Reasons for amniocentesis
An amniocentesis is a procedure in which a long thin needle is inserted into the amniotic cavity (the space around the baby) and used to remove some fluid. They are usually performed at two different times of gestation and for two different purposes.
Genetic amniocenteses are performed between 14 and 18 weeks of gestation to look for the presence of chromosomal anomalies. Tests performed on the fluid include karyotype (a study of the number and shape of the chromosomes), alpha-fetoprotein assays (to exclude a neural tube defect), or enzymatic studies (in cases such as Tay-Sachs disease and other genetic defects).
Third trimester amnios are performed late in gestation to verify that the baby's lungs are mature.

Preparation for the examination
No special preparation is required, and you do not need to have a full bladder. Be as relaxed as possible, do not dread the examination, and wear comfortable clothes. A spouse or close friend is welcomed to hold your hand during the procedure, or you get one of our hand to squeeze for comfort.

Risks of the procedure
By inserting a needle we risk causing some bleeding, creating an infection, and causing the uterus to start labor. All these may result in a miscarriage. The risk of having a miscarriage for a genetic amniocentesis is commonly quoted at one for every 200 amniocenteses. With careful technique it is may be less.
The Procedure
Using ultrasound, the needle is inserted. Although the thought of a needle entering one's belly is not reassuring, the procedure is much less painful than one would anticipate. Local anesthesia is not used since the local anesthesia is more painful than the insertion of the amniocentesis needle, and the baby could move, and require reinjection. A sample of amniotic fluid is obtained.  The baby will regenerate the fluid over the next 4-6 hours

Afterwards
Congratulations!  Hopefully, it wasn't nearly so bad as you might have anticipated.  Women who are Rh negative will need one more shot- Rh immunoglobulin ("rhogam") after the amniocentesis- to prevent the risk of rh sensitization.

As a precaution, for the 24 hours after the amniocentesis avoid physical exercise (no aerobics, do not lift groceries or younger children, avoid intercourse etc.). If you feel any abnormal sensation, fever, loss of fluid, spotting, bleeding, cramping, let your docotr know.

When will you get the results?
Results will be ready in about 10-14 days.  It takes that long to grow the cells, stain them, and analyze them.