Toward the end of pregnancy, the uterus reaches maximum size, the placenta begins to age, and the level of the hormone progesterone begins to drop. Your body becomes more sensitive to the chemical oxytocin, which triggers labor contractions. Finally, the baby itself may release prostaglandins that trigger labor.
Labor has three stages.
Finally, the placenta is delivered during the last stage of labor. Premature Labor Ideally, pregnancy lasts 40 weeks, during which time an embryo multiplies from two cells to literally millions of cells to form a baby who is ready to be born. Obviously, the longer a woman can carry a baby, the better the outcome will usually be. Your health care provider and Yale-New Haven Hospital work together to delay the onset of early labor as long as possible. If at any point in your pregnancy you have unusual or persistent cramping, vaginal bleeding or a rush of fluids from the vagina, call your doctor or midwife right away.