Whats wrong when low variability of fetal heart rate?

Acidosis.: Non-reassuring fetal heart tones include loss of beat to beat or short term variability. They are commonly used to monitor fetal status and determine timing/mode of delivery although they have an exceedingly high false-positive rate in predicting actual neurological impairment in the infant. They can be due to: acidosis; hypoxia; drugs; infection; sleep cycle; dehydration; anemia; thyroidopathies.