39 weeks tomorrow and experiencing a decrease in fetal movement. Is this normal?

No. The perception of decreased fetal movement needs investigation. The most common method to evaluate fetal well being is the biophysical profile which is an ultrasould test that evaluates fetal breathing, fine motions, gross motions and amniotic fluid volume. A non-stress test is done which evaluates the babies heartrate pattern. Two points are awarded for each item and 8/10 or 10/10 is reassuring.
Less fetal movement. Would check with your doctor. Also ask about kick counts.