2nd trimester ultrasound accuracy for dating/conception?

[sallylilly29]

Please help, any insight (no judgement please). I had sex July 5th with my boyfriend. We broke up in the middle of July and reconciled in October. I am not proud of it, but I was with someone else (who I do not love) on July 31st. My periods have always been extremely irregular so when I missed my subsequent periods I thought nothing of it as we had used protection. Exactly 4 months later I started putting on weight and found out I was pregnant. My doctor said per the 20 week ultrasound implantation was most likely in August, not July, meaning the baby would NOT be my boyfriend's. But I have read that 2nd trimester ultrasounds can be 2 weeks off ?- and I am hoping maybe this would mean it is my boyfriends?

Does anyone have any experience with 2nd trimester ultrasound datings being off by 2+ weeks? Any chance the baby is my boyfriend's?

I am so worried , I only want it to be my boyfriend's.

[AnnieBrooke]

What was the latest date in July you had sex with your boyfriend?  You said you broke up "in the middle of" July, but you don't mention any sex after July 5.  Was July 5 the last time in July with him?   I assume you are saying you didn't sleep with anyone in August?

What due date has the doctor given you from your 20th-week ultrasound?

[sallylilly29]

Yes only sex those two exact two times in July and then I didn't have sex again until October when my boyfriend and I got back together.

Due date is April 21, 2017. The only ultrasound I had for dating was this one in my 2nd trimester.

[AnnieBrooke]

For the ultrasound to be off enough for the baby to be from your boyfriend, it would need to be three weeks and a day off.  Ultrasounds can be that far off at 40 weeks pregnant, but I've never heard of them being that far off at only 20 weeks.  By all means get a DNA test when the baby comes in hopes you have an unusual situation, but in your shoes I would mentally prepare for the baby to be from the sex on the 31st.

[sallylilly29]

Thank you very much for the insight. I must be terrible at math/understanding, but how does the ultrasound needing to be three weeks and a day off equate to the 26 day interim when I had intercourse? just trying to understand maybe but I will mentally prepare.

[AnnieBrooke]

You're doing fine at math and understanding, I'm just doing a clunky job of explaining.

First of all, the significant time span is not the time distance between the sex with the two men, it's the time distance between sex on July 5 and the estimated conception date suggested by your ultrasound. (For an April 21 due date, conception would be the 29th of July).  

Essentially, the ultrasound from your 20th week demonstrably isn't pinpoint, since neither the sex on the 5th nor on the 31st is suggested precisely when using your ultrasound-based due date to determine a conception date.  
The question then becomes, which explanation is least likely to be wrong, the one that would require a 2-day error or the with a three-week error?  How far wrong is most logical?

For the sex on the 31st to have produced the baby, the ultrasound would only have to be two days off. I myself had a baby who stuck with the averages for the whole pregnancy (in other words, they don't always come out three weeks off by their 40th week).  So two days off sounds perfectly plausible to me; in other words, not too short.

For the sex on the 5th to have produced the baby, the fast calculation gives a 24-day gap to reach the 29th, but you have a little more wiggle room than that because sperm can live a while in a woman's reproductive tract after sex. I knocked off a couple of days from that 3-week-3-day gap to allow for the long life of sperm in the woman's body.  (That might have been a little too short of a lifespan for the sperm, the max the sperm would have been in your body with enough viability to produce an embryo might have been perhaps as long as five days. So probably the more correct statement is that the ultrasound would have to be 2 weeks 5 days off for the sex on the 5th to have produced the baby.) That said, this is still closer to three weeks than to two, and I have not heard of ultrasounds being that far off in only the 20th week.

But I haven't heard it all, either. I'm sorry this doesn't look more promising for your boyfriend, but remember that you are going to have a wonderful baby to love, and there is nothing wrong with hoping.

[sallylilly29]

I understand now, thank you so much Annie Brooke. I really appreciate this.