Editing Gestational age (section)

==Methods==
According to [[American College of Obstetricians and Gynecologists]], the main methods to calculate gestational age are:<ref name=acog2012>[http://www.acog.org/About_ACOG/ACOG_Departments/Patient_Safety_and_Quality_Improvement/~/media/Departments/Patient%20Safety%20and%20Quality%20Improvement/201213IssuesandRationale-GestationalAgeTerm.pdf Obstetric Data Definitions Issues and Rationale for Change - Gestational Age & Term] {{webarchive|url=https://web.archive.org/web/20131106111500/http://www.acog.org/About_ACOG/ACOG_Departments/Patient_Safety_and_Quality_Improvement/~/media/Departments/Patient%20Safety%20and%20Quality%20Improvement/201213IssuesandRationale-GestationalAgeTerm.pdf |date=2013-11-06 }} from Patient Safety and Quality Improvement at [[American Congress of Obstetricians and Gynecologists]]. Created November 2012.</ref>
* Directly calculating the days since the beginning of the last menstrual period
* Early [[obstetric ultrasound]], comparing the size of an [[human embryo|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.<ref name=acog2012/>
* In case of [[in vitro fertilization]], calculating days since [[oocyte retrieval]] or [[co-incubation]] and adding 14 days.<ref>{{Cite journal | last1 = Tunon | first1 = K. | last2 = Eik-Nes | first2 = S. H. | last3 = Grøttum | first3 = P. | last4 = Von Düring | first4 = V. | last5 = Kahn | first5 = J. A. | title = Gestational age in pregnancies conceived after in vitro fertilization: A comparison between age assessed from oocyte retrieval, crown-rump length and biparietal diameter | doi = 10.1046/j.1469-0705.2000.00004.x | journal = Ultrasound in Obstetrics and Gynecology | volume = 15 | issue = 1 | pages = 41–46 | year = 2000 | pmid = 10776011 | s2cid = 20029116 | doi-access = free }}</ref>

Gestational age can also be estimated by calculating days from [[ovulation]] if it was estimated from related signs or [[ovulation test]]s, and adding 14 days by convention.<ref>{{Cite journal | last1 = Robinson | first1 = H. P. | last2 = Fleming | first2 = J. E. E. | doi = 10.1111/j.1471-0528.1975.tb00710.x | title = A Critical Evaluation of Sonar "crown-Rump Length" Measurements | journal = BJOG: An International Journal of Obstetrics and Gynaecology | volume = 82 | issue = 9 | pages = 702–10 | year = 1975 | pmid =  1182090| s2cid = 31663686 }}</ref>

A more complete listing of methods is given in following table:<ref name=Hunter2010>[http://www.medscape.com/viewarticle/703501_4 A Simple Solution to Dating Discrepancies: The Rule of Eights] {{Cite journal | last1 = Hunter | first1 = L. A. | title = Issues in Pregnancy Dating: Revisiting the Evidence | doi = 10.1016/j.jmwh.2008.11.003 | journal = Journal of Midwifery & Women's Health | volume = 54 | issue = 3 | pages = 184–190 | year = 2009 | pmid =  19410210}}</ref>
{|class="wikitable"
! Method of estimating gestational age !! [[Statistical dispersion|Variability]] (2 [[standard deviation]]s)<ref name=Hunter2010/>
|-
| Days from [[oocyte retrieval]] or [[co-incubation]] in [[in vitro fertilisation]] + 14 days || ±1 day
|-
| Days from estimated ovulation in [[ovulation induction]] + 14 days || ±3 days
|-
| Days from [[artificial insemination]] + 14 days || ±3 days
|-
| Days from known single [[sexual intercourse]] + 14 days || ±3 days
|-
| Days from estimated ovulation by basal body temperature record + 14 days || ±4 days
|-
| First-trimester physical examination || ±2 weeks
|-
| Second-trimester physical examination || ±4 weeks
|-
| Third-trimester physical examination || ±6 weeks
|-
| First-trimester [[obstetric ultrasonography]] ([[crown-rump length]]) || ±8% of the estimate
|-
| Second-trimester obstetric ultrasonography (head circumference, [[femur]] length) || ±8% of the estimate
|-
| Third-trimester obstetric ultrasonography (head circumference, femur length) || ±8% of the estimate
|}

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.<ref name=Hunter2010/> 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 [[standard deviation|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.<ref name=Hunter2010/>

Once the estimated due date (EDD) is established, it should rarely be changed, as the determination of gestational age is most accurate earlier in the pregnancy.<ref name=pmid25244460>{{cite journal |doi=10.1097/01.AOG.0000454932.15177.be |pmid=25244460 |title=Committee Opinion No 611 |journal=Obstetrics & Gynecology |volume=124 |issue=4 |pages=863–866 |year=2014 }}</ref>

Assessment of gestational age can be made based on selected head and trunk parameters.<ref>{{Cite book |last1=Kędzia |first1=Alicja |title=The computer-aided scientific research |last2=Woźniak |first2=Jowita |last3=Ziajkiewicz |first3=Marcin |last4=Dudek |first4=Krzysztof |last5=Derkowski |first5=Wojciech |date=2009 |publisher=Wrocław Scientific Society |isbn=978-83-7374-060-0 |editor-first= |series= |volume=16 |location= |publication-date=2009 |pages=241–246 |chapter=Foetal age assessment on the basis of selected parameters of head and trunk}}</ref> Following are diagrams for estimating gestational age from [[obstetric ultrasound]], by various target parameters:
<gallery>
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)
</gallery>