Questionable heartbeat

Hi.. i'm a first time mom, currently at my 8week of pregnancy. I just had earlier my 2nd ultrasound and the doctor told us that the baby's heartbeat is not identified/recognized yet. My OB also told me that it is normal and it sometimes happen. Does it happen to some of you, too? At what time were you able to know your baby's heartbeat? Thanks in advance. God bless!