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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. If the ultrasound shows that your cervix has shortened, your provider will likely recommend the fFN test to help her gauge 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 (if you've tested positive for it)
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. So for them to be most effective, you want to be pretty sure you're going to deliver in 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: The test won't be useful if you've had sex or a cervical exam within the last 24 hours, because both 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 34 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 expect to see test results within a day or two, or within several hours if you're taking the rapid version of the test.
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 36 weeks pregnant (and in women without symptoms who are 22 to 30 weeks pregnant). Many providers recommend the test for women who are 22 to 34 weeks pregnant with shortened cervixes (20 to 29 millimeters in length) who are having contractions or other symptoms of preterm labor, such as a cervix that has started dilating.
The Society for Maternal-Fetal Medicine does not recommend the test for all pregnant women because studies haven't shown it to be useful for moms-to-be who are not having preterm labor symptoms.
Also, 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
The fFN test is not generally used for women carrying multiples, though occasionally it may be used in women who are carrying twins to tell if they might deliver in the next week or two.
Your provider may skip the fFN test if your cervical length is above or below a certain measurement. If you're having regular contractions and your cervix measures less than 20 millimeters, for instance, your provider probably won't bother giving you the fFN test because it's likely you're already headed into preterm labor. If you're having contractions but your cervix measures 30 millimeters or longer, you may not receive the fFN test because your risk of delivering within the next couple weeks is very low.
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 1 to 5 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 17 to 41 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.