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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Variation of 2-3 days depends on the time of ovulation and fusion between the ova and sperm..
So it is not abnormal..
But in case if you had different partners for copulation during this period then it can be concerning for you..
For testing paternity you can get the DNA Screening done..
If there is no other abnormality found in the scans then nothing to worry about..

Hope this helps..