Fetus heart rate in first, second and third trimester
By week 10, the embryonic heart has developed into the definitive four-chambered heart. The two atria receive blood from the fetal circulation, while the ventricles pump blood out to the lungs and the rest of the baby's body. Valves develop at the exit of all four chambers to ensure that the blood is always pumped in one direction and cannot leak back into the heart. The heart is beating at the rate of 180 beats per minute-twice the speed of your own heart.
Second trimester: The fetal heart rate has slowed considerably from 180 to 140-150 beats per minute by the end of second trimester. From this point onward, monitoring the heart rate pattern on an electronic fetal monitor becomes one of the most useful methods of assessing your baby's well-being.
Third trimester: The heart rate is 110-150 beats per minute. Dramatic changes will occur in the heart and circulatory system at the time of delivery when the baby takes the first breath.