Pregnancy 35 and Older Community
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Avatar universal

Calculating pregnancy

Okay so I am pregnant with my 4th child and I have never understood how they calculate how far along you are. It is not possible to be pregnant on the first day of your period so to count day 1 as pregnancy day 1 seems stupid to me. Now my due date and the Dr.'s due date are only different by a day so it shouldn't matter much except that by putting me 2 weeks ahead of my pregnancy by going from my period would put baby 2 weeks ahead in testing and ultrasound. 2 weeks of a difference is a big deal for a developing baby. Apperently knowing conception date is an oddity but yes I know exactly when I concieved. We were trying, we didn't have sex for days and days prior to ovulation, I have regular cycles and ovulate 14 days before my period (I confirmed this with an ovulation test) we did not have sex for days after ovulation (when ovulation test said I was no longer ovulating. Given sperm living 5 days but egg only living 24hrs conception had to occur within that 24hr period. So why, given all of this, would I be 2 weeks further along then I actualy am? A pregnancy is only 38 weeks but due to the Dr.'s putting you 2 weeks further, they go by 40 weeks so the due date is the same. I do not understand why though, they would put you 2 weeks ahead if you know conception date? At first ultrasound they are expecting to see an 8 week fetus but its a 6 week fetus would that not throw the testing and measuring off?
2 Responses
1701959 tn?1488551541
They will use your first ultrasound as the guide for due date and development. The period due date calculator is truly just an estimate since so many have periods that are not a normal 28 days. They will adjust your due date once they get the results of the sonogram.
Avatar universal
They add the 2 weeks because in the days before scans, they *had* to rely on LMP as they had no more accurate way of estimating due date.  You're right that in this day and age, with ultrasounds it makes no sense to add the additional 2 weeks because they can tell from the scan when the baby was conceived.  However  all the text books/timelines include the extra 2 weeks.  So it's easier just to stick to the old fashioned system.  When they do an 8 week scan, they know they are seeing a 6 week foetus, but I don't imagine they'll change the system any time soon -especially as not all women have an early ultrasound and it would be chaos if some women had their pregnancy dated from lmp whilst others had theirs dated from conception.
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