Expert: Dorothy H, LCCE - 12/18/2009

Question
QUESTION: So I just want to figure this out. I understant that 2 weeks are included in all the calculations, but what if you have irregular or very long periods? That means u/s dating will not fit in with your lmp. What if it's been 10 weeks from your lmp but ultrasound measures 8 weeks? Does that mean that 2 weeks are included in this calculation as well and you conceived around 6 weeks ago or does that mean you actualy conceived 8 weeks ago?
How does that work?

ANSWER: Hi Megan,

Yes, if you have irregular or long periods, the u/s dating will not line up with the dating from your lmp.

Generally, if the u/s is measuring 8 weeks, they are counting 8 weeks from your lmp, which would mean that you conceived 6 weeks ago.  

I hope that helps!

Dorothy H, LCCE

---------- FOLLOW-UP ----------

QUESTION: Well I've seen posts where the the info doesn't add up. Let's say if your lmp was 01/01 and your cycles are 45 days you don't really ovulate on day 14. You probably ovulate on day 30, therefore since ultrasound adds on two weeks it would not match up with your lmp and u/s done an 02/14 would show 4 week pregnancy(2 weeks since conception) and not a 6 week pregnancy. This is why I though u/s is more accurate than lmp since they measure the size of a fetus.  Am I making any sense?

ANSWER: Yes, you are right - if the ultrasound dates  don't match up with the dates going by your LMP,  then they will date the pregnancy by the ultrasound, because it's much more accurate. A first trimester ultrasound generally has a accuracy of +/- 3 days, whereas if you date yor pregnancy by your LMP, especially if your cycle tends to be irregular or long, it can be off by weeks.

With a first trimester ultrasound, they are going by when certain things happen with the embryo- there are several things that happen at week 4, other things at week 5, etc.  which is why it is much more accurate in dating a pregnancy.

I hope that answers your question! If not, please let me know.

Sincerely,

Dorothy H, LCCE

---------- FOLLOW-UP ----------

QUESTION: Thank you for your patience. I know I'm asking so many questions. I guess I have one more. So when u/s matches right with your lmp, does that mean you "go according to the book and ovulate on/around day 14 of your cycle"?

Thank you again, you've been very helpful!  

Answer
Not a problem!

Yes, when your u/s matches right with your lmp, you ovulated that month around day 14.

However, if your cycle is not regular that may not always be the case - you may ovulate on day 14 one month, day 18 the next month, day 12 the following month.  

I hope that answers your question!

Dorothy H, LCCE