The CDC data can tell us a bit of this story. Alongside the drop in the overall number of abortions, the agency also found a decline in the abortion ratio. That figure measures the number of pregnancies terminated for every 1,000 live births. That number dropped too, from 232 in 2008 down to 227 in 2009, a 2 percent decrease. That suggests that the story here isn't just about fewer pregnancies. We can see in the data that the decisions women make after becoming pregnant are playing a role, with more deciding to continue with the pregnancy rather than terminate.
Figuring out the other drivers is a little bit trickier. It would be easy to point to the wave of abortion restrictions that swept through state legislatures in 2011 -- except that this data come from 2009, when those new laws weren't yet in place.
Some academics have suggested that it might be the recession: Americans have gotten more careful with their family planning as budgets have become squeezed.
"They stick to straight and narrow . . . and they are more careful about birth control," said Elizabeth Ananat, a Duke University assistant professor of public policy and economics who has researched abortions, told the Associated Press.
The research, however, suggests that the economy actually has a relatively small impact on the abortion rate, and one that works in the opposite direction of what's suggested above.
A 2004 paper in the journal Health Economics looked at the relationship between the economy and abortion rates at the state level. It found that, "As the economy moves into recession, a 1-point rise in the unemployment rate leads to about a 3 percent increase in abortion rates."
So much for that theory. What seems like a more plausible explanation might be the type of contraceptives women are using. A study published earlier this year looked at the usage of long-acting, reversal contraceptives, methods like interuterine devices, which tend to have much higher efficacy rates than birth control pills (in short, there's a lot less room for user error).
That research, published in the journal Fertility and Sterility, found that use of long-acting contraceptives tripled between 2002 and 2009. Most of that increase happened, however, in the last two years. The proportion of contraceptive users using this method increased from 2.4 percent in 2002 to 3.7 percent in 2007. Between 2007 and 2009, though, it shot up to 8.5 percent.
No research has found a causal relationship yet. What we know is a correlation: At the same time the abortion rate took a big drop, use of more effective contraceptives had recently increased. That seems like it could be one factor explaining why the abortion rate recently dropped, after years of holding steady.