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The period of gestation is calculated from the last menstrual period. add the number of days till today and you would know your gestation.
Based on beta hcg levels there is always a range and they are not good predictors of period of gestation. However a transvaginal ultrasound can also date the pregnancy based on the size of the gestational sac or the crown -rump length , but for all practical purposes LMP is used as standard to calculate period of gestation and thereafter the EDD as well.
I hope i have answered your query in detail.
wishing you safe pregnancy,
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