Avatar universal

conception date - accuracy

My questions is concerning the accuracy of ultrasounds and how close they are to the date of conception. I read that they are less accurate for a conception date the further in trimesters you go??

here's the story:

I had "safe", protected sex with a girl on december 10th. She texted me at the start of January telling me that she was pregnant. She told me she had also had sex with another guy the end of November, but with him, it was completely unprotected sex a few different times. She told me I probably had nothing to worry about because the ultrasound she had and the fact that they didnt use any protection at all points to him being the dad.

she then had another ultrasound either the first of march or end of february to get a better idea on conception date and she told me that it was 99% sure that it was his and that she got pregnant the end of november and that it was his.

I got another call from her this morning saying that in 2 months when the baby comes, she wants me DNA tested because its a 50/50 chance that its mine, and if not it is his. I asked how it went from being absolutely not mine, to a 50/50 chance. She said that now the ultrasound tests and measurements say that the baby is 3 weeks behind, which I guess means conception could've been 3 weeks later putting it right about the time we had sex???

I am obviously just concerned but should I be? I guess what I'm most concerned about is how accurate could "3 weeks behind" be? Isn't that just based off of average size of the baby? Is the ultrasound more accurate in conception date in those first couple weeks/months?
4 Responses
Sort by: Helpful Oldest Newest
134578 tn?1614729226
Early ultrasounds are the most accurate for judging the date of conception, and even they are not foolproof.  Later in the pregnancy (especially this late) they can be as much as a couple of weeks off.  

When she had the first ultrasound, what due date did they give her, do you know?  If you have the date of the earliest of her ultrasounds, and what week and day pregnant she was considered to be then (the GA or gestational age, which is not the same as the date of conception), and especially the due date she was given at that time, we could plug the dates into a calculator and see what the estimated conception date is.  

If she does not know that in the 7th month an ultrasound is not a reliable way to tell who is the father, she is probably not reliable in terms of language like "50/50 chance the baby is yours".  It is not unreasonable for her to ask for you to take a test, but be SURE you go to a lab certified by the court and don't do any kind of cheek swab that you give to her to mail in or anything like that.
Helpful - 0
Avatar universal
thank you for your input. I am not sure of the due date they gave her the first time she had an ultra sound. I'm trying really hard to remember the dates that she told me about everything without asking her right now and making her angry.

I remember it was the last few days of January and the first few days of February when she told me she had an ultrasound and they told her she was about 12 weeks pregnant which meant it was most likely the end of November when she got pregnant. Which makes sense since the guy she was having sex with then didn't use any type of protection and she wasn't on any type of birth control either. She told me she was going to have another ultrasound the first week of March to get a better idea of what week exactly...but she texted me the last week of february and said she already had the ultra sound again and it was almost a for sure thing she got pregnant with him in November and that it wasn't mine.

After that I realized that had to be the most likely situation since we had protected sex in which the condom didnt break or anything on december 10th, and it was a super slim chance it was mine.

Since then I have moved on so this whole thing resurfacing again really threw me a curve ball. I completely understand her asking me for a DNA test and I will absolutely cooperate because in my mind I still don't see how it could be mine given the situation. I feel for her and am very sympathetic. The guy that is most likely the dad is a complete jerk and won't talk to her or see her and wants nothing to do with it. Me on the other hand would own up to my responsibility and take it on like a real man.

I just don't see how it could be mine. I know there is a slight chance but not a very likely one after learning about accuracy of Ultrasounds this late in the pregnancy. Is it fair for me to say that the first ultrasounds were probably correct telling her that she probably got pregnant in November?

I know this will all come out in the wash once the DNA test is done but I'm just trying to put my mind at ease about the situation until that day comes.  
Helpful - 0
134578 tn?1614729226
If you begin to engage in conversation or argument with her about whether there is really a "50/50 chance" at this point, you're going to just spiral yourself down.  Just don't.  Even if you were to (very gently) point out to her that back in February she was clear the other guy was the dad, and she told you she was 12 weeks pregnant and you only had sex on December 10, going further into it when there is no way to test yet will spur an endless cycle of expostulation and self-justifying statements from both of you that won't end until the day comes that you can test.  

You're SO much better off simply saying, "Sure, I'll take a DNA test if it will put your mind to rest."  Don't act nervous or say it is a curve ball, your position is that you don't have anything to worry about.  Frankly it sounds like you don't.  

Don't keep engaging in speculation with her.  Especially, don't tell her that you would be a good dad if you were the dad (even if it is to show you have values compared to the jerk who is dodging her).  It's impressive to have principles and I'm glad you do, but in this situation he more you talk about your responsible nature, the more she will feel like the father had better be you, and she will get halfway to convincing herself that it is true out of desperation.  Part of being a real man is not dangling your willingness to take responsibility where she can see it but can't have it.    

Since I'm not the one with the pregnancy scare, I will say bravo for being a real man, and especially for understanding that the risk one takes when one gets free sex in this world is that occasionally a real man has to stand up when he gets an unexpected new family member.  However, when you are dealing with the one who is actually pregnant with a no-show dad, downplay what she is going to miss by not being wise enough to have gotten pregnant by you.

If you want, you could recommend she write in, and I'll help her parse out her dates.
Helpful - 0
Avatar universal
thanks annie! I appreciate that.

The day she texted me a few days ago just brought back a whirl wind or emotions like it did the first time she texted me. I'm not the type of guy to sleep around a lot, or hardly any at all. So when this happened, I absolutely wasnt ready for it and it scared me to death. I went into freak out mode a little when she first texted me so obviously said some things I shouldn't have to her and bringing up the fact that she told me it was his. It took me a day to calm down and I text her and told her I was sorry and I empathized with what she was going through and that I would take a test anytime she needed me.

The few times I have had Sex, I always made sure to use protection. In this case, I used protection and it didn't break. I know even then its not bullet proof but 99% with a condom is a pretty good chance to not get pregnant. But, just like I said before, if I was unlucky enough that the 1% chance happened...I would take full responsibility for it.

I know it most likely isn't mine, with the fact that the first ultrasounds put it at the end of November, and her and the other guy used no protection at all. I just wasn't sure what the whole "3 weeks behind" meant. Hearing that ultrasounds can be 2-3 weeks off helps put my mind back at ease. After she told me it wasn't mine the first time, I moved on, found an amazing girl, and am working towards a life together and I am very happy. Thats why this scared me even more is because things are finally going so good for me and I have a solid relationship that means the world to me. I appreciate your input and taking the time to help me relax about the situation. I Just needed to hear from somebody else that its a good chance that it isn't mine. I can't wait for the day that I get to have a child...but I want it to be done the right way. Thanks again  
Helpful - 0
Have an Answer?

You are reading content posted in the DNA / Paternity Community

Top Pregnancy Answerers
13167 tn?1327194124
Austin, TX
4769306 tn?1568490209
Learn About Top Answerers
Didn't find the answer you were looking for?
Ask a question
Popular Resources
Get information and tips on how to help you choose the right place to deliver your baby.
Get the facts on how twins and multiples are formed and your chance of carrying more than one baby at a time.
Learn about the risks and benefits of circumcision.
What to expect during the first hours after delivery.
Learn about early screening and test options for your pregnancy.
Learn about testing and treatment for GBS bacterium.