Expert: Catherine Beier MS CBE - 11/24/2008

Question
My last period was on June 13 2008 (LMP = March 20 2009). A couple weeks after my expected period, I took a home pregnancy test around July 22 2008 with a positive result. I scheduled an appointment with my doctor on the 14th of August, which I would've been just a day short of 9 weeks pregnant. At that time she checked for a heartbeat with a doppler, which we heard clear and loud. She told me that she hardly ever gets a heartbeat at 8-9 weeks, so she suggested that I was one or two weeks sooner than we thought. I wasn't able to schedule an ultrasound scan until the end of October (27th). From that scan I was told that I was actually 18 weeks and 6 days. Which is closer, but still almost a week off from my original LMP date of March 20th '09. What confuses me, is that I've been told later pregnancy ultrasounds aren't as accurate as to ones that were scanned earlier in the pregnancy. Should I follow my original LMP date or follow the given ultrasound date? I had believed my conception date was before June 27th, but now with the mixup of dates, I don't know when my conception date was either. Thank you.


Answer
Lily,

You are correct that ultrasounds after 12 weeks can be off as much as +/- 2 weeks.  Due to this, most due dates aren't changed unless the difference is greater than 2 weeks.  

Since you didn't have an early ultrasound, which is the most accurate way to determine a due date, your best bet would be to go with the latest due date.

Your doctor was just guessing that you were further along and couldn't support that assumption without a transvaginal ultrasound to confirm it.


So, I'd take the latest due date and remember that it is only a guesstimate anyway, the baby will come when s/he is ready.


Best,
Catherine