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There are multiple ways of dating a pregnancy.The best and the simplest way to date is to calculate the number of weeks and days from your last menstrual period (LMP)( provided you have good dates as in your cycles have been regular in last 1 year). for example if your beta hcg levels say that you are pregnant between 5-8 weeks then your likely LMP must have been 17th September 2014 or 8th October 2014 (8weeks or 5 weeks respectively).
Second method is to ascertain the ultrasonological age of the conception by doing a dating scan transvaginally which can give an estimated age of the gestation based on haddlock formula .Accordingly the date of conception can be devised.
Beta hcg based levels confirm the pregnant levels but provide a wide range to actually delineate the date of conception.
For all practical purposes the LMP is taken as the marker for counting the period of gestation.It would be advisable to undergo an ultrasound first and accordingly calculate the date of conception , that would be more accurate dating for the conception.
I hope i have answered all your queries in detail.
Wishing you best of health and a safe pregnancy,
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