This interactive presentation is designed to help you learn about the different stages of prenatal development. This material is meant to supplement your text book, not replace it. Please refer to the text book for more information.
By clicking on the hyper-link words below (marked with a red triangle ) you can explore the different concepts and see images of prenatal development. Click on the links to load the images and continue reading. The images will load in the frame to the right ----->
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Have fun exploring and learning about this very important phase in development.
The woman's body secretes hormones to prepare her for potential pregnancy. FSH is secreted by the pituitary gland which starts the production of an egg follicle in the ovary. As the follicle develops, estrogen is secreted by the ovary's follicle before ovulation to prepare uterine tissue (endometrium). Estrogen also stops the secretion of FSH so that egg development is limited to the one currently under development. After ovulation (about the 14th day of the cycle), the burst follicle changes and the ovary secretes progesterone to continue development of uterine tissue and preparation for pregnancy. If conception does not occur, these hormone levels (estrogen & progesterone) drop and the uterine lining deteriorates and is shed. With low levels of estrogen and progesterone, the pituitary gland once again secretes FSH to start the process over. This cycle will repeat itself each month until the egg is fertilized.
If conception does occur, the developing placenta will take over the job of secreting these two hormones. High levels of estrogen and progesterone are necessary throughout the pregnancy to maintain the uterine lining and keep the FSH shut off. In addition to secreting these hormones, the placenta also functions as a semi-permeable membrane in the transfer of material between the mother's blood system and the baby's blood system. After the birth of the baby and the placenta, estrogen and progesterone levels drop and the uterine lining breaks up and is shed.
During ovulation the woman's
ovary usually releases just one fertile egg.
The egg is drawn into the fallopian tube which leads to the uterus. If sperm are present in the tube, the egg will be fertilized (conception) and continue it's journey to the uterus. It will take about 3 or 4 days to reach the uterus.
The newly fertilized egg cell (zygote) needs to divide many times before reaching the uterus.
During ejaculation the male deposits approximately 2 to 3 hundred million sperm which have been under development for about 30 days (meiosis). Almost half of the sperm will die shortly thereafter, and more will die while searching for the fertile egg. The sperm can live about 48 hours inside the womb, so time is precious. Only about 50 sperm will ever reach the egg and compete to be the one single sperm to break through the egg's protective layer.
The Germinal Period...
Once inside the egg, the sperm will release the genetic material it has been carrying.
The sperms genetic material (23 chromosomes) will combine with the eggs genetic material (23 chromosomes).
The newly formed cell now has 46 chromosomes and is called a zygote.
The cell will begin to divide through the process called mitosis (simple cell division). After many divisions, there will be a mass of cells, each cell containing the identical genetic material present in the first fertilized egg cell.
The hollow cell mass will be called a blastocyst, and will be ready to implant in the uterine wall.
Implantation of the blastocyst is usually completed by about the 10th day after conception.
The period of development from conception until about the 21st day is referred to as the germinal period.
The Embryonic Period
Shortly after implantation a row of cells begins to form inside the cell mass. These cells begin to roll into a tube and form the neural tube, the beginning of the embryo.
At one end of the neural tube, a mass of tissue begins to form the head and brain area of the embryo. Down the back of the neural tube cells begin to form that will eventually become the spinal column.
The new embryo will be nourished by the mother, as her blood system exchanges nutrients for waste products through the developing placenta. The placenta keeps the babies blood separate from the mother's blood, but allows the exchange of smaller particles. In other words, small particles (like oxygen, alcohol, and other chemicals) get through but large ones (like blood cells) do not.
At 4 Weeks...
By the 4th week the embryo is about 1/8 inch long. The head and spinal cord still dominate the embryo's growth, but internal organs are forming even without an abdominal cavity to enclose them.
The heart started beating on the 18th day even as the circulatory system was beginning to develop.
The embryo is growing quickly at this time and arm and leg buds will soon sprout.
As seen here, the head and brain area still compose about 1/3 of the embryo's mass.
This is a critical time for healthy development of brain tissue and internal organs. The mothers health will be of primary concern to ensure the health of the embryo.
The embryo is surrounded by the amnion sac which in turn is surrounded by the placenta.
The small embryo is attached to the placenta by the umbilical cord.
The placenta is firmly attached to the mothers uterine wall.
The placenta, a semi-permeable membrane, allows small particles carried by the mothers blood to pass through to the embryo's blood. The embryo also passes waste material through the placenta to the mothers blood.
This way the growing embryo is nourished, receives oxygen and passes waste material. Unfortunately, this is also how harmful material (teratogens) carried by the mother's blood enters the embryo and often causes development problems. For more information on teratogens, visit the Teratology Society web page (a new browser window will open).
At 5 weeks, arm buds and leg buds have sprouted. The embryo is about 1/3 inch long. The head area is still dominant as other facial features begin to develop. The gradual development of the embryo from head area to tail area is referred to as cephalo-caudal development.
Cephalo means head, and caudal means tail.
The introduction of contaminating drugs or viruses (teratogens) at this time could seriously effect physical and mental development.
Here, at 6 weeks, we see the embryo floating in the fluid inside the amnion sac.
The spinal column is clearly visible, which demonstrates the tendency of proximo-distal development. This means that the central axis of the embryo develops first and then proceeds outward in an orderly pattern of growth. Arm buds sprout, then hand buds, and then finger buds in an orderly pattern.
Likewise, legs buds sprout, then feet buds, and then toe buds.
It is about this time that a control gene on the Y chromosome, the SRY gene, causes high levels of testosterone to be released in the embryo. The hormone testosterone causes the development of male sex organs.
This is normal in an embryo with XY chromosomes.
Embryos without the SRY gene (Typically XX type embryos) will develop into females.
The umbilical cord contains 2 arteries (carrying blood from the embryo to the placenta to pick up nutrients) & 1 vein (carrying blood back from the placenta to the embryo). The mothers blood does not mingle with the embryo's blood.
Notice how large the head area is in relation to the rest of the body.
Do you remember what this growth pattern is called? It means the same thing as:
"head to tail" development.
Here we see the embryo (about 1 inch) attached to the placenta via the umbilical cord.
The placenta has been secreting the hormones estrogen and progesterone during this development in order to maintain the uterine wall.
These hormones also influence other changes going on in the mothers body during pregnancy.
At about the end of the 8th week the embryo's cartilage tissue begins to change to bone tissue. This process tends to follow a proximo-distal pattern (from the center of the bone segment toward the joints).
At this time the organism is called a fetus.
The Fetal Period
At 10 weeks
The fetus is about 2 inches in length and growing rapidly. It has eyes, eye lids, nose, lips, fingers and other distinctive human features.
Don't be fooled by his appearance. He will not be ready to enter the outside world for about another 28-30 weeks. There is still a lot of growth to occur before birth.
A crucial part of the developing child is the brain and the connections that are being made in preparation for later interaction with the world. The recent research on brain development will help you appreciate this sensitive period of development.
At 12 weeks
Now about 3 inches long, the systems of the body are already well developed, with many organs more or less complete.
Nerves and muscles are working, and reflexes are becoming established. The heart pumps about fifty pints of blood through the circulatory system each day.
Although the baby can move spontaneously, the mother is not yet aware of the movements.
At 16 weeks
During the 4th month the baby grows so rapidly that it reaches a length of 8 inches, half the height it will be at birth.
The development of neck and back muscles is accompanied by the head being lifted off the chest, and the baby assumes a more upright posture. At this stage, blood vessels are clearly visible through the thin loose skin.
At 18 weeks
In the 5th month, fetal "quickening" movements may be felt as it moves and kicks within the amniotic fluid.
At 20 weeks
A white, cheeselike substance called vernix forms and protects the skin from chapping due to long exposure to the amniotic fluid. The vernix tends to collect under finger nails, in nostrils, around eyes and eye lids.
During the 6th month, the fetus begins to develop a layer of fat beneath the skin, and fine hair, called lanugo, covers the head and body.
About 24 weeks, the baby is said to be "viable," which means that it stands a slight chance of survival should it be born now.
At 32 weeks
During the last two months of pregnancy the fetus increases in weight by just under an ounce daily. Movement inside the uterus is now much more restricted, and the baby settles into a curled position, usually head down.
Labor starts, on average, about 280 days after the first day of the mother's last menstrual period.
In the 1st stage of labor, uterine contractions become noticeable at intervals of between 20-30 minutes. Contractions become progressively closer together forcing the fetus to press against the cervical entrance. The cervix dilates to a diameter of about 4 inches (10 cm). This stage of labor may last from 12 to 16 hours.
The 2nd stage of labor consists of continued contractions which force the baby through the birth canal. This stage may last from minutes to hours and is usually the most painful stage. This is the actual birth of the baby.
The 3rd stage may take about 10 - 20 minutes, as a few more contractions cause the placenta to separate from the uterine wall and be delivered.
Do we have a healthy baby?
As the newborn enters the world outside the womb, the Apgar Scale is used to assess his physical condition. Five characteristics which rate the child's physical health are utilized. A score of zero, 1, or 2 is assigned to each characteristic. A score of 7 to 10 on the Apgar indicates that the child is in good condition. A score between 4 and 6 indicates that assistance is needed. A score of 3 or lower indicates that emergency medical attention is needed. The Apgar assessment is made at 1 minute after birth and then again 5 minutes later. Two ratings are given because some babies have trouble at first, but then perk up and do quite well a few minutes later.
Welcome to the world !!!
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