QuestionQUESTION: How accurate can an ultrasound date a pregnancy at 13 weeks and 3 days.. How many days of error are possible at this time..and also.. Why doesn't my actual LMP match ultrasound LMP I measured 9 days ahead.
ANSWER: Dear Jess,
An ultrasound at that point can already be off by a few days. The later on in pregnancy that an ultrasound is performed, the more inaccurate it is.
It's not clear from your question whether your ultrasound LMP was before or after your actual LMP.
Since women ovulate approximately 14 days BEFORE their NEXT period, if you have a 28 day cycle, you will ovulate on or around day 14 of your cycle, and conceive around then, too. Meaning, 8 weeks after your LMP your fetus will be (approximately) 6 weeks old, but you will be considered 8 weeks pregnant.
However, if you have 35 day cycles, you will ovulate and conceive around day 21 (figuring 35-14=21), so, 8 weeks after your LMP your fetus will be only 5 weeks old.
However, an ultrasound measures how many weeks gestation you are - meaning, how many weeks after conception, and then adds two weeks onto that to figure your LMP, because they just base everyone's pregnancy on the "average" of a 28 day cycle.
This is important because if you have longer cycles, and you count only from your LMP, at 40 weeks your baby has actually gestated shorter than 38 weeks, which is full term. So counting from the later (fake) LMP that your ultrasound calculated buys your baby more time in utero, which is a good thing.
I hope this is clear. Please feel free to ask anything further.
Congrats on your pregnancy, and best of luck!
Sheva
---------- FOLLOW-UP ----------
QUESTION: So it wouldn't be off by like 10-14 days correct.. And my lmp was 10/27/2011 by ultrasound it have me a lmp 10/18/2011..oh and also when they use the ultrasound is it true the first two weeks in not actually pregnant!!
AnswerDear Jess,
At this point in pregnancy an ultrasound is not usually off by 2 full weeks, but it can be off by a few days. (Some technicians are not so good at their jobs, but if you have had a few ultrasounds and they all measured the same, then it is probably accurate.)
It is very possible that what you thought was a period on 10/27 was actually bleeding from implantation, which is when the embryo burrows into the lining of your uterus, and can sometimes cause mild bleeding. Then your actual LMP would have been the month before. Speak to your doctor or midwife about this being the possible explanation for the big difference in dates.
To answer your last question, yes. When they give you the weeks of pregnancy, they are counting from your 'LMP', which, in women with a 28 day cycle, is 14 days before conception, so at 14 weeks pregnancy you would be only 12 weeks after conception, with the first two weeks not being pregnant at all!
Have a great day!
Sheva
