Broadly distributed contraception won't reduce unintended pregnancies, though many resist this conclusion.  Not to be deterred, herein I offer a new approach to demonstrate this fact.

Before walking through the numbers, let's identify the assumptions made by those who believe that more contraception will reduce unintended pregnancies.  For reduction to be the case, (a) contraception must actually reduce the pregnancy risk of new users, and (b) such users' level of sexual activity must not increase materially.  The first statement is obviously true.  The second is true as well because any meaningful increase in sexual activity may offset the effect of contraception, which fails predictably, and result in an increase in unintended pregnancies.

Put differently, the belief that new contraception reduces unintended pregnancies relies upon the assumption that "they're doing it anyway" -- that humans are in perpetual heat and are bereft of self control or dignity.  Thus, the theory goes, only with the great overlay of contraception will the herd of libidinous humanity avoid unintended pregnancies.  Or, as James Lileks put it:

... free birth control protects women from the adverse impact of a strange, mysterious situation that affects millions every year:  sudden-sex syndrome.  We don't fully understand how it works, or what the causes might be, but apparently there's nothing you can do about it.  All of a sudden you're just having sex!  There's not a moment to exercise free will or consider the consequences; it's just like being struck by lightning[.] 

It boils down to this: few people go on contraception without planning for increased sexual activity, whether it be from none to some or some to more.  What is important and typically glossed over in Guttmacher-type analyses is that a person's new embrace of contraception represents a change in sexual plans.  This fact is a source of unintended pregnancies, particularly in the case of first-time contraception users.

Now the numbers. 

Birth control researchers focus on the National Survey of Family Growth ("NSFG"), so let's start there.  According to the most recent NSFG:

The typical (most common) pattern of contraceptive use in the United States is to use the condom at first intercourse, the pill to delay the first birth, and female sterilization when the woman has had all the children she wants.

Based on that, let's meet our three representative females: an adolescent Girl Scout, a 20-something single woman, and a 40-something mother.

We meet our Girl Scout as she matures through the 11- to 17-year-old Girl Scout age group.  Assuming she responds to Planned Parenthood's relentless suggestions that a sexually active lifestyle is "natural" and "sexy," she transitions from abstinence to condomized sex somewhere in the six-year span from eleven to seventeen.  Planned Parenthood targeted 1.1 million adolescents in its last reported year, making this Girl Scout's transition to sexual activity a Planned Parenthood goal and a real possibility.

Our 20-something recently graduated and has taken a job in a big city.  She is eager to meet friends and plans an active social life.  Although she previously used condoms, she is now on the pill, believing it more reliable than both condoms and the men she might meet.

Finally, our 40-something mother has had her third child and wants no more.  Although she and her husband previously used the pill, she recently underwent surgical sterilization.

Note that although all three females are users of new contraception, only the previously abstinent adolescent Girl Scout is a first-time user.

Given the change in contraception for each of our females -- abstinence to condomized sex for the Planned Parenthood Girl Scout, condoms to the pill for the on-the-scene 20-something, and the pill to sterilization for the 40-something mother -- we can summarize the situations and estimate the change in risk of unintended pregnancy for the underlying population as follows:

As shown above in the far-right column, the risk of unintended pregnancy increases despite the flurry of new contraception.  If sexual activity remains the same, population pregnancies will increase +0.2% -- and if sexual activity increases at all, pregnancies will increase even more.  Also note that if the Girl Scout had not taken the Planned Parenthood bait and remained abstinent, the average change in risk would have been a far "safer" negative 5.6% (versus +0.2%).

At this point, I anticipate a few objections.  The first regards the use of failure rates.  A failure rate is the percentage of surveyed women that had an unintended pregnancy in the first 12 months of using each method.  Some might object that typical use failure rates should improve with usage, and thus first-year rates are too high an assumption.  This might be true -- first-year failure rates could improve with time -- but such improvement would reduce Previous Method rates far more than New Method rates.  Assuming our 20- and 40-somethings were better users of their old methods by, say, 2%, the adjusted results show an increase in the risk of unintended pregnancy to +1.5%, from +0.2% in the table above:

A second objection may be that the Simple Average, which assumes equal weighting for each representative, is incorrect.  Previously abstinent adolescents could be underrepresented in a simple average because first-time users might be more common than method switchers.  This is likely, given Planned Parenthood's focus on the adolescent age group.  Consider below the risk effect of increasing the "previously abstinent adolescent" share (the "PP Girl Scout Percentage"):

The table's final row shows that, despite the broad overlay of contraception, the risk of unintended pregnancy increases rapidly if previously abstinent adolescents are meaningful participants in contraception campaigns.  This conclusion is valid whether our first-timers use condoms or the pill.

A final and inevitable objection, particularly from the humans-in-heat/sudden-sex-syndrome crowd, is that our Girl Scout would have begun a sexually active lifestyle anyway.  This need not be true, particularly with positive and persistent parental encouragement to offset Planned Parenthood messaging. 

Like climate modeling and "jobs created or saved," this last objection is purely hypothetical.  Fortunately, we have real data, in the form of numerous studies and industry experts, which data agrees that contraception does not reduce unintended pregnancies. 

In summary, no one goes on reversible birth control without the expectation of increased sexual activity -- and it is usually only an aspiring parent who consciously increases sexual activity without birth control.  Therefore, for virtually all but intending parents, reversible birth control is a critical precondition to increased sexual activity.  Pushing cheap, reversible birth control just sanctions and fuels more sexual activity.  Planned Parenthood's recent "WhereDidYouWearIt" campaign is a perfect example

All scenarios herein are consistent with the NSFG "typical pattern of contraceptive use," and all show increased risk of unintended pregnancy.  Real-world increases in contraception and abortion reflect these scenarios and are the predictable consequence of fraudulently assuring "safe" sex, particularly when that assurance targets first-time users.

¹Failure rates derived from the NSFG "Use of Contraception in the United States: 1982-2008" at www.cdc.gov/nchs/nsfg/nsfg_products.htm.  Female sterilization failure rate derived from NSFG and other sources.

²Ibid.