Abortion was illegal until 1973. Noone got investigated for a miscarriage.
Gynecology is a relatively new field.
Certainly routine annual pelvic exams for healthy women weren't the norm before 1973, as they are today. Neither were monthly or weekly prenatal visits. Nor was prenatal care in general.
Doctors simply didn't know much about the healthy female reproductive system before the second half of the 20th century, not having had much opportunity to study it.
They didn't know- nobody knew, including females, who often do not realize they've miscarried- how incredibly common early miscarriages are.
Gynecology was more an art than a science before the women's liberation movement, when healthy women began to visit gynecologists en masse to receive contraception.
Even today, gynecology is an area that doctors are still learning about.
We live in a very different world than the one we lived in prior to 1973, when women only saw or spoke to doctors- GPs, typically- about their reproductive organs if they were having problems with them, and maybe not even then.
We live in a world where the government likes to keep medical records and compile health statistics, which was not the case prior to 1973, either.
The idea that "prior to Roe, doctors didn't keep track of miscarriages" is not a very compelling argument for why they wouldn't
today.
Neither doctors, nor law enforcement agencies, nor the government, nor anyone else even kept track of
abortions prior to Roe, even though they were illegal.
They didn't even keep track of failed abortions that resulted in women seeking medical attention.
The apparatus wasn't really in place to "keep track" of such things on a national level.
There weren't any computers.
We live in a different world now.