Pregnancy dating scans how accurate
Using the LMP for dating does not account for inaccurate recall of the LMP or irregular cycles.
In a study by Women’s Health (2005), approximately one half of women remember their last menstrual period date correctly.
As the phrase puts it, it’s an estimated date of delivery (commonly termed as EDD).
It’s never an accurate date, you may deliver several days before or after the EDD. The date has traditionally been calculated forwards based on the first day of your last period, but this presumes regular menstrual cycles of 28 days.
Read our report on the 13 best pregnancy i Phone and Android apps of the year here. Very few women actually deliver on their due date, so, while it’s important to have an idea of when your baby will be born, try not to get too attached to the exact date.
Any other suggested dates, especially if based on scans done in advanced pregnancy, can just be ignored.
Even when you can remember when your last menstrual period began, ultrasound can still be more accurate than relying on your last period date alone.
The American College of Obstetricians and Gynecologists state the following: “Ultrasound measurement of the embryo or fetus in the first trimester (up to and including 13 6/7 weeks gestation) is the most accurate method to establish or confirm gestational age” (Committee Opinion, No. Even though using your last period date (LMP) is a traditional dating method, the accuracy of this practice depends on a regular menstrual cycle of 28 days, with ovulation occurring on the 14th day of the cycle.
Subsequent scans done further into the second and third trimester have increasing margins of error, and can be erroneous by a factor of weeks.
ALSO READ: Safe exercises to close your belly gap after pregnancy The due date should be agreed on pretty early in the pregnancy, preferably by a first trimester dating scan.