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When figuring the due date by a sonogram, the doctor will measure the crown-rump length (CRL).
The length of the embryo on the longest axis constitutes the crown-rump length. This is actually considered to be the best parameters to date the embryo. The accuracy of it is plus or minus 3 to 5 days.
A rough rule of thumb is that CRL + 6.5 = Menstrual Age in Weeks.
That said, it doesn't matter what week the sonogram is taken to determine its accuracy, because it is based on the CRL measurement of the fetus. No matter what the length is (because it was done on different weeks of gestation) is will directly relate to the week of gestation and ultimately determine the EDD (estimated date of delivery). The EDD is just that, and educated guestimate (guess/estimate) and is not meant to be written in stone.
When the due date has been set by an accurately measured CRL, it shouldn't be changed by a subsequent scan. If another scan done 6 to 8 weeks later says that you should have a new due date that is further away, they should not normally change the date. Instead, they should interpret this finding as that the baby is not growing at the expected rate.
After more testing has been done to determine whether the baby is growing properly and there is no other cause for concern, then it is feasible that perhaps the original measurement of the CRL wasn't properly measured by the person operating the sonogram. In this case, the new date is the actual date (considering the new measurement had been measured properly).
Keep in mind, however, when trying to figure your conception date that a full term pregnancy is considered to be 38-42 weeks, and that the accuracy of the sonogram being plus or minus 3-5 days. The date you actually give birth is directly related to the CRL and other factors of development of the fetus. Also, an egg doesn't become fertilized and implanted immediately after ejaculation. It can take up to 72 hours post-sex before that happens.
By taking the due date you have been given, count back 40 weeks. The date you get is only a rough estimate of the date of conception. It could be off by about one week. The day you recall having sex should be within that week, either before or after the date you counted back to.
Please note: This really gives you a two week timespan. Example: If the conception date you come up with is May 10 and you know you did not have sex that day, you could have potentially conceived that day (but that is kind of beside the point for the moment). You may have had sex anywhere from May 3 to May 10, or May 10 to May 17 and conceived up to 72 hours after you had sex in between May 3 to May 17.
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∙ 16y ago
Wiki User
∙ 14y agoEstimating due dates by ultra sound in the first 18 weeks are considered very accurate with a margin of error 1.2 weeks. If the ultra sound dates and LMP dates are off more than a week, your doctor will probably do another ultra sound in a couple weeks to compare. Very often a women will conceive earlier or later than they thought due to an irregular cycle.
As long as the repeat ultrasound shows consistent growth and there are no signs of problems, there is no reason to be concerned. Regardless of your due date, it is only an estimate for the baby's actual birthday, as 39 to 41 weeks is still considered within the norm. Only 5% of all babies arrive on their due date.
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∙ 12y agono not yet
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Can a 5 week fetus be seen in an ultrasound?
can a 5 week fetus show on an ultrasound
Had an ultrasound yesterday. LMP was Nov 2 so you should be 7 weeks The ultrasound only showed you being 5 weeks pregnant you are going back next week for another ultrasound Should you be worried?
You have two due dates...one accourding to your last period and one accourding to the size of the child on the ultrasound...i think the ultrasound one is more accurate
Can an ultrasound at 6 weeks be off by 5 days?
Are you sure they didn't say 6 weeks + from conception, at this stage an ultrasound is unlikely to be 10 days off unless you have a long cycle. I had a 36 day cycle and my LMP was December 17 and my EDD from that was September 23. My scan date was October 4 which turned out to be the date my son was born.
How accurate is the ultrasound due date?
They are accurate within 2 or 3 days. But this does not mean that you will have the baby on your due date. Babies can be born any time between 37 and 42 weeks. That is what midwives are taught anyway. With my first my ultrasound said I was due 10 days after my period date but my husband had been away on a trip form the day after conception! Second time around the ultrasound agreed completely, and showed twins. (now 16) == Your last known period date or the date of conception is a better predictor....as all babies are different & you may have an extrememly large or an extremely small baby for it's gestational age, and when compared against babies in the "norm" the due date could be way off.
Can i be pregnant and a ultrasound not detect it?
yes, i had a doctor tell me that an ultrasound will not show a pregnancy until 5 weeks. that is not a fact but just what i was told. when i was pregnant the ultrasound lady had a hard time seeing my baby and i was 5 weeks, i had to go back the next week and she saw the baby perfect.
How many weeks after conception does the first heart beat occur?
week # 10==================== Using a Doppler ultrasound, the fetal heart can be detected at around the 6th - 7th week of pregnancy these days.
I went to a clinic on and the doctor said I was 5 weeks and 3 days. The next day I went to the hospital and they drew blood and did an ultrasound and said that I was 4 weeks which is more accurate?
You must have given them two different guesses on your last period date. But the hospital would be more accurate.
If you dont know your lmp and had ultrasound at 5 weeks 4 days how accurate are the dates they give you?
very accurate (within one or two days)- they tell this by several different things
How accurate are the ultrasounds for dating a pregnancy?
My wife told her doctor she didn't know when her last mp was, so he sent her for an ultrasound! She lied, we just wanted 2 get an early ultrasound, and it was exact 2 the day and that was at 5 weeks 4 days! Our 1st baby measured within 5 days thru the entire pregnancy, and that was as early as 8 weeks and as late as 26!
Could i be father along in my pregnancy than what im told consdering i had irregular period when my periods normally last 5 days but lasted a week with spotting?
Yes. You should tell your Dr so that they can preform an ultrasound to provide you with more accurate dating.
What are the release dates for Week in a Day - 2010 Week of Comfort 1-5?
Week in a Day - 2010 Week of Comfort 1-5 was released on: USA: 2010
Can an ultrasound scan be 2 to 3 weeks out The scan says 7 weeks 1 day but could it be that you are actually 10 weeks 1 day pregnant or are they extremely accurate this early?
Yes, don't worry about the difference.By 4 weeks, a fetus cannot be seen but the thickening of the uterine lining can be detected. By around 5 weeks, an ultrasound can almost always confirm that you are pregnant.Between four and a half to five weeks, the gestational sac is easily seen and can be measured, since the average growth is 1mm per day at this time but not completely uniform this information is considered a tentative date when LMP date is not known.A few weeks later Ultra Sound to determine fetal age is done by taking a measurement of either the femur (longest bone in the body) or the crown of the head to the pelvis called the "crown rump". The measurement is taken and automatically then by comparing your baby's measurements to the data from a large collection of measurements, the ultrasound can then tell how far along your baby is.This is most accurate between week 7 and week 14.Doctors have fallen into using a rule in ultrasound that when the due date based on ultrasound doesn't vary from the mother's dates by more than a week, stick with the mother's dates; if the ultrasound disagrees by more than a week to ten days, it becomes wiser to rely on the ultrasound.Later in pregnancy other things such as babies who are larger than average size, babies growing at different rates among pregnancies, will cause inaccurate dating of pregnancy. But are still an excellent tool for other issues that may develop.The bottom line is, Ultra Sound is a machine made and run by humans, it is an amazing tool for many things, but babies are still going to come into this world when they are ready, regardless of dates. Only 5 to 10 percent of babies arrive on their due date, whether the date is by ultra sound or LMP.
