● Gestational age. To establish gestational age, count the number of weeks and days from the first day of the LMP. Counting this menstrual age from the LMP, although biologically distinct from date of conception, is the standard means of calculating fetal age, yielding an average pregnancy length of 40 weeks. If the actual date of conception is known (as with in vitro fertilization), a conception age which is 2 weeks less than the menstrual age can be used to calculate menstrual age (i.e., a corrected or adjusted LMP dating) to establish dating.
● Expected date of delivery (EDD). The EDD is 40 weeks from the first date of the LMP. Using the Naegele rule, the EDD can be estimated by taking the LMP, adding 7 days, subtracting 3 months, and adding 1 year.
● Tools for calculations. Pregnancy wheels and online calculators are commonly used to calculate the EDD. However, pregnancy wheels vary widely in quality and accuracy, and are often produced as commercial marketing tools. Online calculators may be more reliable, but should be checked for accuracy before routine use.
● Limitations on pregnancy dating. Patient recall of the LMP is highly variable. Even when this date is accurate, the LMP can be affected by hormonal contraceptives, menstrual irregularities, or variations in ovulation that result in atypical cycle lengths. LMP dating should be checked against physical examination markers such as fundal height, and any wide discrepancies should be clarified by ultrasound evaluation. In clinical practice, dating by ultrasound is widespread, regardless of the certainty of the LMP, even though this approach is not currently endorsed by national guidelines.
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