Unprotected Sex & Potential Pregnancy: Seeking Advice & Information

Question
Im not sure if your even the right person to ask. But I figured its worth a shot.So,Im 27 years old. I've never been on any kind of birth control. My LMP was August 16 but it was earlier than normal, 2 weeks. My boyfriend and I had unprotected sex,with him fully ejaculating inside me on August 27 and Sept 2. We split and I had sex on Sept 7 without anywhere near ejaculation, stopped. My doctor says today I am 18 weeks 1 day pregnant. That brings me back to August 29th. Would you think I became pregnant on the 7th despite the guy not ejaculating at all? Thanks

Answer
Since you're 18 weeks 1 day today by ultrasound, that means you got pregnant about Sept. 14 plus or munus 2 weeks (the accuracy of an 18 week ultrasound).  That means you got pregnant from August 31 to Sept. 30, so it could have been Sept. 2 or 7.  Since he didn't ejaculate in you on Sept. 7, it's most likely you got pregnant on Sept. 2.