Eiri, 56% of abortions are done by 8 weeks, 90% by 12. The main reason why any abortions occur later than that is pro-life legislation, ironically enough.
Things like mandatory parental consent causes many teens to delay an abortion for fear of telling their parents, or having to travel to states where there is no such law, or waiting until they turn 18 if they are close to their birthdays.
Also, mandatory waiting periods,like having to wait 24 hours after you see a counselor at a clinic causes delays, because some women have to travel great distances to get to a clinic in the first place. Since the majority of women getting abortions
already have children, they have to arrange childcare and time off from work to get away for two days. Most can't stay overnight (24 hours) so they have to make 2 appointments, hence the delay.