I am 42 years old had a tubal ligation 15 years ago last period was aug 20 2012 could it be pregnancy?

It is doubtful...: The chance of getting pregnant after successful tuboligation is very very slim, but could happen. The easiest way to tell is to do a home pregnancy test (hpt). If negative, check again in 5-7 days. If still negative then, it is likely negative--as these hpts are quite good. At age of 42 is a bit early to start menopause, but it does happen...Consult doc. Good luck.