How accurate is the dating scan in KKH?

Question

How accurate is the dating scan? My hubby and I had our first sexual intercouse with ejaculation on the week of July 25 after I move in to the new house. On 12 July( a day before our ROM),  there was no ejaculation (due to his covid previously which might affected the performance)😅  Kinda remembered these 2 dates as it was a milestone in my life. When I did the dating scan on 13 Sep in KKH, I was extremely shocked that Im on my 9th week of pregnancy. Shouldnt it be the 7th week based on the week of 25th July? Prior to 12th July, we have been doing the deed without ejaculation and it has been fine. The doctor told me based on the size of baby it has to be 9th week. My EDD is dated 18th April 2023. My last menses was from 25th to 30th June. How was the dating scan calculated? Is it accurate? Just more of curiosity,  but I am still  very happy to know that Im pregnant.Some context: I have irregular menses since young,  ranging from 1 to 1.5 or even 2 months. Hence, I didnt discover it earlier.

Vô danh · Answer

Just FYI even without ejaculation sometimes that tiny little bit that we didnt know could be that we are pregnant. Sonogram will indicate the actual weeks of the baby size. That's why it is important to know LMP to estimate the EDD. However, this is exactly right thats why it is estimation. After menses we will ovulate hence when we do intercourse may/may not be pregnant. You may recount the dates based on sonogram and used the calculator apps to see if it is accurate.