Estriol is a metabolite of estradiol, which is secreted by the placenta in large amounts after pregnancy, and therefore can be elevated in pregnancy tests, usually with gestational age, with normal values of 14.01-17.24 nmol/L at 21 weeks, 27.31-43.21 nmol/L at 35 weeks, and 45.39-67.39 nmol/L at 41-42 weeks. In general, the free estriol in the blood is less disturbed and the results are more accurate because it cannot be excreted through the kidneys. It is therefore possible to monitor estriol levels in the blood or urine of pregnant women to determine placental function and to predict fetal status. If estriol levels are abnormal, the risk of fetal abnormalities needs to be considered. In addition to multiple pregnancies, excessive estriol secretion is usually considered as gestational diabetes mellitus, with a higher probability of having a large fetus. Decreased estriol secretion is usually considered for gestational hypertension syndrome, overdue pregnancy, intrauterine growth retardation, fetal adrenal insufficiency, and stillbirth. In addition, patients with moderate or severe hyperemesis, as well as enhanced activity of the renin-angiotensin-aldosterone system, can lead to placental ischemia and a significant decrease in serum estriol. Therefore, pregnant women are advised to have regular maternity checkups to monitor their own and their fetus’ physical condition and to seek timely medical treatment for any abnormalities.