Because the policy of "Two-child" in China, there are more and more women more than 35 years old choose to have another baby. It is necessary to find out a way to prevent them who is older than 35 from preterm labor. About 1500 pregnant women will be recruited at about gestational age of 14 weeks for this study. All of them will be taken samples (i.e. blood, hair, cervical secretion ) to be saved before and after delivery. After all data collected, the investigators will do analysis of all data and find out strategy and mechanism of preterm labor.