Chapter 3 How large a family The number of children is declining In 1960 the average was 3 1 and now it is 1 8 in U S The lowest is in Italy with an average of 1 3 children per couple Major reason for the decline family size of 1 or 2 is more compatible with a woman s decision to divide her energies between family and work Also marital stability Popular advice to limiting family size is their interests of child quality more parental affection attention and material resources per child enhances intellectual development Large families the youngest scored higher than their siblings The larger the family the lower the scores of all siblings Mothers give birth to more children who are low in intelligence Table 3 2 page 96 only child pros and cons between parents and child Prenatal Development The prenatal period 266 days or 38 weeks can be divided into 3 periods germinal embryonic and fetal Period of the zygote takes place in first 2 weeks after conception First cell duplication is long and drawn out completed after 30 hours after conception Gradually new cells are added at a faster rate by the fourth day 60 to 70 cells exist that form a hollow fluid filled ball called the blastocyst The cells on the inside of the blastocyst embryonic disk will become the new organism and the thin outer ring trophoblast will become the protective covering Implantation between 7th and 9th days the blastocyst burrows deep into the uterine lining where it starts to grow At first the trophoblast outer layer multiplies fastest Forming a membrane amnion that encloses the developing organism in amniotic fluid Keeps the temperature of the prenatal world constant and provide a cushion against any jolts The first two weeks are delicate and uncertain 30 of zygotes do not survive this period By the end of the 2nd week cells of the trophoblast form another protective membrane over the amnion chorion which surrounds the amnion Blood vessels emerge from the chorion and burrow into the uterine wall and start to develop the placenta The placenta brings the mother and embryo s blood close together permitting food and oxygen to reach the developing organism and waste products to be carried away Placenta connected to the embryo by the umbilical cord First appears as a primitive body stalk but during pregnancy grows to 1 3 ft Contains one large vein that delivers blood loaded with nutrients and two arteries that remove waste products The flowing blood keeps it firm so it seldom tangles while the embryo is free floating The Placenta and Umbilical Cord The Period of the Embryo Lasts from implantation through the 8th week brief 6 weeks that has the most rapid prenatal changes Embryo is vulnerable to interference with healthy development since body parts are forming Organs and major body systems develop rapidly critical period body is most vulnerable to destructive influences in the prenatal environment Whatever system or organ is developing will most likely be affected by exposure to harm Most likely to have miscarriages during 1st trimester 12 weeks Body will spontaneously abort most result from abnormal pregnancies Last half of the first month Embryonic disk forms 3 layers of cells ectoderm nervous system and skin mesoderm muscles skeleton circulatory system and other internal organs and endoderm digestive system lungs urinary tract and glands At first nervous system develops fastest ectoderm folds over to form the neural tube or spinal cord Then 3 weeks later the top swells to form the brain Then heart begins to pump blood and muscles backbone ribs and digestive tract start to appear by end of first month Second month eyes ears nose jaw and neck arms legs start to form Internal organs more distinct and body posture becomes more upright Can sense its world particularly in mouth and sole feet area Mass differentiates into two layers upper layer ectoderm becomes the outer layer of skin nails teeth sensory organs and nervous system brain and spinal cord lower layer endoderm becomes the digestive system liver pancreas salivary glands and the respiratory system Later a middle layer will become mesoderm become inner layer of skin muscles skeleton and excretory and circulatory system Period of the Fetus Third month muscles and nervous system start to become organized and connected can move kick By the 12th week the external genitals are well developed sex can be identified At the end of the third month the first trimester is completed Second trimester mother can feel its movements Eyes sensitive to light Can be stimulated by sounds Third trimester Age of viability and 26 weeks In the final 3 months the fetus gains more than 5 lbs and grows 7 inches the point at which the baby can first survive occurs sometime between 22 Period of the fetus lasting about 7 months from 9 weeks until birth Appearance of bone cells at about 8 weeks signals beginning of fetal stage Quickening baby s movements are felt by mother 12th week external genitals are well formed Males are more active and move more vigorously More active fetal activity in last weeks of pregnancy predicts a more active infant in first months of life Age of viability born early Last 3 months gains 5 lbs grows 7 inches 22 and 26 weeks age when the infant would have a good chance of survival if Some general principles Teratogens any environmental agent that causes damage during the prenatal period Something that is capable of causing birth defects Depends on the dosage age time of exposure and genetic susceptibility the buffers that may be in place for the fetus other negative influences several negative factors at once can worsen the impact of a single teratogen Males are more likely to be affected than female babies Harm to fetus is greatest in the embryonic period when organs are being formed Once an organ is formed teratogens are not harmful to that organ but will lead to stunted growth or problems with the way organs function Prescription and nonprescription drugs legal drugs can have damaging effects on a fetus Example thalidomide was given to women by doctors in the 60s for morning sickness Many children were born without limbs arms and legs Harm depended when the mother took the drug Alcohol Fetal Alcohol Spectrum Disorders FASD Avoid alcohol entirely 2 ounces a day taken early in pregnancy is related to FAS features Mental retardation distinct physical symptoms widely spaced eyes thin upper lip small head Children with FASD are given one of 3 diagnoses based on severity
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