In any large database, the distribution of the length of pregnancy is skewed because babies are more likely to be born preterm than post-term and at a wider range of gestations into the early preterm period. Thus neither the mean nor median, but the modal value is used to denote the typical length of pregnancy.
Starting from the time of conception, this typical length of gestation and the fetal age at the end of pregnancy is 266 days or 38 weeks (= conceptual age). In most (but by no means all) cases conception occurs in mid-cycle and thus 2 weeks are added to denote menstrual age. By convention, gestational age is also expressed in this manner: the formulae used for dating pregnancies by ultrasound, to determine the length of pregnancy at any point and the expected date of delivery (EDD), also add a standard 2 weeks to derive 'gestational age'. The typical length of pregnancy is 280 days or 40.0 weeks; term is conventionally denoted as 37-42 weeks, preterm as <37.0 weeks and post-term >42.0 weeks. However, these cut-offs may be varied for the purpose of looking at specific issues. For example, prematurity <34 weeks denotes babies that are more likely to require some form of special care; and limits of >290 days or even >287 days (41.0 weeks) have been used to study the effects of post-term pregnancy.
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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.