Dominant gene disorders

In dominant gene disorders, there's a 50-50 chance a child will inherit the gene from the affected parent and have the disorder. Dominant gene disorders include:

• achondroplasia, a rare abnormality of the skeleton causing shorter-than-normal arms and legs
• Huntingdon disease, a disease of the nervous system that causes neurologic deterioration affecting people in their 30s and 40s
   

Recessive gene disorders

Because there are so many genes in each cell, everyone carries some abnormal recessive genes, but most people don't have a defect because the normal gene overrules the abnormal one. But if a fetus has a pair of abnormal recessive genes (one from each parent), the child will have the disorder. It's more likely for this to happen in children born to certain ethnic groups or to parents who are blood relatives. Recessive gene disorders include:

• cystic fibrosis  : a disease that causes the respiratory system to produce thick mucus that clogs the lungs
• sickle cell disease : a disease where red blood cells form a "sickle" shape, rather than the typical donut shape, get caught in blood vessels, and cut off oxygen to tissues
• Tay-Sachs disease : a disorder causing mental retardation, blindness, seizures, and death
• beta thalassemia : a disorder causing anemia

X-linked disorders

These disorders are determined by genes on the X-chromosome of the pair of chromosomes that determine sex. These disorders are much more common in boys because the pair of sex chromosomes in males contains only one X-chromosome (the other is a Y-chromosome). If the disease gene is present on the one X-chromosome, the X-linked disease shows up because there's no other paired gene to 'overrule' the disease gene. Hemophilia is one such X-linked disorder; people who have it lack a crucial clotting agent in their blood.

Chromosomal disorders

Some chromosomal disorders are inherited, but most are caused by a sporadic error in the genetics of the egg or sperm. The chance of a child having these disorders increases with the age of the mother. For example, according to the ACOG, one in 1,667 live babies born to 20-year-olds have Down syndrome; that number changes to one in 378 for 35-year-olds and one in 106 for 40-year-olds. Down syndrome causes mental retardation and physical defects.

Multifactorial disorders

This final category includes disorders that are caused by a mix of genetic and environmental factors. The frequency of these disorders varies from country to country; some can be detected during pregnancy. Multifactorial disorders include neural tube defects, which occur when the tube enclosing the spinal cord doesn't form properly. Neural tube defects include spina bifida and anencephaly. Spina bifida is also called "open spine" and occurs when the lower part of the neural tube doesn't close during embryo development, leaving the spinal cord and nerve bundles exposed. Anencephaly occurs when the brain and head don't develop properly, with the top half of the brain being completely absent. Neural tube defects have been associated with inadequate intake of folic acid during the early part of pregnancy, among other factors.

Prenatal tests

Prenatal tests do several different things. They can identify:

• treatable health problems in the mother that can affect the baby's health
  characteristics of the fetus, including size, sex, age, and placement in the uterus
• the chance that a baby has certain congenital, genetic, or chromosomal problems
• certain types of fetal abnormalities, including heart problems
   

Some prenatal tests are screening tests and only reveal the possibility of a problem existing - they don't provide a definitive diagnosis. Other prenatal tests are diagnostic in nature, which means they can determine with a fair degree of certainty whether a fetus has a specific problem. Many women whose screening tests reveal the possibility of an abnormality have healthy babies, but in the interest of making the more specific determination, the screening test may be followed by a more invasive - and riskier - diagnostic test.

The issue of prenatal testing is further complicated by the fact that approximately 250 birth defects can be diagnosed in an unborn fetus - many more than can be treated or cured. This raises the question of what a parent will do once a defect or problem is detected.