Second-trimester ultrasounds are usually done to more accurately estimate gestational age, confirm the number of babies in the womb, determine the location of the placenta, and to scan the baby's body to look for normal and abnormal anatomy. First-trimester ultrasounds can be done when there are problems with bleeding, possible miscarriage, or to rule out ectopic pregnancies. Third-trimester ultrasounds might be focused on the infant's growth, size, and quantity of amniotic fluid, position, and fetal well-being. It may also be used to check for cervical length if there is a concern about preterm labor. These ultrasound tests can be done abdominally or through the vagina, depending on what the provider is looking for and the age of the pregnancy. For instance, very early ultrasound is more accurate if it is done through the vagina, as is cervical length.

What it tells you. There is no evidence that ultrasounds should be routinely done in low-risk, healthy pregnancies. Rather, they should be done only when there is an indication. However, 60%-70% of all women have at least one ultrasound during their pregnancies. The American College of Obstetricians and Gynecologists concludes that ultrasounds should be performed for specific indications, including unknown last menstrual period (poor dating), suspected twins, unexpectedly large or small uterus, and unexplained vaginal bleeding. The March of Dimes reports that there are no physical risks for mother or baby that have been found to be directly associated with the ultrasound procedure.

Maternal Serum Genetic Screen (Triple Screen or Triple Marker)
Test done at 16-18 weeks
What it measures. This test uses a blood sample from the mother to screen for possible genetic disorders in the baby. For mothers over 35, these tests will be highly encouraged since the risk of these genetic disorders increases with age. It is called the Triple Screen because it tests for three things:

Maternal alpha-fetoprotein (a substance produced in baby's liver that can be detected in mother-s blood)
Unconjugated estriol
Human chorionic gonadatropin-two hormones of pregnancy

The values of the three tests are calculated along with the mother's age to suggest a risk status for such genetic problems as Down's syndrome, neural tube defects, and Trisomy 18.

A fourth test for dimeric Inhibin-A is being evaluated. This substance should increase the test's ability to more accurately screen for Down's syndrome.

What it tells you. This is an optional test. However, if done, it must be performed between weeks 15 and 20 in the pregnancy, and is best when done between weeks 16 and 18. When combined with an accurate due date, it will detect 70% of the babies at risk for Down's syndrome, 75% of those at risk for neural tube defects, and 60%-80% at risk for Trisomy 18. When the fourth test is added, it is estimated that 75%?80% of the babies at risk for Down's syndrome will be identified.

However, while it is fairly effective at screening for actual problems, its ability to detect the absence of problems is low. As a screening test for Down?s syndrome, it has been estimated that as many as 80% of abnormal test are false positives (i.e., the test result indicates a problem, but the fetus does not actually have Down's syndrome).This is due, in part, to the fact that the test is so reliant on accurate gestational dating and maternal age. Other factors such as undiagnosed multiple pregnancies, maternal conditions, and lab errors add to this problem. By the same token, the test will not detect all babies with a genetic defect.

When the test result is positive, further genetic screening tests are offered. Those include high-resolution ultrasound and amniocentesis, which will give more accurate information.

Diabetes Screen, Glucola Screen, Post-Prandial Test
Done at 24-28 weeks
What it measures. During pregnancy, a small number of women (1.5%-2%) will develop difficulty in processing the normal sugars (carbohydrates) in their bloodstreams. This can lead to what is called gestational diabetes mellitus (GDM). If not detected and treated, GDM can cause serious problems for both the mother and the baby.

This test screens for the possible presence of GDM. The basic test is called the one-hour diabetes screen or one-hour glucola test. A sweet drink (glucola) is given to the mother after she has fasted for 12 hours. Then, one hour later, blood is drawn to see if the mother's system appropriately cleared the sugar from her system. For women considered to have a greater than normal chance of developing GDM (women with a family history, prior problems with GDM, or who are overweight), this test will be administered twice: once at the beginning of the pregnancy, and again closer to 28 weeks.

What it tells you. The test will indicate how your body did in clearing that one dose of sugar. It will not tell you whether you have diabetes. If the blood-sugar level was still high after the one-hour test, a second test called the three-hour screening test will be offered. Done on a different day, this test will check your blood-sugar level before the drink (after fasting), and then once each hour for three hours afterwards. If two of the levels measured are high, you are considered to have GDM. Some women with GDM are able to control their blood sugar with diet changes and exercise, while others might require insulin.

Some professionals feel the combination of the required fasting and the use of the glucola is not the best measure of the body's response to normal food and might lead to a high number of false-positives. An alternative test called the post-prandial test (meaning "after a meal") is offered in some areas. In this case, you eat a prescribed diet and a specified breakfast and then have your blood drawn two hours later.

Other Prenatal Tests:

First Trimester Tests
Third Trimester Tests
Genetic Tests
Tests to Evaluate Baby