Third Trimester. Developmental stage. Embryonic Stage. Fetal Stage. Gestational Age weeks. Conceptual Age weeks. Developing Organ s. Central Nervous System. External genitals. Weeks 1 and 2 of Pregnancy. During the first two weeks after the last menstrual period egg follicles mature in the ovaries under the stimulus of follicle-stimulating hormone FSH a hormone secreted by the pituitary gland in the brain.
During the third week, if fertilization occurs , the fertilized egg called a zygote will begin producing the hormone human chorionic gonadotropin the pregnancy hormone Human chorionic gonadotropin first becomes detectable in the mother's blood and urine between 6 and 14 days after fertilization 3 to 4 weeks gestational age.
During the 3rd week the sex of the fetus is determined by the father's sperm, and twins may be formed. Fatigue and swollen or tender breasts are sometimes the first signs of pregnancy. Gestational Age 4 weeks 0. Embryonic Age 2 weeks. The embryo is the size of a pinhead. Most pregnancy tests will be positive at this time. Gestational Age 5 weeks 1.
Embryonic Age 3 weeks. The brain, spine, and heart have begun to form. By the end of the week the heart will be pumping blood. Week 5 is the beginning of the embryonic period which lasts from the the 5th to the 10th week.
It is during this critical period that many birth defects occur in the developing embryo. Most of these birth defects will have no known cause or be due to a combination of factors multifactorial. Embryonic Age 4 weeks. The embryo is now about the size of a pea. Embryonic Age 5 weeks. The embryo is now about 0. By week 7 the trachea and bronchi of the lungs have formed , and the pseudoglandular stage of lung development begins [17] Crown—rump length of 7 mm or greater and no heartbeat, or mean sac diameter of 25 mm or greater and no embryo is considered consistent with early pregnancy loss [20] BACK TO TOP OF PAGE.
Gestational Age 8 weeks 1. Embryonic Age 6 weeks. The average embryo at 8 weeks is 0. The average embryo at 9 weeks is 0. Gestational Age 10 weeks 2.
Fetal Age 8 weeks. The embryo's tail has disappeared and it is now called a fetus. Fingerprints are being formed [11], and bone cells are replacing cartilage. The average fetus at 10 weeks is 1. Gestational Age 11 weeks 2. Fetal Age 9 weeks. The fetus is starting to have breathing movements. It can open its mouth and swallow. The average fetus at 11 weeks is 1. Gestational Age 12 weeks 2. Fetal Age 10 weeks. The fetus is starting to make random movements. The fetus begins to concentrate iodine in its thyroid and produce thyroid hormone at about this time.
The pancreas is beginning to make insulin, and the kidneys are producing urine. The heart beat can usually be heard with and electronic monitor at this time. The average fetus at 12 weeks is 2. Gestational Age 13 weeks 3 months. Fetal Age 12 weeks.
The average fetus at 13 weeks is 2. Organogensis refers to that period of time during development when the organs are being formed. After an egg has been fertilized, and has been implanted in the uterus, the developing form is known as the embryo. Organogenesis takes place during this embryonic phase. In fact, most organogenesis has begun as early as week five in humans remember that a normal human pregnancy lasts an average of 40 weeks. Therefore, damage to any of the organ systems of the body which may ultimately result in some type of birth defect usually strikes during this time frame.
The lymphatic system starts to develop. The primary prenatal development of the sex organs begins. Week Eight By week eight — which is the final week of the embryonic stage — the embryo measures about 20 mm 0. During this week, some of the developments that occur include: Nipples and hair follicles begin to develop.
External ears start to form. The face takes on a human appearance. Fetal heartbeat and limb movements can be detected by ultrasound. All essential organs have at least started to form. Genetic and Environmental Risks to Embryonic Development The embryonic stage is a critical period of development. Children born with this disorder may have cognitive deficits, developmental delays, behavioral issues, and distinctive facial features.
Infection by rubella virus: In adults, rubella German measles is a relatively mild disease, but if the virus passes from an infected mother to her embryo, it may have severe consequences. The virus may cause fetal death, or result in a diversity of birth defects, such as heart defects, microcephaly abnormally small head , vision and hearing problems, cognitive deficits, growth problems, and liver and spleen damage.
Radiation from diagnostic X-rays or radiation therapy in the mother: Radiation may damage DNA and cause mutations in embryonic germ cells. When mutations occur at such an early stage of development, they are passed on to daughter cells in many tissues and organs, which is likely to have severe impacts on the offspring. Nutritional deficiencies: A maternal diet lacking certain nutrients may cause birth defects. The birth defect called spina bifida is caused by a lack of folate when the nervous system is first forming, which happens early in the embryonic stage.
In this disorder, the neural tube does not close completely and may lead to paralysis below the affected region of the spinal cord. Extraembryonic Structures Several structures form simultaneously with the embryo. Placenta The placenta is a temporary organ that provides a connection between a developing embryo and later the fetus and the mother.
It allows the transfer of substances between them. The amniotic cavity is surrounded by a membrane called the amnion, which forms as a sac around the developing embryo. The yolk sac nourishes the early embryo, and the chorion develops into the fetal portion of the placenta.
Chorion, Yolk Sac, and Amnion Besides the placenta, the chorion , yolk sac, and amnion also form around or near the developing embryo in the uterus. Chorion: The chorion is a membrane formed by extraembryonic mesoderm and trophoblast. The chorion undergoes rapid proliferation and forms the chorionic villi.
These villi invade the uterine lining and help form the fetal portion of the placenta. Yolk Sac: The yolk sac or sack is a membranous sac attached to the embryo and formed by cells of the hypoblast. The yolk sac provides nourishment to the early embryo. After the tubular heart forms and starts pumping blood during the third week after fertilization, the blood circulates through the yolk sac, where it absorbs nutrients before returning to the embryo.
Amnion: The amnion is a membrane that forms from extraembryonic mesoderm and ectoderm. It creates a sac, called the amniotic sac, around the embryo. By about the fourth or fifth week of embryonic development, amniotic fluid begins to accumulate within the amniotic sac. This fluid allows free movements of the fetus during the later stages of pregnancy and also helps cushion the fetus from potential injury. Review When does the embryonic stage occur?
Name a few of the major developments that occur during the embryonic stage. What is the embryonic disc? When and how does it form? Define gastrulation. When does it occur? Identify the three embryonic germ layers. Give examples of specific cell types that originate in each germ layer. Harmful exposures during the first trimester have the greatest chance of causing major birth defects.
This is because many important developmental changes take place during this time. The major structures of the body form in the first trimester. These include the spine, head, arms and legs. Some examples of these organs are the heart, stomach and lungs. While the heart and stomach completely form during the first trimester, the lungs continue to develop past the first trimester. What are the greatest risks from harmful exposures during the second and third trimesters of pregnancy?
Harmful exposures during the second and third trimesters can cause growth problems and minor birth defects. Growth is an important part of the second and third trimester. The structures and organs that developed during the first trimester grow larger. Babies with growth problems may be much smaller or much larger than average. This size difference can put babies at risk for certain health problems. Harmful exposures during the second and third trimesters can also cause functional defects like learning problems.
The brain is part of the central nervous system and it develops during the entire pregnancy. Major, structural brain development lasts until about 16 fetal weeks 18 gestational weeks.
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