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6 week ultrasound
On February 8th I had a 6 week ultrasound that gave me a due date of October 4th. That has been consistent with her measuring just 2 days ahead. Is the 6 week ultrasound the most accurate in pinpointing conception? I had sex January 2nd,the 9th and the 12th.

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134578 tn?1483549754
Hi, two days ahead of what?
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Avatar universal
Hi there. She was measuring 2 days ahead of her due date at her anatomy ultrasound. She is growing!
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134578 tn?1483549754
If your main question is how reliable a 6-week ultrasound is, it can be pretty helpful.  Was it 6 weeks 0 days?  If so it might be a little early for the doctor to see everything he or she might benefit from seeing, but it's better than a fifth-week ultrasound.  If it was 6 weeks and some number of days like 6 weeks 3 days or 6 weeks 5 days, that is more helpful.

You named three dates, did you have sex with more than one guy?  Is there a particular date you are trying to rule out of the three?
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Yes I had one at 6 weeks 0 days and they detected a fluttering heartbeat and a little bean. I then had one at 7 weeks 1 day to double check visible free floating fluid which turned out to be nothing of concern. Both dates still gave me October 4th. I am trying to rule out the January 2nd date.
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134578 tn?1483549754
It sounds like you got pregnant later enough from January 2 that the sperm from that event would have been non-viable in your reproductive tract by the time you ovulated and conceived.  It might have been viable in your body up to January 7 or 8, but the early ultrasounds indicate around January 11 for a conception date.  This is not a super-comfortable gap of time, but sperm doesn't live 9 days in the woman's body and so it is hard to see how it could have made the jump from January 7 or 8 to January 11 even if it did last a long time in your body.

Since they gave you the (October 4) due date at 6w0d and 7w1d, that *is* the due date.  I'm still puzzled by why you said, specifically about the October 4 date, "That has been consistent with her measuring just 2 days ahead."  When I asked ahead of what, you said "2 days ahead of her due date."  Do you mean that at a later ultrasound, they changed the due date?  (If so, she is not 'ahead' of her due date," she would simply be on target for her new due date.)    

If they merely commented that the baby is larger than expected for a baby due October 4 (is that what you mean when you say "ahead," that she is bigger?) are you now thinking that in order to be the size of a baby that is bigger by two days than the average, she might have been conceived two days earlier?  And this is worrying you because you think it is pushing the baby into the territory where conception could have come from the sex on January 2?

The trouble with a line of reasoning that uses the word "ahead" or "ahead of her due date" is that it is wrong.  No matter how fast a baby grows compared to average or how large the baby is at a later ultrasound, the ultrasound that matters for trying to guess when conception was is the one in the 6th or 7th week, not later ones. The due date they told you in your sixth week and in your seventh week should remain the baby's due date no matter how fast she grows.  Some babies grow faster or slower than average.  That doesn't change their due date.  It doesn't mean they are "ahead" of anything, it just means they are growing fast or are going to be a large baby.  By the time a baby is full term, it can be three weeks different in size (either direction -- smaller or larger) than "average" 40-week babies.  That does not mean the baby started at a different time, it means that there is wide variation in how fast or slow babies grow.

Anyway, it sounds from your sixth and seventh week ultrasounds like you conceived around January 11, and that sounds like sperm from January 2 would not have still been alive in your body even if it lasted 5-6 days before becoming non-viable.  Get a DNA test when the baby is born, just to so you never have to wonder, and then put the question behind you.
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Thank you so much for your reply! Her due date has not changed, they just said she was measuring two days ahead at her anatomy scan. She is still on track for October 4th
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134578 tn?1483549754
Did you ask them what they meant by saying "two days ahead"?
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