Longitudinal Study on the Influence of Iodine Nutrition Status on Thyroid Function of Childbearing Age Women During Pregnancy and 6 Weeks Postpartum

The goal of this observational study is to provide a reference for formulating individual appropriate iodine intake programs for women during pregnancy and postpartum and improving the monitoring and evaluation system of maternal iodine nutrition status. The main questions it aims to answer are: * To analyse the longitudinal changes of urinary iodine, urinary iodine/creatinine ratio, thyroid function and related factors in different periods of early, middle, late pregnancy and 6 weeks postpartum in Beijing area. * To explore the relationship between iodine nutrition status and the changes of thyroid function and to further clarify the effect of iodine nutrition status on thyroid function of women of childbearing age during pregnancy and postpartum in combination with dietary intake of iodine questionnaire. Participants will be detected there thyroid function, the level of median urinary iodine and urinary iodine/creatinine. Researchers will compare healthy women of childbearing age served as the control group in the same period.