If the Rh- rate is assumed to be 0.2, the odds of finding 6 random Rh- women is approximately 1 in 15,000. Even today, the miscarriage rate is approximately 0.2 and was much higher 600 years ago. So I'm guessing it's far more likely that there were 9/13 miscarraiges (or 7/11) for more than just one reason.