Inclusion Criteria:  Pregnant women aged 18 to 45 years who have received regular antenatal examinations and delivered in the department of obstetrics in Beijing Friendship Hospital, Capital Medical University.  Exclusion Criteria:  (1) chronic or acute disease history (diabetes mellitus, hypertension,or thyroid disease); (2) any mental disorder; (3) abortion (fetal loss before 28 gestational weeks); (4)multiple fetuses; (5) planning to give birth in other hospitals.