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Gestational age (or ''menstrual age'') is a measure of the age of a pregnancy where the origin is the woman's last normal menstrual period (LMP), or the corresponding age as estimated by other methods. Such methods include adding 14 days to a known duration since fertilization (as is possible in in vitro fertilization), or by obstetric ultrasonography. The popularity of using such a definition of gestational age is that menstrual periods are essentially always noticed, while there is usually a lack of a convenient way to discern when fertilization occurred. The initiation of pregnancy for the calculation of gestational age can be different from definitions of initiation of pregnancy in context of the abortion debate or beginning of human personhood. == Methods == According to American Congress of Obstetricians and Gynecologists, the main methods to calculate gestational age are:〔(Obstetric Data Definitions Issues and Rationale for Change - Gestational Age & Term ) from Patient Safety and Quality Improvement at American Congress of Obstetricians and Gynecologists. Created November 2012.〕 *Directly calculating the days since the beginning of the last menstrual period *Early obstetric ultrasound, comparing the size of an embryo or fetus to that of a reference group of pregnancies of known gestational age (such as calculated from last menstrual periods), and using the mean gestational age of other embryos or fetuses of the same size. If the gestational age as calculated from an early ultrasound is contradictory to the one calculated directly from the last menstrual period, it is still the one from the early ultrasound that is used for the rest of the pregnancy.〔 *In case of in vitro fertilization, calculating days since oocyte retrieval or co-incubation and adding 14 days. Gestational age can also be estimated by calculating days from ovulation if it was estimated from related signs or ovulation tests, and adding 14 days by convention. A more complete listing of methods is given in following table:〔(A Simple Solution to Dating Discrepancies: The Rule of Eights ) 〕 As a general rule, the official gestational age should be based on the actual beginning of the last menstrual period, unless any of the above methods gives an estimated date that differs more than the variability for the method, in which case the difference cannot probably be explained by that variability alone.〔 For example, if there is a gestational age based on the beginning of the last menstrual period of 9.0 weeks, and a first-trimester obstetric ultrasonography gives an estimated gestational age of 10.0 weeks (with a 2 SD variability of ±8% of the estimate thereby giving a variability of ±0.8 weeks), the difference of 1.0 weeks between the tests is larger than the 2 SD variability of the ultrasonography estimate, indicating that the gestational age estimated by ultrasonography should be used as the official gestational age.〔 Following are diagrams for estimating gestational age from obstetric ultrasound, by various target parameters: File:Gestational sac diameter by gestational age.png|By gestational sac diameter File:Crown-rump length by gestational age.png |By crown-rump length (CRL) File:Biparietal diameter by gestational age.png|By biparietal diameter (BPD) 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「gestational age」の詳細全文を読む スポンサード リンク
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