Could my ultrasound and conception date be off by 3 weeks?

[alamb7]

I am so worried and hoped some input would put my mind at ease. I'm young and I've been with my amazing boyfriend for a while now. We're committed and excited. But a part of me is still worried. On June 21st, 2016 I was sexually assaulted at a party I attended without him. We already discussed the event and I've gone to therapy to cope. On July 9th I had what felt like a period, that lasted my usual three days. Then he and I had unprotected sex twice that week after.I had an ultrasound on December 19th that put me at 24 weeks. I went in for my ultrasound at 27weeks and 3 days (january 11) and the doctor said I looked small to be 27 week's but the baby's measurments gave him the 27 week's and 3 days. Our due date is April 10th. The dates line up for the baby to be my boyfriends. But the anxious side of me is worried. Is there anyway it could be the other guys? The period could have been implantation? Maybe the ultrasound was three weeks off? My boyfriend has been so amazing about all this. He is aware of my worries and tries to mollify my concerns but I still wonder. Any input would be appreciated. Please respond.

[AnnieBrooke]

Implantation bleeding is largely a myth fostered on the internet by women wanting to get pregnant or worried about their dates.  For all the women on this site who write in about it, either worried or excited, nobody ever has written back and said they actually determined that they had in fact had implantation bleeding.  An embryo is smaller than the dot on the letter i in this sentence when it implants.  It is not going to cause gushers of blood, if anything it might cause a couple of drops.  Even women obsessively watching for signs that they are pregnant don't ever see it, it is simply not common.

If all your dates are lining up properly, take the period as a period (it sounds like it was one) and keep going to your therapist for the anxiety.  

[alamb7]

Thank you so much. I didn't think of it that way. That helps.