How many weeks pregnant would i likely be if i had unprotected intercourse 19 weeks ago. i also have irregular periods?

It usually takes 3-4 days for sperm to join with an egg after a sexual encounter and a few more for that material to implant in the womb. The 'gestation' estimate is usually based on the date of the last menses, which here may be off. Ultrasounds generally give a good estimate of how far along the baby is which is as good as your going to get when periods are irregular.