Completely confused about my due date!

[NETTEYJC]

I am pregnant with my first child, when I had my very first doctors appointment he told me that my baby would be due on 20/09/10, after this my midwife confirmed that this was my expected due date, however at a recent scan I was told that my baby is due on 27/09/10, I know that there is only a weeks difference but I am totally confused at the due date now? Surely the doctor and midwife have it right as they went by my period dates, the first day of my last period being 14/12/09. Could anyone shed any light on this and tell me what my due date is??!

Here the scan done at 12 weeks is a dating scan and that is the date used as EDD. The LMP dates are just a guide here.
Not sure about everywhere else though sorry.

your scan date is the most accurate. i had 3 due dates for dd1 but they took then scan as the actual date and i had her on her due date. dd2 had 2 due dates and i had her on her scan date. my children are very punctual

An EDD based on the first day of your last menstrual period (LMP) assumes that you have a 28 day cycle of which you will ovulate on the 14th day, as per the "average" woman, whoever she may be!

An EDD based on a 12 week scan looks at various measurements of your baby (IIRC there's at least Crown-Rump Length, Femur length and possibly head and/or abdominal circumference) and calculates a date assuming that your baby has been growing at the same constant rate as all other babies to that point.

This is where it gets tricky, because whichever method you use you're making assumptions. I think the current wisdom is that babies only grow at a constant rate for the first 8 weeks and after that you can only look at an average - IIRC the error margin at 12 weeks based on "average" growth (ie scan) is +/- 1 week. Very few women follow the theoretical "average" model and ovulate on day 14, and even if they do you can't factor in exact time of fertilisation or implantation, both of which will impact on growth. I think around here they tend to use LMP unless the 12 week scan is out by more than a week - at which point they'll use the scan date.

Best thing to do is to remember that "E" in EDD stands for Estimated. Some babies do arrive on their EDD, but they are in the minority. A "normal" gestation for a human is anywhere between 37 and 42 weeks so try not to get too fixated that your baby "should" arrive on a given day as you're more likely than not to be disappointed!

Oh - and congratulations

Many thanks, things much more clearer now - the hospital didn't explain anything at all! It's just a pain in the ass when according to midwife dates another month has passed and then all my dates are a week behind if I go by scan dates!