This page addresses errors in estimating gestational age: "The accuracy of ultrasound in predicting gestational age gets worse as the pregnancy advances. By 20 weeks, ultrasound is accurate only to within plus or minus two weeks, and by the third trimester, its accuracy falls to plus or minus 3 weeks."
I found preemie survival rates here:
With the range of error of properly performed ultrasounds, a "20 week" baby with no chance of survival might actually be a 22-week baby with up to a 10% chance of survival. A "22-week" baby might actually be a 24 week baby with a 40-70% chance of survival. Once we enter the third trimester, a "24-week" baby might actually be a 27-week baby with a greater than 90% survival rate.
Ultrasound is the most common method of estimating gestational age in abortion practice. Many women are going in for late abortions because they'd lost track of their cycles and weren't sure they were pregnant at all, so the LNMP method of dating is lost with these women. The doctors are having to rely almost entirely on ultrasound.
Here are some cases of abortion doctors miscalculating gestation age: a supposedly 21-week baby who turned out to be 30 weeks, a supposedly 11-week baby that turned out to be 27 weeks, a baby that was believed to be about 12 weeks and turned out to be 28 weeks, and a supposedly 6-week baby that turned out to be 26 weeks. Given the frequent examples of abortion facilities having inadequately-trained staff performing ultrasounds, such screw-ups are to be expected.
Some abortion facilities openly advertise elective abortions to 26 weeks -- babies that have an 80-90% chance of survival, and who may actually be three weeks older than estimated. They might be as old as 29 weeks, with a greater than 90% chance of survival.
Of course, of you wrap them in a towel and stick them in a closet, rip out their throats with Bierer's forceps, or "snip" their spinal cords, they have a 0% chance of survival.
As for the assertion by abortion advocates that banning later abortions would prevent women from aborting babies that they learned have a disability, I can only ask why we passed the Americans with Disabilities Act to prohibit discrimination against those with disabilities, then insist that we should inflict fatal discrimination prenatally. Sick and disabled babies should get the same protection under the law that we give to healthy babies. And it's about time we did away with Roe and Doe and the open season on viable babies.