Your baby's heart starts beating very early. By the 6th week of pregnancy (age of fetus is 4 weeks), the heart tubes fuse and contractions of the heart begin. This can be seen on ultrasound.
Occasionally you may listen to your baby's heartbeat at the doctor's office and hear it skip a beat. An irregular heartbeat is called an arrhythmia. Arrhythmias in a fetus are not unusual, so don't be overly concerned. The equipment could be faulty or there may be some other problem transmitting the sound.
Arrhythmias are not usually serious in a baby before birth; many disappear after the baby is born. If an arrhythmia is discovered before labor and delivery, you may require fetal heart-rate monitoring during pregnancy. When an arrhythmia is discovered during labor, it may be decided to have a pediatrician present when the baby is born.