Boys and girls possess many differences on the outside; the inside has differences as well. Heart rates for both are different, changing constantly as your child grows and develops. According to Parenting.com, boys are more active and accident prone, leading to valuable clues as to why their heart rates are slightly higher during activity and lower during resting periods.
Boys are notorious for their rambunctious ways. Jumping, running, hitting and falling seem to be the male specialty. According to Michael Thompson and Teresa Barker's book, "It's a Boy!: Understanding Your Son's Development from Birth to Age 18," boys' heart rates tend to be higher than girls because of the more active lifestyle. Noting that boys' hearts are larger in diameter than girls', more blood is pumped per minute, thus increasing boys' heart rates during activity.
Resting Heart Rate
On the flip side of the heart rate question, Michael Thompson and Teresa Barker also state that boys have lower resting heart rates than girls. This could be indicative of most boys' laid back nature and ability to lower stress naturally. Boys also tend to have a higher systolic blood pressure rate than girls, during activity and resting periods.
Folklore says that if your fetus has a lower heart rate, that means you are giving birth to a boy. According to KidsHealth.org, this is not true. Girls possess higher heart rates during labor, however heart rates during pregnancy are no different between males and females. In fact, near the fifth week of pregnancy, KidsHealth.org states, the baby's heart rate will closely match the mother's heart rate.
Boys' hearts and lungs may outgrow the female heart and lungs, but male development is slower, overall. According to Parenting.com, girls grow faster than boys and reach puberty earlier. It is important to make sure that you do not overly worry about your son's development when you notice the girls getting taller and developing faster. This is perfectly normal and your son will grow into his own with time.