How accurate are ultrasounds to determine gestational age?

Hello, I have a question: How accurate is the ultrasound concerning determination of gestational age and the date of conception? I had my LMP on 01.12.16, cycles 26-29 days (usually 27-28). The first ultrasound showed GA 7.4 hbd (CRL 13.2 mm) and the gestation age coincided with the LMP date, fertilization around 14 XII, which matches perfectly – unprotected ratio 14.XII. However, all subsequent ultrasounds shows that the pregnancy is 2-3 days older. USG 11.4 according to LMP => CRL 52.5 = 12 week, USG 13.1 hbd according to LMP, CRL 74 = 13.4 hbd. Another 14.4 according to LMP according to BPD (28.2), HC (105) and AC (87) = 15 hbd. At the last USG 18.6 according to LMP => FL (2.95) 19.1 hbd. The ultrasound at 24 weeks – terms according to Ultrasound and LMP are equal. Does that mean I got pregnant earlier, had a previous ovulation? 09/12/16 I was at a party, I drank too much and I do not remember how I got home. By these results I began to fear that maybe something happened to me and I do not realize it and I do not even know who the father is. I am already in 8 months and the stress of this situation becomes unbearable. Of course I’m going to do DNA testing after birth but until then I would like to know the expert’s opinion. In fact, if I had an ovulation on 14 day of my menstrual cycle the GA according to Ultrasound and LMP must match ideally? (In the previous cycle I think my ovulation was on the day 13 but this cycle was quite long – 30 days). I also did Beta Hcg examination: 4 + 1 (according to LMP) 158; 5 + 1 (according to LMP) 3299. Thanks in advance for help, this is very important to me

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