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.

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