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The length of your pregnancy is calculated from the first day of your last menstrual period (LMP), even though you don't actually conceive until 14 days after your LMP, and later than this if your cycle is longer than 28 days.

The LMP is used because most women know when their last period was, but may not know when they actually conceived.

The timing of certain tests, the monitoring of the baby's growth, and the correct diagnosis of premature labor, or being truly "overdue," (postdates), as well as many other situations that arise in the course of a typical pregnancy, all depend on a correct determination of the EDC for appropriate management.

In the past, the EDC was calculated by using Naegele's Rule, which determined the date by subtracting 3 months from the 1st day of the last period and then adding 7 days.

But ultrasounds can also lift the lid on a few worries you might be having. The main reason for the ultrasound is to work out how many weeks pregnant you are, and to estimate your due date.I went in to the OBGYN today to go over blood tests that I'd submitted a few weeks ago, and for what I thought was an ...but, turns out I'm farther along than I thought, and I'm 10 weeks and 1 day (instead of 8 5)!I checked online and it seems ultrasounds are pretty accurate for dating for the first trimester...anyone else have similar results and a change of dates?However, it is extremely common to encounter patients who have irregular or infrequent menstrual cycles, or have fibroid tumors that cause their uterus to feel enlarged on pelvic exam, or who conceive shortly after a pregnancy ends (without ever actually having had a period after the last pregnancy), or who got pregnant while taking birth control pills, and these situations often render the above methods useless and misleading when trying to figure out a reliable EDC.This is where our wonderful ultrasound machines can make a crucial difference.Hi, I think with the very early scans, 5-6 weeks they can be a little bit off.Mine sas I was 5 weeks but I knew from my calculations that I was a week further on.The cookies contain no personally identifiable information and have no effect once you leave the Medscape site.A registered nurse with more than 25 years of experience in oncology, labor/delivery, neonatal intensive care, infertility and ophthalmology, Sharon Perkins has also coauthored and edited numerous health books for the Wiley "Dummies" series.x what i don't understand is how babies measure different CRL yet that is how they measure the EDD on the ultrasound?If one baby is measuring say 67mm and one 76mm, and they date them both at 13w, how does that work??

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