Just as a baby’s body size and shape change dramatically during the fetal period, her heart rate also changes with gestational age. As a baby grows in the womb and develops features for life after birth, her heart rate will change to reflect this. A child's heart rate is also affected if she is born prematurely or if there is difficulty during labor and delivery.
The heart is formed and begins beating around the 5th week of gestation. Your baby's first heart rate is about 100 beats per minute, which is relatively low compared with later weeks. After the 5th week, her heart rate begins to accelerate and reaches a high average rate of approximately 175 beats per minute by the 10th week of gestation. It is often during these weeks that you may have an initial exam to confirm pregnancy, which includes an examination of the fetal heart rate by Doppler, and you are able to hear the heart’s rapid pace.
After an initial rise in the early weeks of pregnancy, your baby’s heart rate then declines after approximately the 10th to 12th weeks. This decline drops by about 25 to 40 beats per minute between the 10th and 20th weeks of gestation or until almost the end of the second trimester, which lasts until the 24th week.
Between the 20th week of gestation until the time, when your baby is considered to be full-term, his heart rate declines again slightly and then stabilizes between 100 and 160 beats per minute. According to the University of New South Wales Embryology department, some babies have a heart rate between 160 and 180, and this is still considered normal. Your baby’s heart rate should remain in this zone until the time of delivery.
During labor, a baby’s heart rate is typically monitored to check for variations in the rate. Although an infant born between 37 and 40 weeks gestation often has a heart rate between 120 and 160 beats per minute, the contractions that occur during labor may cause her heart rate to speed up or slow down. This can mean a sign of distress, which is why doctors often keep track of the baby’s heart rate to determine if she is transitioning through birth normally. If a baby’s heart rate slows down during a labor contraction and then remains slow after the contraction has stopped, this can be a sign of a decrease in oxygen for her, and intervention is necessary.
A baby who is born at less than 37 weeks gestation is considered to be premature. Premature babies often have faster heart rates than babies born at full term, and depending if illness is present or the baby is crying, a heart rate as fast as 200 beats per minute may still be considered normal. This occurs because after birth, premature babies use oxygen more quickly as their bodies adjust to life outside of the womb.