Testing: One, Two, Three

Pregnancy is the most natural thing in the world, but it's a good idea to monitor just how nature is taking its course. That's where prenatal tests come in. They keep tabs on the baby's development and detect problems early on. But not all tests are routine, so you need to be informed. This rundown will help you study for the tests before you take them. Here's what to expect each trimester:

First Trimester

Rubella Immunity Who All women When The first visit How A blood test Why To detect antibodies to rubella (German measles), which indicate immunity to the disease. If antibodies are not present, exposure to the disease during the first trimester could cause birth defects.

Rh Factor Who All women When The first visit How A blood test Why To determine the mother's Rh status. The Rh factor is a protein found in red blood cells. If she is Rh-negative (she doesn't have the protein) and the fetus is Rh-positive, the mother's body may produce antibodies that attack the baby's red blood cells. If an Rh difference is detected, a doctor can provide shots to prevent this type of reaction.

HBV Screening Who All women When Anytime during pregnancy, but usually in the first trimester How A blood test Why To check for the hepatitis B virus (HBV), which can be transmitted to the baby at birth

STDs Who All women When The first visit How Blood tests and swabs of the vagina and cervix Why To test for sexually transmitted diseases (STDs) which, if untreated, can cause birth defects. Be aware that your doctor may only offer to test you for syphilis and HIV. Be sure to request tests for chlamydia, gonorrhea, and herpes, as these can also be dangerous to a fetus.

Chorionic Villus Sampling (CVS) (optional)

Who Women over 35 and those who have -- or whose partners have -- a family history of genetic diseases such as cystic fibrosis When Between 10 and 12 weeks How Using ultrasound as a guide, a doctor inserts a catheter through the cervix or a needle into the abdomen to retrieve sample cells from the placenta. Why To check for genetic abnormalities early in the pregnancy Note: The procedure carries a 1 in 100 risk of causing miscarriage.

Second Trimester

Maternal Serum Multiple-Marker Screening (Triple Screen) Who Offered to all women When Between 15 and 18 weeks How A blood test Why This test checks for genetic abnormalities by measuring the level of a protein produced by the fetus called alpha-fetoprotein (AFP) as well as that of two hormones, human chorionic gonadotropin (hCG) and estriol. A high reading of AFP may signify a neural-tube defect. A low level may indicate Down syndrome. A new fourth screen can test levels of the hormone inhibin to better determine Down syndrome risk. Another test that would detect Down syndrome risk earlier in pregnancy is in development. Note: False positives of this test are common. Only 2 or 3 women in 100 who have low levels will actually be carrying a baby who has Down syndrome.

Glucose Screening Who Most women, but especially those who are overweight, are over 30, or have a family history of diabetes When 24 to 28 weeks How A blood test, which is done one hour after the mother drinks a sugary liquid Why To measure the blood-sugar level, which indicates the risk of gestational diabetes

Ultrasound (optional) Who Women whose doctors want to check fetal and uterine structure and development When Anytime, but usually between 15 and 20 weeks How A device called a transducer is placed either on the abdomen or in the vagina. Sound waves from the transducer create pictures of the fetus on a monitor and pick up the sounds of the fetal heartbeat. Why To establish the due date, check for multiple fetuses, monitor fetal growth, and look for structural abnormalities. New 3-D ultrasound machines can offer doctors a clearer image to better detect birth defects.

Amniocentesis (optional) Who Women over 35, those who have -- or whose partners have -- a family history of genetic diseases, and those whose multiple-marker test results are abnormal When Usually between 14 and 18 weeks How Guided by ultrasound, a doctor inserts a needle into the abdomen to draw a sample of amniotic fluid. Why To detect neural-tube defects, Down syndrome, and other genetic disorders.

Third Trimester

Group B Strep (GBS) Who Most women When Between 35 and 37 weeks How Cells are swabbed from the rectum and vagina. Why To screen for Group B streptococcus (GBS) bacteria, which can cause meningitis or inflammation of the baby's blood, lungs, brain, or spinal cord. Some doctors only screen women considered at risk, meaning that they have a history of urinary tract infections, a fever near delivery time, or have delivered a previous baby with GBS. Ask to be tested even if you're not at risk. GBS is treated with antibiotics during delivery.

Non-stress Test (optional) Who Women who have complicated pregnancies, medical problems that could affect the baby, or who have had problems with previous pregnancies When Weekly How A monitor is strapped to the mother's abdomen. Why To monitor the fetal heart rate

Stress Test (optional) Who The same as those who undergo a non-stress test When Usually after a non-stress test that doesn't show a change in fetal heart rate when the fetus moves How A monitor is strapped to the mother's abdomen. Contractions are then triggered with an intravenous drip of Pitocin or with nipple stimulation. Why To monitor fetal heart rate during a contraction

Biophysical Profile (optional) Who Women whose doctors have a concern about the fetus's well-being When Anytime during the third trimester How An ultrasound is performed. Why To measure the amniotic fluid and the baby's muscle tone, "breathing" movements, and activity level.