Hello,
Precum is basically prostatic fluid. Sometimes there are active sperms in the prostatic secretions or precum which can cause pregnancy. If your girlfriend misses her periods, then she should do a urine for pregnancy test.
I hope it helps. It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.