The heart is one of the earliest developing organs in a new baby, and hearing that tiny heartbeat is one of the first pregnancy memories for many new parents. During pregnancy, a baby's heart goes through many changes during development that affect its size, function and rate.
Early Heart Formation
In the rapidly growing fetus, the heart begins to beat at approximately 22 days after conception, often before the mother even realizes she is pregnant. During this early stage of development, the heart grows from a simple tube shape into a four-chambered beating heart. It is during this period that the growing heart is at most risk for cardiac defects and the baby is at highest risk for a miscarriage from those defects.
Shortly after formation, the tiny heart has a beat that is similar to an adult's heart rate, then it begins increasing at a few beats a week through the ninth week. The heart first begins to beat at a rate of about 80 beats per minute around five-weeks gestation, then steadily increases to a mean of 175 bpm by the ninth week. The increase in fetal heart rate at this point in the pregnancy is so steady that a study by medical sonographers published a formula to help determine the fetal age based on heart rate: Embryonic age in days = EHR(0.3)+6.
After that point, it begins to slow again throughout the pregnancy.
Pregnancy myth suggests that an unborn baby's heart rate can predict the baby's gender, with faster heart rates (above 140 bpm) indicating a girl and slower heart rates indicating a boy. The basis for this is the faster metabolism of baby girls. There have been several studies, including a large study by Austin Radiological Associates in the 1990s, but none has shown a statistical correlation.
Second and Third Trimesters
Starting at the end of the first trimester (around 10 weeks) the baby's heart rate should be audible by a hand-held Doppler instrument. Prior to this, the only way to confirm the heartbeat is by visualization on a sonogram. With the Doppler instrument, the heartbeat will sound surprisingly fast to most people, with an average rate of 120 to 160 bpm. New parents often describe the sound as similar to galloping horses.
By 20 weeks of gestation, the baby's heart rate will be audible by a stethoscope or fetoscope, but this method can be more difficult if the placenta is attached to the anterior of the uterus or if the baby is positioned in such a way that her back is to the mother's back.
Labor and Delivery
Labor and delivery are hard work for both mother and baby. The work that the baby does to move into position and withstand uterine contractions is evidenced by the normal fluctuations in heart rate that occur during this time. Fetal heart rate can be monitored either externally or internally, continuously or intermittently, during this time. A baby's heart rate will normally accelerate during a contraction, then slow as the mother and baby recover. If the baby's heart rate fails to recover adequately, shows significant deviations from his normal baseline, or does not respond to the work of the contraction, it could indicate a problem that warrants medical attention.