How to know my LMP

Sorry if medyo confusing ang subject ng topic. I just cant figure out the exact words na pwedeng maging title kasi. :) Anyway, here's my case: Month of januara, feb, march hindi na ako nagkakaroon. So meaning I'm 3 months delayed na. I took a PT and it turned out positive! no wonder, hehe. . And since this is an unplanned pregnancy I really cannot recall my last LMP date but I'm almost positive or let's say I'm 100% sure that I did have one pa in december, can't remember lang the date kung kelan. So here's my question. should it be possible ba na madetect sa ultrasound if how many weeks na ang baby? and mas accurate ba yun? I heard kasi from my friend's doctor na bihira daw ang nagbabase sa US, more on LMP daw coz its more accurate daw compared to ultrasound. is this true? I got confused lang. Sana may makakapagsagot. Thanks everyone.