I just found out im 12weeks pregnant... Trying to work out when we conceievd, I had sex sex with my partner on the 3rd October and then had my period on the 15th October, we had sex again nearer the end of oct going into november. When did I conceive? Im assuming it would def be after my LMP? So maybe end of October?
You'd have conceived the end of October.
Your LMP started on 15 October. That is correct for 12 weeks pregnant. It is based on your LMP, not when you actually conceived which would be about two weeks or so later.
You would have likely ovulated between 26 October and 31 October. The egg is viable for up to about 48 hours max, sperm can remain viable for up to about 5 days max.
If your last period was "normal" then you were very likely not pregnant then.
Hope it helps.