Need A Lil Help!

Ok so I'm 17w3days exactly I'm kinda confused as to how many months am I at this time? My friends tell me I'm 4 months & will be 5months on Saturday (which is the day I turn 18weeks) but I've read that you can't go by 4weeks in each month because not every month has only 4weeks in fact the only month with 4weeks total is February all the other months has 5weeks so I'm not sure how to calculate how many months I am, I also read that on the day that you missed your last period you won't turn a month until the next month on that day so my last period was due Dec.5 so I'm thinking on the 5th of each month I'll be another month. So Confused plz help if you can...thanks