Duration Pregnancy

Duration

Though pregnancy begins at conception, it is more convenient to date from the first day of a woman's last menstrual period (acronym = LMP), or from the date of conception (if known). Starting from one of these dates, the expected date of delivery (acronym = EDD) can be calculated. Counting from the LMP, pregnancy usually lasts between 37 and 42 weeks, with the EDD at 40 weeks,[9] 38 weeks after conception. 40 weeks is a little more than nine months and six days, which forms the basis of Naegele's rule for estimating date of delivery. More accurate algorithms which take into account other variables, such as whether this is the first or subsequent child (i.e. mother is a primip or a multip, respectively), ethnicity, parental age, length of menstrual cycle and menstrual regularity form the basis for more sophisticated online calculation methods, e.g. here.
Pregnancy is considered 'at term' when gestation attains 37 complete weeks but is less than 42 (between 259 and 294 days since LMP). Events before completion of 37 weeks (259 days) are considered pre-term; from week 42 (294 days) events are considered post-term.[10] When a pregnancy exceeds 42 weeks (294 days), the risk of complications for mother and fetus increases significantly.[9][11] As such, obstetricians usually prefer to induce labour, in an uncomplicated pregnancy, at some stage between 41 and 42 weeks.[12][13]
Recent medical literature prefers the terminology pre-term and post-term to premature and post-mature. Pre-term and post-term are unambiguously defined as above, whereas premature and postmature have historical meaning and relate more to the infant's size and state of development rather than to the stage of pregnancy.[14][15]
Fewer than 5% of births occur on the due date; 50% of births are within a week of the due date, and almost 90% within two weeks.[16] It is much more useful to consider therefore a range of due dates, rather than one specific day, with some online due date calculators providing this information, e.g. here.
Accurate dating of pregnancy is important, because it is used in calculating the results of various prenatal tests (for example, in the triple test). A decision may be made to induce labour if a fetus is perceived to be overdue. Furthermore, if a difference of predicted due date is highlighted between dates based on LMP and ultrasound dating, with the latter being later, this might signify slowed fetal growth and therefore the need for closer review.