There are many different reasons to have genetic counseling done, as well as different ways to have genetic testing done. I am going to deal mainly with genetic testing as it relates to pregnancy.
First a quick look at genetics...
Every human being has genes, these tiny little switches that determine hair color, eye color, and other traits are packed into 46 chromosomes inside our cells. The human sperm and egg cells are different from other cells in that each only has 23 unmatched chromosomes inside. When pregnancy begins and the egg and sperm are joined you start with a brand new cell, with 46 chromosomes.
Each gene's instructions are either dominant or recessive. Examples of dominant disorders would be: high cholesterol, Huntington disease, extra fingers or toes, glaucoma, etc. Problems caused by dominant disorders can be either nonexistent or quite severe. If a parent has a dominant gene for a certain condition there is a 50% chance that each child would have the disorder.
If only one parent has a recessive disorder gene, then the dominant gene from the other parent will prevent the disorder. If both parents were carriers of a recessive gene then there would be a one in four chance that each child would inherit the disorder. Examples of recessive disorders: sickle cell anemia, Tay-Sachs, phenylketonuria (PKU). Recessively inherited disorders are often more serious.
There are other types of disorders, such as X linked disorders, and carriers.
Types of Testing
Maternal Serum Alpha-fetoprotein (MSAFP) - This is a blood test done between 15 - 17 weeks of pregnancy. There is no risk to the baby during this screening. The mother's blood is screened for one or more substances (alpha-fetoprotein, hCG, estriadol). Higher than normal levels may indicate a neural tube defect, while lower values may indicate certain chromosomal disorders, usually Down Syndrome. The problems with this screening is that there are many false positives. This can cause more invasive testing, worry, etc. When the reason may simply be that you are further along than anticipated, you might be having twins. However, a normal test should help relieve anxiety.
Ultrasound - This screening can show if the baby has defects such as kidney problems, heart defects, and limb defects. This procedure does not detect all defects and has not been shown to be helpful in determining Down Syndrome in a fetus. A good ultrasound does not indicate that you will not have a baby with a defect, just decreases the likelihood.
Amniocentesis - This test will screen for all known chromosomal defects by sampling fetal cells in the amniotic fluid. It is done with the placement of a needle, guided by ultrasound, into the uterus to collect the fluid. It is usually done between 15 and 18 weeks of gestation, although some practitioners are doing early amniocentesis as early as 9 weeks. It normally takes two weeks to receive the results. The results can be very accurate, however, they cannot tell you the severity of a present defect. There is also risk to the baby from this procedure. About 1 in 200 women will miscarry after the amniocentesis, even if the baby was unaffected, and about 1 in 1,000 will experience infection.
Chorionic Villus Sampling (CVS) - CVS can be done earlier in pregnancy, some centers are doing it as early as 8 weeks, while most are doing them around 10 weeks gestation. A small tube can be placed through the vagina, or it can be done abdominally and a tiny tissue sample is taken from the outside of the sac that contains your baby. CVS results can be done as soon as ten days. This is less accurate than amniocentesis and the rates of complications are higher. Miscarriage is 1 in 100 or 200, small risk of missing digits (fingers and toes) for 1 in 2,000 or 3,000 of babies. These risks are higher the earlier the CVS is done.