Preterm labor test: Fetal fibronectin

Can the fetal fibronectin test really tell me whether I'm in preterm labor?

The fetal fibronectin (fFN) test can't tell you for sure that you are in labor, but it can tell you that you're not. This is useful because in the early stages of preterm labor, it's very difficult to tell if a woman really is in labor based on her symptoms and a pelvic exam.

A negative result on the fFN test means it's highly unlikely that you'll give birth in the next week or two, which can set your mind at ease and allow your provider to hold off on treatments that may not be needed.

A positive result, on the other hand, has somewhat limited value. It means you're at a higher risk of giving birth early, but it doesn't guarantee that you're about to give birth or that you'll deliver early at all. (About 30 percent of preterm labor cases resolve on their own, and half of women who are hospitalized for preterm labor end up delivering full-term babies.) When considered with other factors such as cervical dilation or length, however, a positive result can help your provider decide how to proceed with your care.

If you're having symptoms of preterm labor, your provider may order an ultrasound to see whether your cervix is shortening. During true labor, the cervix shortens, effaces (thins out), and dilates (opens) so that your baby can come out. Sometimes, fFN will be collected as a part of the initial evaluation of preterm labor symptoms. and other times, it may be used in combination with other test results, such as that ultrasound, to help your provider assess your risk of delivering early.

(Read more about the stages of labor.)

What kind of treatments can a negative fFN test result help me avoid?

A negative fFN test result can allow your provider to hold off on admitting you to the hospital and giving you the following:

• Medications to stall preterm labor for a few days
• Corticosteroids, which are used to make your baby's lungs mature faster
• Magnesium sulfate, to help lower your baby's risk of cerebral palsy if you're delivering before 32 weeks of pregnancy
• Antibiotics for group B strep 

These drugs can be critically helpful to your baby's health and survival, if they're needed, but they can have side effects – and it's always best to avoid treatment that isn't necessary. In the case of corticosteroids, you may only be able to take them once. Steroids are most beneficial for your baby if they're given when your doctor is concerned you have a reasonably high chance of delivering within the next week. The benefit of steroids wears off over time, and they can only be given 1-2x during pregnancy, so the use of steroids is typically reserved for when there is true concern for preterm delivery within the next week.

How does the fetal fibronectin test work?

Your provider inserts a speculum into your vagina and takes a sample of your cervical and vaginal secretions. You'll feel no more discomfort than you do when you get a Pap smear. (Note: If you've had sex or a cervical exam within the last 24 hours, your test may need to be repeated 24 hours after your original test because both sex and having a cervical exam can cause a false positive fFN result.)

When lab technicians analyze your sample, they measure the amount of fetal fibronectin, a protein produced by the fetal membranes. This protein serves as the "glue" that attaches the fetal sac to the uterine lining. It's normally found in increased amounts in your vagina during the first half of pregnancy and again late in pregnancy as you near labor. But if fFN leaks from the uterus and more than a small amount shows up in the vagina too early (between 22 and 35 weeks), it may mean that the glue is disintegrating ahead of schedule because of contractions or an injury to the membranes.

If this happens, your risk of going into labor and giving birth prematurely is significantly higher. More important, if no fetal fibronectin is found in your sample, it means your risk of giving birth in the next two weeks is very small. You should ask your doctor when to expect your test results, as this can vary from hospital to hospital and office to office. However, for patients with symptoms of preterm labor, the results are usually available within minutes to hours

Who should take this test?

The fFN test is approved by the Food and Drug Administration for use in women with symptoms of preterm labor who are 24 to 35 weeks pregnant (and in women without symptoms who are 22 to 31 weeks pregnant). Many providers recommend the test for women who are 22 to 35 weeks pregnant with shortened cervixes who are having contractions or other symptoms of preterm labor, such as a cervix that has started dilating.

However, you won't receive the fetal fibronectin test if:

• Your amniotic membranes have already ruptured
• You're dilated more than 3 centimeters
• You have moderate or severe vaginal bleeding
• You currently have a cervical cerclage

fFN is not always necessary, and sometimes your provider can use your cervical length as a guide to determine if an fFN test is needed. If your cervix is too short, your provider may be concerned about your risk of having your baby in the next week, so they may recommend steroids or medicine to stop contractions even if the fFN test were to be negative. In these cases, your provider may opt to skip the fFN test. On the other hand, if your cervix is very long, your provider may feel reassured that your risk of delivery within the next week is low, so they may also forego the fFN if they feel adequately reassured, based on your whole clinical picture and your ultrasound results.

What do the results of my fetal fibronectin test mean?

According to the Global Library of Women's Medicine, your risk of delivering early based on your fFN test result is estimated to be as follows:

• Negative fFN test result: You have a .8% percent chance of delivering in the next two weeks. Even though preterm labor is unlikely in the near term, you may be kept under observation for several hours to monitor for symptoms and further changes to your cervix. (If there are no further changes, you'll probably be discharged.)
• Positive fFN test result: You have a 16.7% percent chance of delivering in the next two weeks. Depending on your risk factors, your provider may begin treating you with medications to stall labor and steroids to speed up your baby's lung development.