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OT: Is ultrasound or LMP more accurate?

hollyissad's picture

Hi all! I'm just wondering if you've had experience with whether your ultrasound or your last monthly period are more accurate for calculating your due date.

My last monthly period would put me at a due date of July 23rd. My first ultrasound was at the emergency room, and that put me at a due date of July 18th. I had a second ultrasound that also put me at a due date of July 18th. My OB first said that we would go by the July 23rd due date, but then after learning that two ultrasounds both agreed with one another, decided to go with July 18th, with the knowledge that I might be a few days later. Anyone have experience with this, and with what is more accurate? I really want to be sure of my due date! Smile


Teas83's picture

With my DD (now 2) I went with my LMP and she was born on her due date. I live in Alberta, Canada and we don't get ultrasounds for due date purposes unless you have no idea when your last period was. I just had the regular scheduled ultrasound at 18-20 weeks and they didn't say anything about confirming or denying my due date. I'm pregnant again now so I'll have to see if it works out again.

z3girl's picture

I've had 3 children, and I would say LMP is the "best" way to calculate your due date only because your due date is not very accurate to begin with! The difference between the 18th and 23rd is very minimal.

Due you know the date you may have conceived? With my first child, I knew when I ovulated, so based on LMP, I was due 4/5, but I ovulated later so my doctor opted to have my due date 4/10. Ultrasounds are not entirely accurate, so unless the measurements are way off, my doctor prefers to go by my cycle. We went by LMP with my second, and he came 2 weeks early on his own. With my third, we ended up inducing. Once you are past 37 weeks, you hope to last at least 39, but can happen anytime.

Your doctor hopefully knows you and your body, so trust him/her. Hopefully you won't have any complications, so you'll be wishing to get the baby out the 18th!

Maxwell09's picture

I used my LMP but it really didn't matter. They said to think of the due date as the middle of a two week window of when the baby could come. The baby can come two weeks before, on, two weeks after or anywhere in between. They turned out to be right in my own pregnancy and all that I've known about. SS4 was two weeks late, both my bestfriends were 3 weeks early as was I. I was really hoping I would have had BioBaby on his due date because nothing else happens in that month but baby's come when they want.

Sunflower1's picture

With DD I was positive of my dates, first ultrasound showed DD's due date three days earlier, but we kept the original due date. I still have to be induced at 41+5. They just grow at their own pace.