Although the pregnancy begins in the implantation, often is advisable for the date of the first day of the last menstrual period of the woman. This is used to calculate the considered date of childbirth (EDD). Traditionally (according to the rule of Naegele, that is used to calculate the considered date of delivery, or EDD), a human pregnancy is considered that lasts approximated of 40 weeks (280 days) from the last menstruation (LMP), or 37 weeks (259 days) as of the date of the fertilization. Nevertheless, a pregnancy is considered that it has arrived upon maturity between weeks 37 and 43 from the beginning of the last menstruation. The babies who are born before the mark of 37 weeks consider premature, whereas the babies born after the mark of 43 weeks consider posmaduros. According to Merck, the norm for the human pregnancy is that it lasts 266 days from the date of the fertilization. This it is of 38 weeks, or approximately 8 months Gregorian and days 22,5, or 9,0 months lunar). To count from the beginning of the last menstrual cycle of the woman, the norm is of 40 weeks (the base of the Naegele rule).

In agreement with the same reference, less than 10% of the births take place in the date of victory, 50% of the births are within one week of the date of victory, and almost 90% within two weeks. But it is not clear if this talks about to the date of victory calculated from sonograma or principles of the last menstruation (to see more down). Although these are the averages, the real duration of the pregnancy depends on several factors. For example, the first pregnancy tends to last more than the later pregnancies. The exact date of the fertilization is important, since it is used in the calculation of the results of diverse prenatal tests (for example, in the test of triple).

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