fetus swallows amniotic fluid. The meconium appears from 20th week and at term, it is distributed uniformly throughout the gut up to the rectum in-dicating the presence of intestinal peristalsis. In the intrauterine hypoxia, the anal sphincter is relaxed at the meconiummay be voided into the liquor amni.
2.5 Urinary System
From the 11 Gestational Weeks onwards, the fetal kidneys produce urine, Urine usually is found in the bladder even in small fetuses. This plays an important role in the formation and circulation of amniotic fluid.
2.6 Endocrine system
The thyroid gland is able to synthesize hormones by 12 weeks, thyroxine has been detected in fetal serum as early as 12 weeks. Insulin which is secreted by the pancreas in fetal plasma can be detected at 12 weeks.
2.7 Development of genitalia
The differentiation and development of testis of male fetus starts at 9 gestational weeks, Testicular migrates to the scrotum at about 32 gestational weeks.
The differentiation and development of primary ovary of female fetus starts at 11 to 12 gestational weeks. The two mullerian ducts develop into female genital tract.
（冯祥 潘苗苗）