Unobstructed intercourse at the fertile time does not always result in pregnancy.
However, the probability of conception occurring for a couple not using anti-conception devices (condom, diaphragm, and spermicides) is at least 25% in any given cycle among normally fertile couples of average sexual activity,(40) and it ranges up to 68% for couples who have relations every day during the fertile time.(41)
The lower figure (25%) will be used in describing the magnitude of early abortions with the IUD and the Pill; it is conservative relative to the coital patterns stated by Kinsey for American couples where wives are under 40.(42)
Thus, a pregnancy rate of .25 in each cycle among one million women using IUDs every cycle would result in 250,000 conceptions per month. An average of 12 menstrual-fertility cycles per year would yield 3,000,000 IUD-caused early abortions.
However, since the IUD has about a 5% surprise pregnancy rate, among 1,000,000 IUD users in the USA, there would be approximately 50,000 recognized pregnancies each year, many to be killed later by surgical abortion.
Subtracting these 50,000 from the directly IUD caused abortions yields an estimated 2,950,000 early abortions each year caused by the IUD. Multiply that by 84 for the estimate of the world total – 247,800,000 early abortions each year.
Estimates about the number of abortions caused by the Pill are more difficult because of the triple-threat action of the Pill. The older high dosage pills had “breakthrough ovulation” rates of between 2% and 10%;(43) given the lower dosage in today’s Pills and the numbers of women using the minipill which apparently has almost no suppression of ovulation [ed. this means that the primary effect of this progestin minipill does not prevent fertilization, but rather prevents the implantation of the tiny developing embryo (blastocyst) into the uterine, and thus results in the abortion of that embryo], the 10% figure does not seem unreasonable to use, but we can calculate it both ways.
Among the 13.8 million American women using the Pill,(44) the 10% rate would yield 1,380,000 ovulatory cycles each month. Applying the 25% overall conception rate would then yield 345,000 conceptions each month, or 4,140,000 new lives each year, almost all of whom would be aborted by the implantation-resisting effects of the Pill.
A 4.7% rate of breakthrough ovulation was observed and reported in 1984.(45) Applying that rate to the 13.8 million American women on the Pill would yield 648,600 ovulations and an estimated 162,150 new lives conceived each cycle, or 1,945,800 each year, almost all of whom would be denied implantation and thus aborted.
Even if you want to use the low 2% breakthrough ovulation rate, a figure I think is too low considering the lower dosages and the minipill today, you will end up with over 800,000 early abortions per year from the Pill alone, about half as many as from surgical abortion.
And again, such figures are only for the United States and would need to be multiplied by 4.3 times for the rest of the world.
40.C. Tietze, "Differential fecundity and effectiveness of contraception," Eugenics Review 50 (1959) 231. Thirty percent of couples discontinuing contraception achieved pregnancy in the very first cycle thereafter. Cited in C. Tietze, "Probability of Pregnancy Resulting From a Single Unprotected Coitus," Fertility and Sterility (1960) 485-488.
41. J.C. Garrett and J. Marshall, "The Risk of Conception on Difference Days of the Menstrual Cycle," Population Studies 23 (1959) 455-461. The authors calculated the probabilities of conception based in coital frequency as follows: once per week = .14; every sixth day = .17; every fifth day = .20; every fourth day = .24; every third day = .31; every second day = .43; and every day = .68.
42. A.C. Kinsey et al., Sexual Behavior in the Human Female (Philadelphia: Saunders, 1953) Table 93. For 2,200 white married women, coital frequency per week by age: 21-25 yrs = 3.0; 26-30 yrs = 2.6; 31-35 yrs = 2.3; 36-40 yrs = 2.0. The coital frequency rate of every fourth day is lower than any of the above Kinsey rates, and Barrett and Marshall calculated a .24 pregnancy probability for such a rate. Therefore, the .25 rate used in my calculations is conservative relative to published American coital patterns and the conception probability rates published by Tietze and Barrett=Marshall.
43. John Peel and Malcolm Potts, Textbook of Contraceptive Practice (Cambridge: Cambridge University Press, 1969) 99.
44. Robert A. Hatcher and others, Contraceptive Technology: 1990-1992 (New York: Irvington, 1990) 228.
45. "We are close to lowest steroid dosage in the Pill," News and Views, 30 November 1984. Excerpts from the second annual meeting of the Society for the Advancement of Contraception, Jakarta; ed. W. Korteling, (West Orange, NJ: Organon International).
[John F. Kippley, 1994, The Couple to Couple League International, Inc., www.ccli.org]