Body Proportions At Different Ages

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Figure 23.17

Structures in the developing embryo and fetus are sensitive to specific teratogens at different times in gestation.

Figure 23.17

Structures in the developing embryo and fetus are sensitive to specific teratogens at different times in gestation.

Development Fetus Month Month

2 month 3 month Newborn 2 years 5 years 13 years 22 years embryo fetus

2 month 3 month Newborn 2 years 5 years 13 years 22 years embryo fetus

Proportion Newborn Body

Figure 23.18

During development, body proportions change considerably.

Figure 23.18

During development, body proportions change considerably.

During the sixth month, the fetus gains a substantial amount of weight. Eyebrows and eyelashes appear. The skin is quite wrinkled and translucent. Blood vessels in the skin cause a reddish appearance.

In the seventh month, the skin becomes smoother as fat is deposited in the subcutaneous tissues. The eyelids, which fused during the third month, reopen. At the end of this month, a fetus is about 40 centimeters long.

In the final trimester, fetal brain cells rapidly form networks, as organs elaborate and grow. A layer of fat is laid down beneath the skin. The testes of males descend from regions near the developing kidneys, through the inguinal canal, and into the scrotum (see chapter 22, p. 881). The digestive and respiratory systems mature last, which is why infants born prematurely often have difficulty digesting milk and breathing.

Approximately 266 days after a single sperm burrowed its way into an oocyte, a baby is ready to be born. It is about 50 centimeters long and weighs 2.7 to

3.6 kilograms. The skin has lost its downy hair but is still coated with sebum and dead epidermal cells. The scalp is usually covered with hair; the fingers and toes have well-developed nails; and the skull bones are largely ossified. As figure 23.20 shows, the fetus is usually positioned upside down with its head toward the cervix (vertex position).

Premature infants' survival chances increase directly with age and weight. Survival is more likely if the lungs are sufficiently developed with the thin respiratory membranes necessary for rapid exchange of oxygen and carbon dioxide and if they produce enough surfactant to reduce alveolar surface tension (see chapter 19, p. 795). A fetus of less than twenty-four weeks or weighing less than 600 grams at birth seldom survives, even with intensive medical care. Neonatology is the medical field that cares for premature and ill newborns.

Some Causes of Birth Defects


The idea that the placenta protects the embryo and fetus from harmful substances was tragically disproven between 1957 and 1961, when 10,000 children in Europe were born with flippers in place of limbs. Doctors soon identified a mild tranquilizer, thalidomide, which all of the mothers of deformed infants had taken early in pregnancy, during the time of limb formation. Although some women in the United States did use thalidomide and had affected children, the United States was spared a thalidomide disaster because an astute government physician noted adverse effects of the drug on monkeys in experiments, and she halted use of the drug. However, thalidomide is used today to treat leprosy and certain blood disorders.


The virus that causes rubella (German measles) is a powerful teratogen. Australian physicians first noted its effects in 1941, and a rubella epidemic in the United States in the early 1960s caused 20,000 birth defects and 30,000 stillbirths. Exposure in the first trimester leads to cataracts, deafness, and heart defects, and later exposure causes learning disabilities, speech and hearing problems, and type I diabetes mellitus. Widespread vaccination has slashed the incidence of this congenital rubella syndrome, and today it occurs only where people are not vaccinated.


A pregnant woman who has just one or two alcoholic drinks a day, or perhaps many drinks at a crucial time in prenatal development, risks fetal alcohol syndrome or the more prevalent fetal alcohol effects in her unborn child. Because the effects of small amounts of alcohol at different stages of pregnancy are not yet well understood and because each woman metabolizes alcohol slightly differently, it is best to avoid drinking alcohol entirely when pregnant or when trying to become pregnant.

A child with fetal alcohol syndrome has a characteristic small head, misshapen eyes, and a flat face and nose (fig. 23B). He or she grows slowly before and after birth. Intellect is impaired, ranging from minor learning disabilities to mental retardation.

Teens and young adults with fetal alcohol syndrome are short and have small heads. Many individuals remain at early grade-school level. They often lack social and communication skills, such as understanding the consequences of actions, forming friendships, taking initiative, and interpreting social cues.

Problems in children of alcoholic mothers were noted by Aristotle more than twenty-three centuries ago. Today, fetal alcohol syndrome is the third most common cause of mental retardation in newborns. One to 3 in every 1,000 infants has the syndrome.


Chemicals in cigarette smoke stress a fetus. Carbon monoxide crosses the placenta and plugs up the sites on the fetus's hemoglobin molecules that would normally bind oxygen. Other chemicals in smoke prevent nutrients from reaching the fetus. Studies comparing placentas of smokers and nonsmokers show that smoke-exposed placentas lack important growth factors. The result of all of these assaults is poor growth before and after birth. Cigarette smoking during pregnancy raises the risk of spontaneous abortion, stillbirth, prematurity, and low birth weight.

Nutrients and Malnutrition

Certain nutrients in large amounts, particularly vitamins, act in the body as drugs. The acne medication isotretinoin (Accutane) is a derivative of vitamin A that causes spontaneous abortions and defects of the heart, nervous system, and face. The tragic effects of this drug were noted exactly nine months after dermatologists began prescribing it to young women in the early 1980s. Today, the drug package bears prominent warnings, and it is never prescribed to pregnant women. A vitamin Abased drug used to treat psoriasis, as well as excesses of vitamin A itself, also cause birth defects. This is because some forms of vitamin A are stored in body fat for up to three years after ingestion.

Malnutrition during pregnancy causes intrauterine growth retardation (IUGR), which may have health effects on the fetus, and on the per-

Shier-Butler-Lewis: I VI. The Human Life Cycle I 23. Human Growth and I I © The McGraw-Hill

Human Anatomy and Development Companies, 2001

Physiology, Ninth Edition

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