As Trigg stated, poorly written. But without a way to tell the "exact" time, it's not a bad idea to go by the last period. Worst case is off by 2 weeks and best case could be 24hrs or so.
I would ask you where you get this crap from but we know you won't be back.
The "arguers" are angry at the 20 weeks, saying that if a woman is only pregnant for 18 1/2 weeks, this law will be extending it that extra week and a half an banning the abortion. They want to scratch and claw for every last minute available to them in order to have an abortion.