Preterm birth is defined in relation to miscarriage and full-term birth. A birth before 28 weeks of pregnancy is considered a miscarriage; a birth after 37 weeks of pregnancy is considered full-term; and a birth between 28 weeks and 36 weeks and six days is considered preterm. Since preterm birth occurs in the late stages of pregnancy, the fetus has a certain chance of survival. However, efforts should be made to delay the timing of preterm birth as much as possible to increase the survival rate of the fetus. It is generally believed that births between 28 and 34 weeks are considered early preterm, during which the survival probability of the fetus is relatively low; whereas births after 34 weeks up to 36 weeks and six days are considered late preterm, during which the maturity of the fetus is higher and the survival rate is also relatively high.