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Hello. I wanted to know how off can an ultrasound be to calculate conception. I took a u/s 9/15/2017 & I was 10 weeks and 6 days. Then one on 9/22/2017 that’s dates 11 weeks and 6 days of course. To figure all possible dates of conception how do i count back? I am unsure when my last cycle was but I went to the hospital and the drs did and ultrasound to determine how many weeks I was and when my due date would be. Conception would have occurred around July 15th maybe even with me being unsure of my last period or the cycle length Can I count on that conception date?
7 Responses
134578 tn?1546634665
Hi, Bella, the weeks and days are somewhat useful, but it helps a lot and cuts the confusion to use the estimated due date instead. When you had the ultrasound on the 15th of September, what due date did they give you from the measurements and developmental markers of the embryo at that time?
Also, who told you conception on July 15? Was it a doctor? Because that doesn't really line up with 10 week 6 days on September 15.
Yes I’m 9/15/2017 I got an ultrasound and they told me I was 10 weeks and 6 days. Which put my lamp at July 1st and conception occurs two weeks after that right? July 15th
And yes at the hospital they told me my due date was April 7th I switched drs once and the due date went from April 7 to the 8 so only one day. Can I count on accurate conception from my first due date at 10 weeks? How far off will it be? When did conception actually occur
OK, well, at the hospital the first time, they told you April 7 was your due date, and at the doctor's the second time, you got another ultrasound, one week later, and they told you that April 8 was your due date? None of this was based on you telling them a first day of your last menstrual period, correct? Just on the measurements of the embryo and its developmental markers as seen on the ultrasound?

The answer to the question of "how far off will it be?" depends on how early in the pregnancy the ultrasounds were. At about 6 weeks GA, there is very little margin for error, because all embryos start as one cell and the cells divide at a known rate, for a while. But after a while, some embryos diverge from the average, some grow slightly faster than average and some more slowly than average. By 12 weeks GA, most doctors will give a margin for error of plus or minus 7 days either way, if asked to give an estimate of when conception occurred based on an ultrasound measurement of the baby. For that same reason, by the time you are at 40 weeks, if asked to estimate when conception was by looking at an ultrasound of the baby, the doc will say the margin for error is +/- 3 weeks.

So a correct answer for you depends on how much of what you said was told to you by the doctor from an ultrasound, or if it came from you taking what the doctor said about weeks and going home and putting it into a conception calculator, or if the doctor didn't tell you the due date from looking at your ultrasound but instead looked at a calendar or little cardboard wheel and based everything on when when your last period was.

What dates are you trying to cross off the list?
Sex that produced the pregnancy could have been anywhere from the 6th of July to the 20th or so.
Avatar universal
Yes my ultrasound on September 15 2017 read 10 weeks and 6 days. I did not know my last cycle so the dr did do an ultrasound and tell me how far along I was and gave me a due date. If I go on the calendar myself and go to 9/15/2017 & count 10 weeks back I do land on July 1st. If I would have gotten pregnant in April that would mean I was pregnant for a year because I had my baby APRIL 6th. Impossible.
134578 tn?1546634665
OK, very sorry about the mixup. I do occasionally mix up months because of the scrolling nature of online communications -- can't see the top when writing at the bottom. Here is how I figure things. Baby's due date was April 7 or 8. (And this in fact was confirmed by baby's birth, if the doc did not say the baby was premature or overdue.) Conception date given on a calculator for the April 7 date is July 15 and the April 8 date is July 16. Add and subtract possibly 3 or 4 days either way, because of the ultrasound being at ten weeks and not earlier. That makes a range of July 11 or 12 to July 19 or so. Then also add at the front end, 5-6 days to account for the long life of sperm in a woman's body if she has unprotected sex. That pushes the front out to July 6, giving you a range of around July 6 through the 20th, at the max.

So now that I presumably sound more rational, could you say when the sex you are trying to rule out, was?
Avatar universal
Yes I had sex on father’s day June 18th 2018 with one man and I had my period after that . A period means I am not pregnant correct ? And after my period went off (it was not all the way off but it’s was the last day so it was light) I had sex with a different guy. I had sex with this guy July 1st the 7th the 16th and the 29th . I later found out I was pregnant and gave birth April 6th 2018 I have not taken a dna test yet but baby looks like my mom so I can not rule out who the father may be. Could it be my June guy? My baby was actually born by a planned c section 2 days before my due date he was 8 pounds and I’m sure that’s not premature & I’m sure they would have scheduled me for March if I got pregnant in June to prevent me from going in labor but Based off these dates , is there any way my ex from Father’s Day could be the father? I would have had my baby in March right?
134578 tn?1546634665
You have enough of a time gap between guys (mid June and mid July) that you can be totally confident that the ultrasounds, even with some margin for error, are correct in showing you that your June guy is not the dad. Besides, as you say, a period indicates not being pregnant. Get your DNA test with the July guy.
Avatar universal
Just an update mr July is the father
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