Final answer:
The nonstress test is used primarily in the third trimester to assess fetal well-being by measuring the baby's heart rate in response to its movements.
Step-by-step explanation:
The nonstress test is a common prenatal test used to check on a baby's health. This noninvasive test gives insight into the baby's oxygen supply by measuring the heart rate as a response to its movements. Typically, the nonstress test is used for fetal well-being during the third trimester of pregnancy. The purpose of this test is to ensure that the fetus has an adequate oxygen supply and to look for any signs of distress, which is particularly important as the pregnancy advances and the baby prepares for delivery.
The average duration of pregnancy is divided into three trimesters, each posing different aspects on fetal development and maternal health. During the first trimester, the groundwork for all critical bodily systems is laid, and signs and symptoms such as morning sickness might occur. The second trimester often brings a feeling of renewed energy and noticeable weight gain as well as the exciting moment when fetal movements can first be felt. The third trimester, however, is characterized by more rapid fetal growth and more pronounced movements, which can cause discomfort for the mother and make fetal monitoring through nonstress tests more relevant.