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Prenatal Development Chapter 8 Prenatal Development… • The process by which a baby-to-be grows inside the mother • The process usually takes about 9 months for a full-term pregnancy • Or 280 days • Or 40 weeks Timeline of Pregnancy • The timeline of pregnancy can be divided into 3 trimesters • Trimester3 month time period First Trimester • Begins at the moment of Conception • This is the first 3 months of pregnancy • The baby-to-be has 3 names during this period…the zygote, the embryo, the fetus The Zygote • A fertilized egg is a single cell, but it soon begins to divide • After just a few days, the zygote grows to about 500 living cells that form a tiny ball • The zygote travels thru the fallopian tube to the uterus where it implants itself in the lining of that organ The Embryo • It is an embryo from the time of implantation into the uterus until the 8th week of pregnancy • During this time, the cells multiply rapidly • In a few weeks after implantation, the embryo is the size of the head of a pin The Embryo • As the cells multiply, they begin to take on different functions…3 layers of cells form that will eventually develop into all the structures of the human body • Ectoderm outer layer of cells…becomes skin, hair, nails, also forms nervous system, brain, spinal cord, and nerves • Mesoderm middle layer…becomes bones, muscles, blood vessels, some internal organs • Endoderm internal cell layer…becomes internal organs such as stomach, liver, and lungs Embryo • Midway thru the first trimester, the embryo has grown to the size of a raisin. • 2 weeks later it is the size of a grape. • Then the eyes and inner ears begin to form on the sides of the embryos head, and the mouth and nostrils begin as small openings. Next the eyelids begin to form and the nose, upper lip, and outer ears begin to take shape. The heart begins to pump blood cells, although the heartbeat cannot yet be heard. The digestive and respiratory systems start to form, and buds that will become arms and legs emerge. The Fetus • Fetus name from the beginning of the 9th week until birth • At the beginning of the fetal stage, major organs are present, but still forming • Heartbeat flutters rapidly and can be heard using a special instrument • Bone begin to form taking on a skeletal shape • Skull and jaws begin to harden • Muscles begin to develop, as do tissues that connect muscles to bones • Arms and legs strengthen • A very thin skin covers the fetus The Fetus • At then end of the 1st trimester, the fetus is about 3 inches long and typically weighs 1 ounce. • All the fetus’s major organs are growing and developing – Intestines move from the umbilical cord to the abdomen – Ears and eyes move closer to their final positions – Liver begins to produce bile – Kidneys produce urine – Fetus can open and close its mouth and fists, but pregnant woman cannot feel these movements Support System • As the baby develops, so do structures that will protect and nourish the developing baby – Amniotic Fluid fills the sac that surrounds the developing baby…it cushions the developing baby from outside pressures – Placenta tissue, shaped somewhat like a pancake, is attached to the uterine wall and contains a rich network of blood vessels – Umbilical Cord connects the baby to the placenta – For the Duration of pregnancy, blood will flow from the mother’s body thru the placenta and umbilical cord, carrying oxygen and nutrients to the developing baby. Wastes will pass from the baby thru the umbilical cord and placenta into the mother’s body for processing and disposal – The placenta acts as a filter between the mother’s and baby’s bodies 2nd Trimester • 2nd Trimester— 4th, 5th, and 6th months of pregnancy • By the end of this period, the fetus will weigh nearly 2 pounds! • Eyebrows and lashes appear and Lanugo (aka fine hair) begins to cover the fetus’s skin • Fat tissue, which will later keep the newborn warm, begins to form under the skin 2nd Trimester • The fetus can move around and kick • The pregnant woman usually can feel these movements, a milestone often called, QUICKENING • Prenatal thumb-sucking begins during the 2nd trimester • The lungs strengthen and the fetus ‘breathes’ amniotic fluid in and out • The fetus swallows amniotic fluid, and nutrients in the fluid pass thru the digestive system • The senses—taste, touch, smell, sight, and hearing— steadily develop • The eyes now open and close • The fetus begins to hear sounds including mom’s heartbeat and other loud noises from outside mom’s body 3rd Trimester • 7th, 8th, and 9th months of pregnancy • Brain and other parts of nervous system continue to develop • Brain scans have shown that the fetus sleeps and may dream • Heart and circulatory system mature • Lungs almost fully developed by 9th month • Fetus steadily grows and gains weight until birth • Vital fat tissue increases all over the fetus’s body • Fetus triples its weight 3rd Trimester • Average birth weight 7.5 lbs. • Head is largest part of the body, accounting for ¼ of newborn’s weight • Bones of skull are somewhat soft and pliable allowing the fetus to pass thru the birth canal • During 8th month, fetus has less freedom of movement • During 8th month, fetus turns head downward in preparation for a typical birth • In 9th month, the still growing fetus settles further down in the woman’s pelvis