Trying to estimate conception date and understand ultrasound dating! Help!

[chloehope01]

Okay, so I'm trying to figure out when conception most likely occurred and understand how all of the dating is done. My LMP was March 28, 2015 but my period was due March 26, had positive pregnancy test that day and then got my period two days later on the 28th (so I had a chemical pregnancy of sorts ) so Idk if that will throw off ovulation days. I have a mostly regular 30-31 day cycle. I had my first scan at 8 weeks from LMP and baby's ACTUAL measurement was 7 weeks. At my 12 week scan from LMP baby's ACTUAL measurement was exactly 12 weeks and they said that was great because it was right along with LMP dating of 12 weeks. What I don't understand is why should you match up with LMP when that would mean conception happened on the first day of my cycle which is pretty much impossible. So, would it mean that I conceived around the 4th of April which coincides with baby's actual measurement at my 8 week scan? Is there are chance I conceived a week later around the 11th? I have read that the ultrasound measurements usually aren't off by more than 3-7 days. Does that mean the actual measurements aren't usually off by that much or the measurement from the LMP? It's all kind of confusing to me so any input you gals can give would be great!

 

[Blu10]

Id go by the date at your dating scan, 12 wreks is supposed to be most accurate for dating purposes. Unless you can be certain when you ovulated then its hard to oredict and even then you can implant later. Congrats on your pregnancy x

 

[minties]

The first 2 weeks of pregnancy, you aren't actually pregnant. They used to work out your due date from LMP as it was a date that most women would know and there were no scans in the "old days". So 4 weeks pregnant is about 2 weeks post ovulation.

For example, say your period was on the 1st July, had sex on the 13th (or thereabouts) then you ovulated around the 15th, and got a positive test on the 28th you would be called 4 weeks pregnant.

 

[bunyhuny]

The first two weeks of pregnancy are before ovulation, so you take the date that you measured 12 weeks, and you count back 10 weeks to find your approximate O day.

 

[chloehope01]

Thanks for your help!!! Understanding it all better now!

 

[bunyhuny]