Does menopause starts with a sudden lost of menses or does it lose gradually?

How will someone know if a woman started menopause?Does it mean that if you are in your early 50's and you had a regular menstration every month and suddenly it stopped it means you are going through the process of menopause? Or the menses stopped gradually by having irregular menses or spottings only? Please help me my mother who is 53 years old stopped to have menses this year but she is active in unprotected .And she is worried she can be pregnant and she does not want to have a baby anymore.Please advise us. Thank you>