35-37 weeks
What it measures. Although maternal and infant infections during birth and the postpartum period are rare, they can be very serious. Group B (beta-hemolytic) streptococcus (GBS) is a bacteria that has been identified as being one of the main causes of these rare infections. This organism is normally present in the vagina or rectum in about one of every four women. In nonpregnant woman, GBS rarely causes any problems. During pregnancy and birth, however, the bacteria can possibly cause an infection in the mother's uterus or in the baby. Out of 100 women who have the bacteria, less than 1% of the infants will actually get an infection from it.

New guidelines from the Centers for Disease Control and Prevention state that all pregnant women receive a test for GBS at 35-37 weeks. This involves swabbing the area around the opening of the vagina and the rectum to screen for the presence of the bacteria. If detected, the mother will be offered antibiotics while in labor.

What it tells you. The effectiveness of the GBS test is dependent upon both the performance of the test and on the processing at the lab. One study found that it was about 70% sensitive (positive test, bacteria present) and 90% specific (negative test, no bacteria present). However, this means that about 30% of women who have the bacteria will test negative. The latest guidelines from the CDC recommend that all pregnant women be tested for GBS and women who are positive be offered antibiotic treatment during labor.

Third Trimester Hemoglobin and Hematocrit (H&H)
28-32 weeks
What it measures. These tests check for anemia in the last part of the pregnancy. Some practices do a regular CBC blood draw, while others use a finger stick hemoglobin test.

What it tells you. It is normal for pregnant women to have a drop in their H&H around 28 weeks in their pregnancies. If the H&H have dropped lower than expected, iron supplementation and possibly more tests before and at the time of labor will be suggested.

Fetal Fibronectin (fFN) Testing
What it measures. Fetal fibronectin (fFN) is a biochemical marker used as a predictor for the risk of preterm delivery. The presence of fetal fibronectin in the cervical or vaginal secretions is highly correlated with preterm labor.

The test involves placing a simple swab briefly at the back of the vagina, behind the cervix. It is then sent to a lab where the sample is checked for the presence or absence of fetal fibronectin.

It is used when a mother may be having, or be at risk for, premature labor. Not a routine test, it may be performed between 24 and 35 weeks when a woman is having regular contractions or her cervix is softening, shortening, or opening too soon in the pregnancy. It may also be used between 22 and 30 weeks as a screening test for women with multiples or premature rupture of membranes, or those with a history of a previous preterm delivery.

What it tells you. Fetal fibronectin testing is new. It is used both for women with symptoms and for women who have risk factors for possible preterm labor or delivery.

If the fFN test is negative it is considered to have a predictive value of 99.2% in women with symptoms. Thus, a negative result reassures both mother and provider that the baby will not be born in the next two weeks. The test's positive predictive value is significantly less accurate. A positive test is considered to be accurate only about 16.7% of the time in predicting that the baby will be born early (i.e., in the next two weeks).Vaginal bleeding and/or recent intercourse will cause a false positive test.

This test is also often used in conjunction with a special kind of ultrasound called trans-vaginal ultrasound (TVUS), in which a small probe is placed in the vagina to measure the thickness and length of the cervix. The practitioner can do a manual check of the cervix to determine if it is open or closed, and its length in the vagina. The TVUS can actually get a picture of the portion of the cervix one cannot feel on exam. The combination of a negative fFN test and a normal TVUS are very reassuring.

Since the test is only predictive for a two-week time period, someone with continued symptoms or risk factors may have repeat tests at two-week intervals.

*taken from "On the Road to Motherhood," by Mayri Sagady Leslie, CNM, Every Baby magazine, Issue Four.