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Avatar universal

How accurate are ultrasounds in determining gestational age and conception dates

So, a quick recap. I lived apart from my wife for a year. She is in the Navy. She was out to see for a while, but pulled back in of March 10th. I came down and we had sex March 11th. 2 weeks later, (without any symptoms) she takes a pregnancy test, and it turns out, she is in fact pregnant. We go to the doctor for an ultrasound, and she tells him that the last day of her menstral was on March 5th. The doctor tells her that she must be mistaken because the baby in measuring out to be a little over 14 weeks old. (EDD was 11/27/06 Conception date would have been 03/06/06) He suggests scheduling another ultrasound because 2 weeks is a big difference, and that could mean a lot down the road in the pregnancy, in regards to the babies health and proper stage of development. We get a second ultrasound (after I initially wrote you) to confirm the babies age and fetus measures out to be a little over 18 weeks at this point (EDD was 11/28/06 and conception date of 03/07/06)
so, I know that you can't predict anything, or tell me whether my wife may have made a mistake. I just want to know, if it were you, and being very knowledgable about pregnancy, would you be concerned? Would you have questions? If so, what questions would you ask?

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Avatar universal
I think early ultra sounds, that is with in the first trimester, are very accurate. Within 5 days. It is most likley that you did concieve earlier than you thought. I wouldn't be concerned, I would be excited. Pretty cool to shave off two weeks like that!! Good luck to you both!

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Avatar universal
I worked 5 years in an ob-gyn office and can tell you that u/s are very accurate in determining gest age and concept dates.
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Avatar universal
You said you had sex with her March 11 and the doctor is now saying the baby was conceived March 7th. That's only a difference of 4 days and ultrasound can definitely be off by 4 days or more. Actually they can be off by as much as two weeks in either direction but are generally more accurate the earlier they are done. But 4 days is so little of a difference that it shouldn't be of any concern.
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Avatar universal
hi my friend who is prego now has same story ... she got her periods in april n missed her periods in may she took a preg test it was positive she went to doc .. her blood work n ultrasound showed  she was 2 months prego  i mean she got prego in mid of april but had her periods. she was litrally shocked.what i also heard that some woman  have their 1st cycle like normal periods  during pregnancy .i hope it ll help u
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Avatar universal
At my ultrasound early on in the 7th week I measured right on. At my 14th week, because I had some trouble, theyhad to do an ultrasound and it was measuring a week n half ahead. The further along you get the bigger the distance is that they can be off. 4 days is no biggie at all! They say in early pregnancy they can be off by 5 days give or take either way. And once you're past like 12 weeks it can be 2 weeks either way! Good luck and congrats on the little one!!
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Avatar universal
I didn't explain things clearly in my first question.  What I really want to know is...should I be concerned about not being the father.  Due to the conception date...if it is in fact as accurate as you say, I wouldn't be the father.  I came into town on the 11th and the conception date is on the 6th.  And from what I've been reading online, the actual sexual act accures days before conception;  Possibly putting the actual sexual act anywhere between March 2nd and the 5th. I didn't come into town until March 11th.  I know that this may not be the right place to ask these questions but...if the ultrasound is in fact accurate, then I have something to really be concerned about.
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Avatar universal
according to a quick calculation i did on the calendar from counting from day of last period, march 5th, your wife would be in her 19th week of pregnancy this week. I'm confused as to when your last u/s was please say when you went in for last u/s. if your wife's last period was march 5th and you conceived on march 11th then she ovulated pretty early in cycle - at least on her 7th day of her period maybe that's why there's confusion in her weeks i don't know. what week does she think she's in?
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Avatar universal
From what you say it is possible that you may not be the Father. BUT I would not fly off the handle yet, you can always take a paternity test once the baby is born and future u/s may also show conception as being a few days later when you were actually in town. Sorry that you are going through this. It makes it hard to get excited when you have doubts.
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Avatar universal
well i would think if her last period was march 5th then it makes no sense with the dates. unless she never really had a period. if she actually did get her period march 5th and you were together march 11th and she didn't ovulate till a few days later (which makes more sense as women usually (not always) ovulate mid cycle (if 28 day cycle probably day 14), but sometimes ovulate earlier or later too)then it makes sense that you would be the father and they could be a few days off in their calculation. like the others said 4 days off isn't that big a deal in pinpointing day of conception.
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93532 tn?1349370450
If i am understanding your post right, the u/s showed only a 5 day difference from what you expected? If so that u/s seems to be within the margin of error of =/- 5 days or so. Honestly, they are NOT good at determining actual conception dates. It cannot be predicted which swimmer got there first or when it exactly happened, how long it took the egg to travel, or when the egg implanted. Those can all be off here and there.

My first u/s was a quicky one to check for a heartbeat and he did a fast measurement which had me a week later, which was impossible, literally. I had found out 11 days after I ovulated, which if the u/s would have been construed as right, would have had me testing + on a urine test at 4 days past ovulation, not even blood tests pick it up that fast.

As mentioned before, if you do suspect something is "off" a paternity test is the best suggestion. But given the dates, I am not sure that is warranted and that by asking for one may cause more problems.

I am going to congratulate YOU, as my guess you are the dad! BTW, conception can take place within hours of intercourse. Many women on here utilize ovulation predictor kits, which will tell you the egg is releasing in anywhere from, what, 12-24 hours? You only have a small window of time to catch that little bugger. So she could ovulate early in her cycle, released the egg, you had sex, caught the egg, and voila! Baby!!

Good luck, keep us posted on everything.

Andi
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Avatar universal
we had our first u/s on March 31st and the Doctor said that she was a little over 14 weeks then.  We had the 2nd u/s on June 26th and they said that she was around 18 weeks then.  The reason I'm asking about the accuracy of a u/s is because both ultrasounds gave extremely close due dates of november 27th and November 28th.  Both of those dates gave a conception date that was before I came into town.  Maybe I'm mistaken, I thought that the conception date would match the gestational age, because I thought that that is what the doctors are able to accurately determine based on the size of the child.  Both Doctors determined the gestational age to be virtually the same as the other.  I know that they can't tell me what night we had sex, that led to the conception of a baby; but I did think that they could accurately tell me the age of the fetus (like the date it began growing or something) based on it's size.  After the first ultrasound...I thought like everyone else: "Well, the ultrasound could be off by a week or two, and the next one will say something completely different."  Well the RED FLAG went up in my head when the second ultrasound confirmed the accuracy of the first. I thank all of you for your feedback...it has been insightful.  Please continue to let me know what you think.  2 heads are better than one...so hell, this forum is the best thing going right now.  None of the Doctors at the hospital will answer my questions unless my wife is present.  And the last thing I want to do is cause her to worry and stress while she's pregnant, whether it's my child or not...I don't want to make her worried.
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130384 tn?1221593027
Hi there,
Like Andi said, ultrasounds have a margin of error of almost a full week.  This is why they let a woman go into labor anywhere from 38 to 42 weeks - because no one can tell the EXACT date of anything that happened.  So even though the ultrasounds provided dues dates within 1 day of each other, they could still be off by almost 1 week either side of the due date.

I understand your concern.  I highly recommend a paternity test, unless you simply just want this baby and she says its yours.  You'll have to ponder that decision and decide what's right for you.  Good luck, hope this helps!
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Avatar universal
are you sure you don't mean that your first u/s was May 31st that showed at 14 weeks?
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121828 tn?1333464491
That must be a horrible feeling and I wish I had some techical answers for you. First though, have you asked your wife? There may be something you forgot, a date in question, she may be able to clear things up for you, maybe? Good luck.
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Avatar universal
So, you're saying that she was 14 weeks on March 31st and 18 weeks on June 26th?  How is that possible?  She should be about 26 weeks if the first date is correct.
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134578 tn?1642048000
I would never use size on an ultrasound to accuse someone of infidelity, especially if the difference is only about 5 days.  The embryo could have moved along slightly more slowly than standard or it could have implanted a couple of days later than people who write the charts counted on.  The charts naming baby size are a standardized measure, not a paternity proof.  It sounds to me like the baby is yours.
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127124 tn?1326735435
Do you have any other reason to doubt your wife was faithfyl. Please do not jump to conclusions and assume the baby isn't yours based on the ultrasounds.  They are not error proof.  With both my kids I knew exactly the days we had sex.  The ultrasounds showed I was 1 wk further along than would have been possible. With my 2nd pregnancy they had me 4 wks behind at the 1st ultrasound which meant I would have had a positive pregnancy test before conceiving.
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Avatar universal
i apologize ...the ultrasound was May 31st not March...sorry.
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Avatar universal
then it does sound like you're right on target - give or take a few days. don't worry, relax and just enjoy this wonderful time in your life. is your wife going to be able to be home now that she's pregnant or will she still have to be away with the navy? i hope that you get to spend the time with her so you can have all the fun and joys of pregnancy too. just be ready to run out in the middle of the night for any cravings. good luck.
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Avatar universal
actually i was thinking about it again and it really makes the most sense that you're the father. you bded probably within a 24-72 hour time frame of conception if her period was march 5th and she probably ovulated a little early. they're probably just a few days off with their calculations. Enjoy!!
Helpful - 0
Avatar universal
I understand your concern, but I have to say I really dont think you have anything to worry about.  First of all, it would take an extremely cold and calculating woman to have unprotected sex right before she came home to her husband.  Its not like she came back four months pg.  As for her taking the test without any symptoms - thats more normal than unusual.  Ill bet every single woman on this site does it almost every moth if she just thinks she mught be pg.  And Ill bet that after spending a year away from you, your wife was ready to start a family and just excited and anticipating it.  Honestly, the nedical establishment does not know as much as it thinks it does.  There are SO MANY stories on this site alone of ridiculous mistakes in gest. age, missing twin fetuses, etc.  Just relax and try to let these doubts go.  If you are saying you dont want to stress your wife, its obvious you two have a very good relationship, dont do anything to mess it up.  Women are at our weakest during pg, and an accusation as big as infidelity will leave a scar that will probably never go away.  Estimated gest. date is not a reason to accuse someone.  I got pg with my daughter (after three years of trying) on the day I was excpecting my period, according to my dr. it was impossible to have happened then, but I know it did because my husband was abroad for three weeks and we didnt have sex till then.  He could have easily started second guessing the situation, but he would have been wrong.  Anyway, sorry about rambling, but I really think YOU are the father, and try to enjoy this time with your wife.  Im sure when the baby is born hell be looking up at you with your nose, your fathers hairline, and your unlces toes, and ll doubts will vanish:):):)
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Avatar universal
Not sure if i understand all the dates being thrown out here but i can tell you that just because a doctor says that a woman is "supposed" to ovulate 14 days after her period i know for a fact that i don't,when i had gotten pregnant with my 3rd child i know i conceived her within a 4 day span of the last day of my period,we are always using protection expect for the 1st couple of days after my period because we were "informed" that we can't get pregnant then, well they were wrong! I wish you all the best..please keep us informed.
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130265 tn?1450983381
I was scrolling down, and I want to applaud you for learning and trying to understand all this pregnancy stuff. I wish my hubby would've been so interested. I've had 4 ultrasounds out of 6 pregnancies. They've all been accurate. From my personal experience, the computer generated fetal age tends to coinside (can't think of the corrrect spelling right now) with when implantation occurred. My u/s weren't accurate based upon my last period, it was accurate based upon when I knew I ovulated and implanted. I'm one of the very few women who always has implantation bleeding.

I was trying to follow the dates you gave us. It sounds way too early for her to have ovulated, unless she has short cycles. Is there any way for you to question her about them? The 'period' she experienced may have been implantation bleeding, which would explain the date that the ultrasound's giving you.

I really hope this works in your favor. That's really awesome you're supporting her despite your doubts. BTW, does she know your concerns? Please, please, please just don't jump the gun quiet yet. Weird things do happen.
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93532 tn?1349370450
Maybe this can help put things into perspective, my LMP was Jan 22nd, the computer generated due date was October 31st based on an average of 29 day cycles. My doc's EDD has me at October 28th, which adjusted things radically (doesn't seem like much, but it throws things off a bit in early u/s). My most in depth u/s to date has my EDD right on with my doc's, even though the baby is a bit big. I am in my 24th week, your wife is only about 4 weeks behind me. She may either have longer/shorter cycles or be mistaken about her last period. If you think about it from a normal 28 day cycle, her LMP was most likely later in Feb, she ovulated right around the time you were together, and the dates match up well for that. Just about either way, it looks like the time you had sex was when she was fertile and conceived.

Again, like other ladies, I applaud your interest. But, again, like the other ladies, I do not think you have much to worry about by way of paternity. If, down the road, you have concerns, you can always ask for a test. In the interim, I think she is carrying your baby : )

Andi
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