Take it down to 10 weeks (about 2 months).
A sexually active woman should be monitoring her monthly menstrual cycle. If she misses a visit from Aunt V (month 1) then she should immediately test for pregnancy. The pregnancy test can detect whether or not she is pregnant as early as 21 days after having sex. The best thing to do is test early on. Most likely you will receive a pill. If you wait until the baby is bigger, then the abortion procedure will involve surgical removal and it could take longer to get an appointment.
I research this stuff because the left makes such a big stink about it. It's the hill they want to die on so I gotta know my stuff. They're just so ridiculous.
My recommendation to women who don't want a baby is to remain abstinent or be very vigilant about monitoring your body each time you have sexual intercourse. Don't assume the birth control or contraception is secure. And if you had unprotected sex, then don't be lazy about it!
These women who want late-term abortion or an abortion after 20 weeks are not being pro-active. They are most likely reacting to a bad breakup or unmet expectations from a lover.