QuestionQUESTION: So when you get an ultrasound and it reads 13weeks and 5days on the screen what does that mean? i heard you take 2weeks off that and that is conception date but how does a ultrasound machine add two weeks to the fetal age? and why do they give you time since lmp instead of conception?
ANSWER: Dear Holly,
Usually, when they measure dates, they count from the LMP and not conception. The reason they do this is because the average cycle length is 28 days, and a woman with a 28 day cycle would ovulate and probably conceive on day 14. This is not a great practice, however, since every woman's cycle has different lengths, so not all women conceive on day 14. But many medical practices are based on the 'average' or 'normal', and don't allow for variations.
So the ultrasound is not adding two weeks to the fetal age, they're just adding two weeks to the 'weeks of pregnancy'. A pregnancy is counted as 40 weeks, counting from the LMP, for a real total of 38 weeks of actually carrying a baby.
If you want to know your real conception date, you would figure out your regular length of your cycles, and subtract 14 days from that number. So if your normal cycles are 35 days, then you probably conceived on day 21 or so, meaning, 21 days after your LMP.
If you don't know your LMP, they'll use the dates they figure out on the ultrasound.
Best of luck, and if you want any clarification on anything I mentioned, feel free to ask again.
Sheva
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QUESTION: So actually when it say 13wks and 5days on an ultrasound screen its not the actual fetal age its the weeks pregnant you are so fetal age is really 11wks and 5days
ANSWER: Yes, usually that is what it means. But it is a good idea to confirm that with your practitioner to be 100% sure that this is how they set up their computers.
Thank you for your question,
Sheva
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QUESTION: so one last question when the measurements start popping up on the ultrasound screen thats measurements from lmp or how big the baby is im just confused on the ultrasound print out if it says 13wks and 5days if thats not really how big your baby is if its only 11wks and 5days old it just confuses me can you help me understand this better like the ultrasound i had and didnt tell them my lmp she said i was 13wks and 5days then she used the wheel and said i prob conceived nov 6 but it was just and estimate..how is the ultrasound gonna show 13wks and 5days if i didnt memeber my lmp?? i hope this isnt too confusing
AnswerYes, you're right. If it says 13 weeks, 5 days, it means your baby is about 11 weeks and 5 days old.
They use the wheel to takes off two weeks from those 13 wks, 5 days, and that shows when you probably conceived.
Since you don't know your LMP, they have to guess how old your baby is, based on the ultrasound, (11 wks, 5 days old) and then count back two weeks from there to estimate your LMP, since on average, women ovulate and conceive about 2 weeks after their period starts (so that would add up to 13 wks, 5 days).
(Of course, there are variations to this, but since you don't know your LMP they estimate it using the average numbers.)
Remember, your due date is always an estimate, even if a woman does know her LMP, and your baby can safely come between 38 to 42 weeks.
So even if you don't know your LMP exactly, you can still have a healthy pregnancy and a healthy baby.
Good luck,
Sheva
