What is and what cause a slow heart rate in a fetus?

Several reasons. Normal fetal heart rates are 120-160bpm & as low as 110 at term. If lower, there are reasons that can be significant so an obstetrician should be able to evaluate these or make a referral to a high risk pregnancy specialist.

Related Questions

What is the risk of having a slow heart rate in a fetus?

Depends. There are many variables involved to answer this question appropriately. A slow heart rate can be normal very early on. Later a very slow rate can signify a heart block in the baby. In labor the rate can be ominous or not depending how slow and the pattern on the tracing.