Curious about Estimated Due Dates

geminimommy

So I know an estimate is just that... an estimate.  But there are a few ways to figure out the estimated due date, and they don't seem to add up to what my obgyn told me at the last appointment.

Based on my last period, which was 9/21/10... my due date is calculated as 6/28/10. 

Had my ultrasound appt last week and the dr. said I was due 7/4.  We heard a heartbeat and everything looked ok... but she said I was measuring at just about 7 weeks. 

A whole week's difference confuses me.  Could she just have made a mistake (she got it off the wheel of estimated due dates)? 

I'm just concerned that it throws off some of my appts, ultrasounds and testing.  I want to be able to calculate my trimesters too.

Anyone thought they were at one point in the pregnancy and told something different later on? 

Shelleybell

based on my lmp i was a week later then initally thought. Being off a week is totally normal. I wasn't charting, didn't know exactly when i o'd and as long as the baby is growing normally it is totally fine.  I asked my uncle who is a doctor how the doc pin-pointed i was  6 weeks 4 days when i thought i was 7 weeks 1 day- he said it is very very scientific and they can tell pretty accurately how far you are based on the baby's measurements.

ohfour

Going by the first day of your LMP, it assumes ovulation around day 14 and adds 2 weeks on to how "pregnant" you are. For example, if you are 7 weeks according to your LMP, then your baby is actually only 5 weeks in reality. For me, that would be off because I know I ovulated on CD 23, so going by my LMP would be 1 week off of their calculations since I O'd a week "late". I don't know if that makes any sense.

Anyway, like a pp said, the baby's measurements can usually give them a pretty good estimate.

LauraOTR

Calculating from LMP is really pretty inaccurate, unless you have "perfect 28 day cycles" which few women have. You probably just ovulated later than the assume 14th day. I ovulated on day 19 or 20 and my ultrasound matched up with that. Many women don't know their ovulation date, which is why they use LMP as an estimate.

Go by the ultrasound measurement. It's more accurate.

gymnst1013

LauraOTR:

Calculating from LMP is really pretty inaccurate, unless you have "perfect 28 day cycles" which few women have. You probably just ovulated later than the assume 14th day. I ovulated on day 19 or 20 and my ultrasound matched up with that. Many women don't know their ovulation date, which is why they use LMP as an estimate.

Go by the ultrasound measurement. It's more accurate.

Exactly.  And even with a 28 day cycle, that doesn't necessarily mean you ovulated on CD14.  Could have been days later.

cherylanddoug

LauraOTR:

Calculating from LMP is really pretty inaccurate, unless you have "perfect 28 day cycles" which few women have. You probably just ovulated later than the assume 14th day. I ovulated on day 19 or 20 and my ultrasound matched up with that. Many women don't know their ovulation date, which is why they use LMP as an estimate.

Go by the ultrasound measurement. It's more accurate.

this exactly.

kg_08

The ultrasound this early in the pregnancy is the most accurate date. Not everyone ovulates at exactly the same time, so you could have just ovulated late. I was a few days off as well.

hurricane1212

If it makes you feel any better... If I had gone by my LMP, my LO would be measuring almost 6 weeks behind. We obviously had to go by what the ultrasound showed and baby is still very healthy and totally normal!