What are some influences that impact on healthy prenatal development? 

Teratogens are the broad range of substances (such as drugs and pollutants) and conditions (such as severe malnutrition and extreme stress) that increase the risk of prenatal abnormalities. These abnormalities include obvious physical problems (such as missing limbs) and more subtle impairments such as brain damage that first appears in elementary school. A specific teratogen may damage the body structures, the growth rate, the neurological networks, or all three. Teratogens that harm the brain, and therefore make a child hyperactive, antisocial, retarded and so on, are called behavioural teratogens; their effects can be far more damaging over the life of a person than physical defects. (Berger, 2000)

What are the factors that influence the degree of affect?  

One crucial factor is when the developing organism is exposed to which teratogen. Some teratogens cause damage only during specific days or weeks early in pregnancy, when a particular part of the body is undergoing formation. Others can be harmful at any time, but how severe the damage is depends on when the exposure occurred. The time of greatest susceptibility is called the critical period. Each body structure has its own critical period. As a general rule, for physical defects the critical period is the entire period of the embryo. (Berger, 2000) A second important factor is the dose and/or frequency of exposure to a teratogen. For most teratogens, experts are reluctant to specify a threshold below which the substance is safe. One reason is that many teratogens have an interaction effect; that is, one poison intensifies the effects of another. (Berger, 2000) A third factor that determines whether a specific teratogen will be harmful, and to what extent, is the developing organism's genes. In some cases, genetic vulnerability is related to the sex of the developing organism. Generally, male embryos (XY) embryos and fetuses are at a greater risk than female in that more male embryos are more often aborted spontaneously. In addition, newborn boys have more birth defects, and older boys have more learning disabilities and other problems caused by behavioural teratogens. (Berger, 2000)

What are some of the specific influences that may affect prenatal development?

Radiation, chemicals and other hazards in the environment can endanger the fetus. Chromosomal abnormalities are higher among the offspring of fathers exposed to high levels of radiation in their occupations. Environmental pollutants and toxic wastes are also sources of danger to unborn children. Among the dangerous pollutants and wastes are carbon monoxide, mercury and lead. Another environmental concern is toxoplasmosis, a mild infection that causes cold-like symptoms or no apparent illness in adults, but can cause eye defects, brain defects and premature birth. Cats are common carriers of toxoplasmosis, especially outdoor cats who eat raw meat. The expectant mother may pick up the virus through the cat litter box. (Santrock, 1999)

In terms of the mother’s age, two time periods are of special interest: adolescence and the thirties and beyond. Infants born to adolescents are often premature. The mortality rate of infants born to adolescent mothers is double that of infants born to mothers in their twenties. Down Syndrome, a form of mental retardation, is related to the mother's age. By age 40, the probability is slightly over 1 in 100. By age 50, it is almost 1 in 10. The risk is also higher before age 18. Women also have more difficulty in becoming pregnant after the age of 30. (Santrock, 1999)

A developing fetus depends completely on its mother for nutrition, which comes from the mother's blood. Among the important factors are the total number of calories and the appropriate levels of protein, vitamins and minerals. The mother's nutrition even influences her ability to reproduce. In extreme instances of malnutrition, women stop menstruating. Also children born to malnourished mothers are more likely to be malformed. (Santrock, 1999) Another common reason for slow fetal growth - and hence low birthweight - is maternal malnutrition, a problem that has many specific causes. Women who begin pregnancy underweight, eat poorly during pregnancy, and consequently do not gain at least 1.5 kilograms per month in the second and third trimesters run a much higher risk than others of having a low-birthweight infant. Indeed, women who gain less than 7 kilograms, even if they are non-smokers who begin pregnancy overweight, still have a higher risk of preterm and smaller babies than those who gain at least 7 kilograms. (Berger, 2000)

Maternal diseases and infections can produce defects by crossing the placental barrier. For example, the greatest damage to the fetus from the mother contracting German measles occurs during the 3^rd and 4^th weeks of pregnancy. Syphilis is more damaging later in pre-natal development - 4 months or more after conception. Rather than affecting organ development as Rubella does, syphilis damages organs after they have formed. The importance of the mother's health to the health of their offspring is nowhere better exemplified than when the mother is infected with HIV. (Santrock, 1999)

Drugs includes the use of tobacco, alcohol, prescription or illegal drugs. For example, the effects of thalidomide during the fourth week of development had devastating effects. Heavy drinking by an expectant mother can also be devastating. Fetal alcohol syndrome is a cluster of abnormalities that appear in the offspring of mothers who drink alcohol heavily during pregnancy. The abnormalities include facial deformities and defective limbs, face and heart. Most of these children are below average in intelligence. In one study, however, even mothers who drank moderately during pregnancy had babies who were less attentive and alert, with the effects still present at 4 years of age. Cigarette smoking by pregnant women can also adversely influence pre-natal development, birth and postnatal development. Fetal and neonatal deaths are higher among smoking mothers. Also prevalent are a higher incidence of preterm births and lower birthweights. Respiratory problems and sudden infant death syndrome are also more common among the offspring of mothers who smoked during pregnancy. Tranquilizers taken during the first three months may cause cleft palate or other congenital malformations. Mothers who take large amounts of barbituates may have babies who are addicted or may exhibit tremors, restlessness and irritability. (Santrock, 1999)

Drug	Usage	Effects
Alcohol	3 or more drinks daily, or binge drinking of 5 or more drinks on one occasion early in pregnancy	Causes fetal alcohol syndrome (FAS). Symptoms include abnormal facial characteristics (small head, wide spacing between the eyes, a flattened nose, a narrow upper lip, unusual eyelids), overall growth retardation, learning disabilities and behaviour problems.
	More than ½ ounce of absolute alcohol a day	Causes fetal alcohol effects (FAE). FAE does not obviously affect facial appearance or physical growth, but it affects brain functioning.
	Moderate drinking: less than 1 or 2 servings of beer or wine or 1 mixed drink on a few days per week	Probably has no negative effects on prenatal development, although this is controversial
Tobacco	Maternal smoking early in pregnancy	Increases risk of abnormalities, including malformations of the limbs and the urinary tract
	Maternal smoking late in pregnancy	Reduces birthweight and size. Babies born to habitual smokers weigh, on average, about 250 grams less than they would otherwise be expected, and they are shorter, both at birth and in the years to come. They may have childhood problems, particularly with respiration and, in adulthood, increased risk of becoming smokers themselves.
	Paternal smoking	Reduces birthweight by about 45 grams on average

Compared with women of higher socioeconomic status, pregnant women at the bottom of the economic ladder are more likely to be ill, malnourished, teenaged, and stressed. Physical difficulty like malfunction of the placenta or the umbilical cord is likely when pregnancies are closely spaced and close spacing correlates with poverty. Poverty helps explain the wide national and international variations in the following statistics:

·         Of the more than 25 million low-birthweight infants born worldwide each year, the overwhelming majority are in developing countries.

·         Developing countries in the same geographic region, with similar ethnic populations, have markedly different low-birthweight rates when they have different average incomes.

·         Within nations, differences in low-birthweight rates among ethnic groups follow socioeconomic differences among those groups.

·         Within the United States, low birthweight rates in the poorest states are almost twice those in some richer states. (Berger, 2000)

The mother's stress can be transmitted to the fetus. When a pregnant woman experiences intense fears, anxieties and other emotions, physiological changes occur in the fetus. These include changes in respiration and glandular secretions. For example, producing adrenaline in response to fear restricts blood flow to the uterine area and may deprive the fetus of adequate oxygen. Also, reassuring the mother of fetal well-being has positive outcomes for the infants in the study. (Santrock, 1999)

Are there times during pregnancy when the effect of teratogens is especially important?

Not only is the specific teratogen important in determining the effects on prenatal development, but so is the time during pregnancy when the teratogen influences the fetus. This is referred to as a critical period. The table below describes the critical periods for fetal development for the major organs and body systems.