The thought of an amniocentesis makes many women nervous, probably because it involves two critical undertakings: a needle being inserted through the abdomen and breeching your baby's watery environment. It does carry some risk of complications, including a slight chance of miscarriage. However, the amnio (as it is commonly referred to) is one of the best tools available for diagnosing genetic disorders and chromosomal abnormalities. An amniocentesis is typically performed in the second trimester sometime between week 15 to 20 of pregnancy, although a later amnio may be done depending on the indication (or reason). At this point in the pregnancy, there are enough fetal cells present in the amniotic fluid for withdrawal and analysis.

In addition to the wealth of genetic and health information it provides, an amnio can also tell you definitively if your baby is a boy or a girl. If you want to be surprised about the sex of your child, it may be a good idea to restate your wishes before you receive your amnio results.

Your provider may suggest you meet with a genetic counselor prior to having an amniocentesis performed to weigh the risks versus the benefits of the procedure, given your specific medical background and family history. He will also lay out any alternative procedures that are options, such as a level-two ultrasound. If you do decide to go with the amnio, you might be sent to a perinatologist (or maternal-fetal specialist) who is experienced in the procedure.

An amniocentesis is a relatively short outpatient procedure. First, an abdominal ultrasound is performed to look for an easily accessible pocket of amniotic fluid. Your abdomen is swabbed with an antiseptic solution, and the doctor will then insert a needle into the amniotic sac and draw an amniotic fluid sample into a syringe. The fluid contains sloughed-off fetal cells, which will be analyzed in the lab.

Here are reasons why an amniocentesis may be planned:

Age: Women over thirty-five have an increased risk of carrying a baby with a chromosomal disorder like Down syndrome. When the father is over fifty, amniocentesis could also be advised because there may be a connection between paternal age and an increased risk of Down syndrome.

Family history: If you've already had a baby with a hereditary or chromosomal abnormality or neural tube defects, or you or your partner have a family history of any of these conditions, an amniocentesis may be recommended.

Rh status: Rh-negative women may have a amnio to determine the Rh status of their fetus. If your baby is RH isoimmunized, you can administer an exchange blood transfusion through the fetal cord during an amniocentesis.

AFP, triple-screen, or quad-test results: If your screen is abnormal, you may be offered an amnio.

Ultrasound abnormalities: If your ultrasound turned up indications of the possibility of a chromosomal disorder, an amnio may be recommended for further evaluation.

Lung maturity: If you're experiencing symptoms of preterm labor or other medical complications (placenta previa, prior classical C-section) that point to an early birth, your provider may perform an amnio to check for markers of fetal lung maturity.

Suspected intra-amniotic infection: If you are experiencing preterm labor, you may be offered an amnio to rule out an intra-amniotic infection.

After the amnio procedure, the baby will be monitored by ultrasound and will have his heart rate checked for a few minutes to ensure that everything is okay. Minimal cramping may follow the procedure. You will be advised to restrict strenuous exercise for a day (no step aerobics and no sex), although other normal activity should be fine. If in the days following amnio you experience fluid or blood discharge from the vagina, let your provider know as soon as possible.

The CDC estimates the chance of miscarriage following an amnio at somewhere between one in 400 and one in 200, and the risk of uterine infection at less than one in 1,000. There is also a slight risk of trauma to the unborn baby from a misplaced needle or inadvertent rupture of the sac.

If there are problems, the information you get from an amnio can prepare you for providing your child with the best possible care at birth. However, the timing can make other decisions difficult, such as whether to continue a pregnancy. Results from your amniocentesis won't be available for up to two weeks. And by the time the test results are back, you will be halfway through your pregnancy (about week 20). For this reason, some women at risk for chromosomal abnormalities will instead choose the chorionic villus sampling test, which can be performed in the first trimester.

Some studies suggest that the experience and associated skill level of the physician performing a CVS or amniocentesis can make a big difference in the risk of complications. Many physicians will refer you to a specialist for just this reason. If your provider is performing the procedure, ask for an estimate of how many amnios or CVS procedures she has performed.