Throughout pregnancy the uterine cervix needs to remain firm and closed while the body of the uterus grows by hypertrophy and hyperplasia but without significant fun-dally dominant contractions. For labour to be successful the cervix is converted into a soft and pliable structure that can efface and dilate and the uterus becomes a powerful contractile organ. There is no single endocrinological or biochemical switch in the human which changes the uterus from its no-labour state to its labour state. The onset of labour is a gradual process which begins several weeks before delivery itself with changes in the lower pole of the uterus which cause cervical ripening and effacement. The onset of clinically identifiable contractions is a relatively late event in this process. Cervical ripening occurs through breakdown of collagen, changes in pro-teoglycan concentrations and an increase in water content. The lower segment of the uterus also stretches and relaxes and behaves physiologically more like the cervix than the contractile upper segment of the uterus. These changes in the lower segment of the uterus are associated with an increase in the production of inflammatory cytokines, particularly interleukins-1, -6 and -8 and prostaglandins from the overlying fetal membranes and decidua and from the cervix itself. Cervical ripening is associated with an influx of inflammatory cells into the cervix which release matrix metalloproteins which contribute to the anatomical changes associated with ripening. The later increase in fundally dominant contractility in the upper segment of the uterus is associated with an increase in the expression of receptors for oxytocin and prostaglandins, in gap-junction proteins which mediate electrical connectivity between myocytes, and in more complex changes in the intracellular signalling pathways which increase the contractility of the myocytes.
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The first trimester is very important for the mother and the baby. For most women it is common to find out about their pregnancy after they have missed their menstrual cycle. Since, not all women note their menstrual cycle and dates of intercourse, it may cause slight confusion about the exact date of conception. That is why most women find out that they are pregnant only after one month of pregnancy.