Even for a healthy young couple trying to conceive a child, there is a natural rate of loss after conception (after fertilization). What percentage of conceived prenatals are lost, and therefore do not result in a live birth?
Terminology: prenatal refers to any and every stage of development from conception to birth. A prenatal is a human person, from the first moment of their existence at the single cell stage of fertilization all the way to the completion of birth.
After the pregnancy is established, that is, after the 6th week (counting from the start of the last menstrual cycle), the rate of natural miscarriage is about 10%. The couple is not doing anything wrong. The process of development in the womb is complicated and can sometimes fail naturally.
But from conception up to the 6th week, there is a higher rate of natural loss. How high is a matter of dispute among physicians and researchers. This is important because an over-estimate of this rate of natural loss is being used to justify the killing of prenatal human persons via abortifacients.
Let’s consider the current evidence.
This study found that one third of conceptions result in natural loss, and the other two thirds proceed to a live birth
“Considering live births only (which accounted for two thirds of the conceptions), the approximate probability would be 0.25 with daily intercourse” 
The 25% figure is the chances of a conception that leads to live birth with each cycle (and daily sex while fertile). The one third loss includes about 10% of miscarriages after the 6th week, and the rest of the loss prior to the clinical recognition of pregnancy.
The same author, with different co-authors, has an earlier study which found a total of 31% natural loss, including 9% miscarriage after recognized pregnancy. The early loss figure was 22% .
This study concluded that 70% of conceptions fail to reach live birth, with 10% lost after clinical recognition of pregnancy, 30% lost before implantation, and 30% soon after implantation.
This figure is difficult to believe, since 25% of attempted pregnancies per cycle reach live birth . A 70% loss implies that 93.3% of cycles with attempted conception succeed (70/75). But data from several sources puts the actual maximum successful conception rate at 40% . If so, then 25 percentage points are the live births, and only 15 points of the 40 are losses: 15/40 = 37.5% natural losses.
One issue with the Macklon study is the use of detected levels of hCG to determine hidden pregnancies. And the authors admit “The detection of `background hCG’ in non-pregnant women may have further confused the issue” .
This study concluded “an estimated natural post-fertilization survival rate of 36.7%”. The researchers called this figure “low, yet not inconsistent with estimates from other investigators.” . The natural loss rate would then be 63.3%.
This study found a conception rate of 40% per cycle. The natural loss rate was 7.9% for miscarriages (after the point that the woman would know she was pregnant) and 24.6% for early losses (prior to the point where the woman would know she was pregnant) for a total of 32.5%.
“The conception rate per cycle was 40% over the first 12 months. Of the 618 detectable conceptions,
49 (7.9%) ended in clinical spontaneous abortion, and 152 (24.6%) in EPL.” 
Wang’s findings agree with Wilcox and Stanford. Note that, if the conception rate is 40% and the well-established rate of conceptions that lead to live births is 25%, the loss must be about 15/40 or 37.5% as a good approximation. Wang, Wilcox, and Stanford all have values that more or less fit this calculation.
This study is often cited as the high value for possible early pregnancy loss . However, there are a number of factors that make this estimate of pregnancy loss unreliable. First, there were “28 cycles in which intercourse took place at the time of ovulation, EPF was detected in 18.” (EPF is early pregnancy factor, a chemical present 24 to 48 hours after conception.) The number of pregnancies that proceeded to birth were only two. The low numbers in this study make the data prone to statistical noise.
Second, the 2 out of 18 result would represent a natural loss rate of 88.9% — much higher than any other study. When a study with small numbers deviates greatly from other studies with larger numbers of women, cycles, and conceptions, the former is more likely to be in error.
Third, the claim of 18 conceptions out of 28 cycle attempts is too high at 64.3%. We know that the absolute maximum for a best case scenario is about 40% successful conceptions for each cycle. This implies that the EPF detection has a high false-positive rate (37.8% false positives, at least). So the Rolfe study should not be used as the basis for a claim on the percentages of early pregnancy loss.
Recurrent Pregnancy Loss (RPL)
“RPL is estimated to occur in 2%–4% of reproductive-age couples.” 
“Recurrent pregnancy loss affects up to 5% of couples trying to establish a family. Evaluation classically begins after 3 consecutive miscarriages of less than 10 weeks of gestation….” 
If the odds of pregnancy loss were 70%, for any one pregnancy, then the odds of a second loss would be 0.70 x 0.70 = 0.49 and the odds of 3 consecutive losses would be 0.21 or 21%. Instead, the odds of 3 losses is at most 5%, making the single pregnancy loss odds 36.8% (the cube root of 0.05). And this figure agrees with Wilcox and Wang on the prevalence of early pregnancy loss, and contradicts higher estimates.
Natural Loss Rate in Peer-Reviewed Medical Literature:
Wilcox 1995: 33%
Wilcox 1988: 31%
Macklon 2002: 70%
Stanford 2002: 63.3%
Wang 2003: 32.5%
Rolfe 1982: 88.9%
RPL calculation: 36.8%
Relevance to Theology
In discussing abortifacient contraception, some Catholic authors have proposed that the high rate of natural pregnancy loss justifies the use of abortifacient contraception. Their claim is that the deliberate knowing choice to make use of an abortifacient, while sexually active, is justifiable because the natural rate of loss is so high. As a matter of medical fact, the evidence to-date weighs against very high numbers for natural loss. As a matter of moral theology, you are responsible for every deliberate knowing choice before the eyes of God. The fact that many people die in car accidents does not justify deliberately running over someone with your car, nor does it justify driving recklessly, knowing that you are increasing the chances of killing someone.
I must also point out the bias, in some medical sources, against admitting that various forms of contraception destroy the prenatal human person after conception. They do not want to admit that many forms of contraception are actually abortifacient. The culture and their own thinking favors societal and legal approval for abortion, abortifacients, and contraception. So there are numerous sources claiming that these forms of abortifacient contraception are not abortifacient — because they only destroy the conceived prenatal before implantation.
But as a matter of facts provable from peer-reviewed medical studies, the various types of chemical contraception and certainly also IUDs of every design work by abortifacient mechanisms a substantial percentage of the time. For IUDs, they are effective as emergency contraception (EC) even when inserted 5 days or more after intercourse — a timing that would almost always be subsequent to conception; and yet no pregnancies occur.
Catholic moral theologians have been remiss in condemning abortifacient contraception. In many cases, they treat abortifacient contraception as if it were mere contraception, and they even approve of its use, despite the deaths of innocent prenatals caused by that deliberate knowing choice. Abortion is genocide. And more than half of the deaths in that genocide are from abortifacients. And none of the most popular Catholic authors on the topic of contraceptive can be bothered to publicly condemn abortifacients and the deaths they cause.
Please take a look at this list of my books and booklets, and see if any topic interests you.
 Wilcox 1995
Allen J. Wilcox, et al., “Timing of Sexual Intercourse in Relation to Ovulation — Effects on the Probability of Conception, Survival of the Pregnancy, and Sex of the Baby”, December 7, 1995; N Engl J Med 1995; 333:1517-1521.
 Wilcox 1988
Allen J. Wilcox, et al., “Incidence of Early Loss of Pregnancy”, N Engl J Med 1988; 319:189-194.
 Macklon 2002
N.S. Macklon, et al., “Conception to ongoing pregnancy: the `black box’ of early pregnancy loss”, Human Reproduction Update, Vol. 8, No. 4 pp. 333-343, 2002.
 Stanford 2002
Stanford and Mikolajczyk, “Mechanisms of action of intrauterine devices: Update and estimation of postfertilization effects”, Am J Obstet Gynecol 2002;187:1699-708.
 Diedrich, et al., “The role of the endometrium and embryo in human implantation,” Human Reproduction Update, Volume 13, Issue 4, 1 July 2007, Pages 365–377.
 Wang 2003
Xiaobin Wang, et al., “Conception, early pregnancy loss, and time to clinical pregnancy: a population-based prospective study”, Fertility and Sterility, Vol. 79, No. 3, March 2003.
 Rolfe 1982
Barbara E. Rolfe, B.Sc., “Detection of fetal wastage,” Fertility and Sterility, Volume 37, Issue 5, May 1982, Pages 655-660.
 Jaslow 2009
Carolyn Jaslow, et al., “Diagnostic factors identified in 1020 women with two versus three or more recurrent pregnancy,” Fertility and Sterility, Vol. 93, No. 4, March 1.
 Stephenson 2007
Mary Stephenson and William Kutteh, “Evaluation and Management of Recurrent Early Pregnancy Loss,” Clinical Obstetrics and Gynecology: March 2007 – Volume 50 – Issue 1 – p 132-145.