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Dr. C.
Dr. C., Board Certified
Category: OB GYN
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My wife ultrasound out her at 7 weeks 4 days, a conception

Customer Question

My wife ultrasound out her at 7 weeks 4 days, a conception date of July 20. Not possible because we were on vacation we did have intercourse on 7/28. How can she be 7 weeks 4 days.
Submitted: 1 year ago.
Category: OB GYN
Customer: replied 1 year ago.
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Expert:  Dr. C. replied 1 year ago.

Hi, It's Dr. C. Board certified Ob-gyne. I can help you today.

The reason for the confusion is that the doctors are referring to "gestational age". Gestational age differs from conception age by 2 weeks. When the doctor says 7 weeks 4 days, they are talking gestational age which is actually 5 weeks 4 days from conception.

This is a very confusing historical convention that dates pregnancies from the start of the last period, which is usually about 2 weeks before conception.

The ultrasound conception date is an estimate with an error that gives a 7 day window of possible conception range. The most likely date isn't exact.

What day was the ultrasound done? Given the information you posted, I'm thinking that it was done today or yesterday.

Please let me know if the situation is still confusing to you and if there is anything that I can explain more clearly. That 2 weeks language difference is a source of huge confusion.

Expert:  Dr. C. replied 1 year ago.

Any questions?

Customer: replied 1 year ago.
It was done today and her last cycle was on the 7/6/2015. We have been keeping track. Confusing things like that could make men wonder.
Expert:  Dr. C. replied 1 year ago.

Yes can do. I answer many questions about this very concern.

The most likely conception date also depend on the length of the cycle. Most women average a cycle length of 28 days. That places the most likely conception date as July 20th (with a range of 7 days). But if her cycle runs a little longer, say 32 days, that would push the conception date later to July 24th. On top of that, the ultrasound has an error range of 3 days in either direction. The egg lives for 24-48 hours, so that puts July 28th well within the window of conception.

Notice that July 6th counting forward to today is 7 weeks 4 days. That is gestational age counting from the period. But it's 5 weeks 4 days from the most likely conception date.

Is the confusion cleared up now? Please reply if there is any more detail needed.

Expert:  Dr. C. replied 1 year ago.

Any questions?

Customer: replied 1 year ago.
It's a scary thought because now I'm starting to think on our vacation were she went. Just don't really get how she is 7 weeks already and if not will the next ultrasound be different
Customer: replied 1 year ago.
This is the ultrasound
Expert:  Dr. C. replied 1 year ago.

Thanks for the image. I'm going to rework all the calculations and explain once more what the possible range of conception is so that we can be as clear as possible.

August 29th 1st trimester u/s = 7w4d

That makes the most likely conception day 5w4d earlier. That date is July 21. Yesterday I used a "wheel" so was off by 1 day. Today I counted back day by day.

The error range on a first trimester ultrasound is plus or minus 3 days. With older machines it's 5 days. The error range is written on the final report. Because of the geographic location you live, I think 3 day range is more likely than 5. So I'll use 3. The reason for the error range is that the measurement is somewhat technician dependent. The technician needs to get the best side view possible. Sometimes the correct angle can be difficult to get. Also the technician needs to place a small caliper mark on the edges of the embryo. This caliper mark has a certain width too.

With a 3 day + or - range added, the range for possible conception dates is July 18th to 24th. If 5 days + or - range is added, it's July 16th to 26th.

Sperm can live for up to 7 days inside a woman so we need to add that to the front of the range which gives us July 11th-24th or July 9th-26th.

You say the sex was on the 28th. That is technically outside the ultrasound determined range, but it's just 2 days off which is actually quite little. If the technician was a bit sloppy, that would be enough to throw it off by that much. The image you sent is certainly not the best side view for measuring but also I don't see caliper marks on the image so the technician probably didn't give you one of the better images.

If you have other reasons to suspect that she was unfaithful, then do go ahead and request a paternity test. This can be done using her blood and either blood or a swab from your cheek. But tread carefully. This can be a relationship changing accusation. If you want a paternity test, be sure to use a company approved legally in your state. In most states, that information can be found on your states website using "paternity testing" in the search engine.

This is a tough situation. I wish it was clearer one way or the other. But the numbers are just quite too tight. If she has another ultrasound, it should be done in about 2 weeks. The larger embryo size would reduce some of the technician error but still be in the range where the growth of the embryo is very consistent without variation of genetics and environment influencing the growth yet.

I hope this was clear and helpful. Let me know if I can add more details.

Expert:  Dr. C. replied 1 year ago.

Any questions?

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Expert:  Dr. C. replied 1 year ago.

It's Dr. C. checking in to see how things are going. Do you need any more clarifications?