I grew up with a girl who was a surprise pregnancy for her parents. Her siblings were all 15-20 years older than us. Her mom was 47 when she was born and had just assumed that her early pregnancy symptoms were menopause.
Uncommon, yes. Not so sure that I would call it "rare". It might be in this day and age, with widespread access to and use of birth control. But, biologically speaking, a woman can get pregnant up until menopause, whenever that may be. Of course, odds of complications or issues with the fetus increase with maternal age.