There was a University of Ottawa study which estimated that up to 50% of all pregnancies may end in spontaneous abortion, usually before the woman knows she's pregnant. Other studies have shown somewhere in the area of 25% miscarrying by the 6th week of gestation.
These seemingly high numbers aren't really that surprising when you take into account the wealth of factors that can cause a miscarriage, particularly in very early pregnancy. Couple that with abortion, stillbirths, etc., and yeah, a good chunk of the would-be generation ends up never being born. I'm not sure why that's any more disturbing than the number of pregnancies and would-be children which never come to be due to contraception, though, as well as all of the people who simply won't exist because the right sperm and egg never met.
Also, I wonder how the stats for these would-be people are affected when one factors in the children who reach adulthood due to lower infant and childhood mortality rates and advances in care for premature babies, not to mention all of the kids who had the opportunity to be born thanks to better care (including safe, reliable access to abortion) for their mothers.