Place the stages of prenatal development in order from earliest to latest.
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FERTILIZATION Show
Biologically speaking, fertilization (or conception) is the beginning of human development. Fertilization normally occurs within several hours of ovulation (some authors report up to 24 hours) when a man's sperm combines with a woman's egg WEEK 2
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Hello, BabyThis mother-to-be is holding an ultrasound image of her fetus. She is nearly nine months pregnant, so the fetus is fully developed and almost ready to be born. The fetus has grown tremendously and changed in many other ways since it was a tiny embryo seven months previously. Defining the Fetal StageA fetus is a prenatal human being between the embryonic stage and birth. The fetal stage extends from the beginning of the ninth week after fertilization to about 38 weeks after fertilization, which is the average time of birth. The fetal stage lasts a total of approximately 30 weeks. Figure \(\PageIndex{2}\) shows a seven-week-old embryo that is just getting ready to begin the fetal stage of development. At 7 weeks the embryo is about 10 mm long and has a big forehead. It is developing the inner ear but not the outer ear. The limb buds are visible. Fetal DevelopmentThe image in Figure \(\PageIndex{3}\) shows a fetus at the start of week 9, the first week of the fetal stage. The fetus is shown larger than its actual size, which from crown to rump is only about 3.2 cm (1.3 in.) long. Even at this early age, however, the fetus has developed to the point of being recognizable as a human being. It possesses virtually all of the major body organs. However, most of the organs are not yet fully developed and functional, and some are not yet situated in their final anatomical locations. These final developments will occur during the remainder of the fetal stage. Weeks 9 to 15During weeks 9 to 15, the fetus’s reproductive organs develop rapidly. The external genitals of male and female fetuses are rather similar in appearance at first, but they will be clearly differentiated by week 12. At that point, the biological sex of the fetus can be determined with almost 100 percent accuracy using obstetric ultrasound. Figure \(\PageIndex{4}\) s shows a fetus at 11 weeks. Other developments that usually occur in the fetus during weeks 9 to 15 after fertilization include the following:
Weeks 16 to 26Many important changes occur in the fetus during weeks 16 to 26 after fertilization. Some of the specific developments that occur during weeks 16 to 26 include the following.
Weeks 27 to 38During weeks 27 to 38 after fertilization, the bones of the fetus complete their development. The fetus also grows rapidly during these final weeks, and its body fat increases substantially. Its formerly wrinkled skin starts to plump out as layers of subcutaneous fat are deposited. Additional changes that occur in the fetus during weeks 27 to 38 include the following:
By the end of week 38, the fetus measures about 51 cm (20 in.) long. A 38-week fetus is pictured in Figure \(\PageIndex{5}\). Fetal CirculationThe heart and blood vessels that form the cardiovascular system are among the earliest organs to develop in the embryo. They continue to develop in complexity and grow in size during the fetal stage. However, until birth, the circulation of blood in the fetus is different than the postnatal circulation will be, primarily because the lungs are not yet in use. The fetus cannot breathe the air because it is floating in amniotic fluid. Instead, the fetus obtains oxygen from the mother’s blood via the placenta and umbilical cord. Prenatal CirculationThe fetal circulation before birth is illustrated in Figure \(\PageIndex{6}\). Oxygen-rich blood from the placenta is carried to the fetus by the umbilical vein. Some of the blood flows through a fetal vein called the ductus venosus, which carries the blood to the inferior vena cava. In turn, the vena cava carries blood to the right atrium of the heart. Throughout the fetal stage, there is an opening between the right and left atria, called the foramen ovale, which allows most of the blood reaching the right atrium to flow directly into the left atrium, thus bypassing the pulmonary circulation. Blood that enters the left atrium is pumped into the left ventricle, and from there through the aorta, the major artery that carries blood to the rest of the body. Blood that reaches the umbilical arteries flows back through the umbilical cord to the placenta, where carbon dioxide and other waste products from the fetus enter the maternal circulation. Not all of the blood reaching the right atrium through the ductus venosus passes directly into the left atrium via the foramen ovale. A small amount of blood is pumped from the right atrium into the right ventricle, and from the right ventricle into the pulmonary arteries. A fetal artery called the ductus arteriosus directs most of this blood away from the nonfunctioning lungs by shunting blood from the pulmonary trunk to the aorta. Postnatal CirculationAfter birth, as the newborn takes the first breath, the blood circulation suddenly changes. There is decreased resistance in the lungs now that the infant is surrounded by air instead of amniotic fluid. The lowered resistance allows more blood to flow into the pulmonary arteries from the right atrium and ventricle, and less to flow through the foramen ovale into the left atrium. Blood now travels to the lungs through the pulmonary arteries and then back to the heart through the pulmonary veins to the left atrium. This produces an increase in pressure in the left atrium that forces the foramen ovale to close. Once the foramen ovale closes, blood can no longer flow through it and bypass the pulmonary circulation. The ductus arteriosus is no longer needed to shunt blood away from the lungs, and it normally closes within a day or two of birth. The ductus venosus usually closes within another couple of days. Birth WeightThe fetal growth rate is one of two major factors that determine the weight of the fetus at birth, or birth weight, which averages about 3.4 kg (7.5 lb.) in a full-term infant. The other factor that determines birthweight is the length of gestation. Infants born before full term, which is defined as 36-40 weeks after fertilization, are usually smaller than full-term infants because they have spent less time growing in the uterus. Pre-term birth is one of the major causes of low birth weight, which is defined as a birth weight lower than 2.5 kg (5.5 lb.), regardless of gestational age. Low birth weight increases the risk of death shortly after birth. As many as 30 percent of deaths in the first month of life occur in preterm infants. Holding the length of gestation constant, a newborn may be classified as small for gestational age, appropriate for gestation age, or large for gestational age. Fetuses that did not grow adequately before birth may end up being small for gestational age, even when they are born at full term. Viability of the FetusFetal viability refers to the point in fetal development at which the fetus is likely to be able to survive outside the uterus. When babies are born too early, they have an elevated risk of dying within the first few hours to weeks of life. The main causes of early mortality in pre-term infants are inadequately developed respiratory and nervous systems. For babies born at 23 weeks of gestation, the chances of surviving are only between 20 and 35 percent, and survival is possible only with intensive, advanced medical care. For babies born at 25 weeks of gestation, the survival chances are much greater — as high as 70 percent — but again, intensive medical intervention is needed (see the newborn infant in Figure \(\PageIndex{7}\). The chances of survival are much better after 26 weeks of gestation. More than 90 percent of babies survive if they are born after 26 weeks and receive any necessary medical care. What a difference just three weeks makes! Review
Explore Morehttps://bio.libretexts.org/link?17806#Explore_More Attributions
This page titled 23.4: Fetal Stage is shared under a CK-12 license and was authored, remixed, and/or curated by Suzanne Wakim & Mandeep Grewal via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request. What is the order of the stages of prenatal development?There are three stages of prenatal development: germinal, embryonic, and fetal. Keep in mind that this is different than the three trimesters of pregnancy.
What is the order of the stages of prenatal development quizlet?This period is generally divided into three stages: the germinal stage, the embryonic stage, and the fetal stage.
What are 3 stages of prenatal development and characteristics of each?Normal prenatal development lasts about 38 weeks and is divided into three stages: germinal, embryonic, and fetal. During these three stages of gestation, the original single-celled zygote develops into an embryo and then fetus [1, 2].
What is the first stage of the three stages of prenatal development?Early Stages
The germinal stage of pregnancy starts at the time of conception, when the sperm and the egg combine to form a zygote. During the germinal stage, the zygote begins to divide in order to implant into the uterine wall. Once implantation is complete, the embryonic stage begins.
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