You don't need to do anything special to prepare for the fetal fibronectin test.
To avoid a false-positive result, the test will be done before any pelvic exam or vaginal ultrasound. Avoid having sex for 24 hours before the test. Also, avoid using douches, lotions and soaps, which can affect test results, for at least a day before the test.
Vaginal bleeding can affect the test results, too. If you experience vaginal bleeding, the test probably won't be done.
If the amniotic sac ruptures — your water breaks — the test won't be done. The amniotic sac is a fluid-filled membrane that cushions your baby in the uterus.