number but most estimates are that more than half, quite a few more according to some, either don't implant or spontaneously abort. In most cases this happens very very early and isn't even known to the woman.

Here's a link to a 2004 Discover article that does a pretty good job with the question.

http://discovermagazine.com/2004/may/cover/article_view?b_start:int=2&-C

If I recall the stats correctly, about 30 to 50% of fertilized eggs fail to attach.

A miscarriage is a pregnancy loss that occurs before 20 weeks, well before the fetus is able to survive outside the womb. About 10 to 15 percent of recognized pregnancies end this way (1, 2). As many as 40 percent of all pregnancies may end in miscarriage, because many losses occur before a woman realizes she is pregnant (3).

http://www.marchofdimes.com/baby/loss_miscarriage.html

These stats are for miscarriages due to any cause, not just failure to implant. I've always wondered how people could claim that each fertilized egg is so precious to god when so many of them are shed so early in pregnancy, sometimes before a woman even knows she is pregnant.

"Actually, the statistics are more grim than Pence admits. Only about 50% of all zygotes successfully implant in the uterine wall and become embryos. And only about 65% of all embryos lead to live births; the rest (about 35%) are lost to natural miscarriage. Therefore, slightly less than one third of all zygotes lead to live births. These statistics come from Gregg Easterbrook in "Abortion and Brain Waves" (2000) and from Morowitz and Trefil, The Facts of Life. Science and the Abortion Controversy (New York: Oxford University Press, 1992). Easterbrook: only about 32.5% (65% of 50%) of all fertilized ovum are carried to term. Morowitz and Trefil: "slightly fewer than a third of all conceptions (72 percent of 45 percent) [or 32.4%] lead to a fetus that has a chance of developing." Morowitz and Trefil point out that not all fertilizations result in successful cell division -- some fertilized eggs don't even become blastocysts. "

eom

I'm not medical expert, but I have given birth and I have had a miscarriage.