Why does fetal heartbeat decrease in third trimester?

In the third trimester, a decrease in fetal heartbeat can occur due to several factors, including maturation of the autonomic nervous system, which helps regulate heart rate. Additionally, increased size and activity of the fetus can lead to more efficient cardiovascular function, resulting in a lower resting heart rate. It's important to note that variability in fetal heart rate is normal, and any significant changes should be monitored by a healthcare provider.