Women are being asked to participate in a research study that is looking for new methods to understand and predict premature birth (birth before 37 weeks of pregnancy—full term pregnancy is 40 weeks). In this study the investigators will be looking at proteins in vaginal fluid, tissue from the placenta after the baby is born (fetal membranes), and a new form of ultrasound of the mouth of the womb (cervix) that measures its length, volume, and stiffness, in order to try to know who will have a preterm birth and who will reach full term. The investigators will collect the vaginal fluid samples and perform ultrasounds both in the clinic throughout pregnancy in normal women, and from women admitted to the hospital because they are in early labor. The investigators will collect the placental tissue after the baby is born (this is normally discarded). This study will help us to understand the process of preterm birth and the investigators hope that this will eventually allow us to develop better methods to treat preterm labor and prevent the birth of premature babies.