The normal progesterone level in the fourth week of pregnancy is between 16-60 nmol/L, although the average should be around 30 nmol/L. During pregnancy, progesterone plays an important role in stabilizing the fetus, and a low progesterone level indicates the possibility of preterm miscarriage, and a low progesterone level of less than 10 nmol/L indicates the possibility of ectopic pregnancy. Therefore, during the fourth week of pregnancy, progesterone must be strictly monitored to avoid a persistent decrease, which may lead to preterm abortion. In the fourth trimester, if the progesterone is below the normal range, for example, below 30 nmol/L, oral progesterone can be used for fetal preservation to avoid the chance of preterm miscarriage. If it is within the normal range, you can wait until about 50 days to go to the hospital for an ultrasound to determine if the fetus is in a stable stage.