Fetal heartbeat

ChocolateDrop💙👶🏽 • Patiently awaiting the day I get to snuggle & kiss mY baby boy!💙 Baby Manuel "the bean" Pena III
Sorry if this is a silly question but this is my first pregnancy and I was concerned when my dr told me the hear beat today. At around 16wks my baby's heart beat was 140-146. Today  at 20wk I went his heart beat was 120. Is that normal for it to drop like that?