MORNING AFTER PILLS

[Ilyanna]

HotD, thanks for posting the NY article.
As for your excellent explanation, I would like to add one thing if I may (it gets a bit technical/pharmacological, ymmv. If it's tl&dr for you, check the cliff's note at the end ):

The modes you listed are dependant on the hormonal active ingredient that is used. There currently are three different emergency contraceptive pills, containing:

levonorgestrel (progestin),

• mono-preparation containing only progestin, as used in the OTC brands Next Choice One Dose©, and Plan B One-Step©
  -> a variety of studies over the years have shown no effect on the endometrial growth. Its method of action lies solely in the prevention of fertilization/ovulation.
  
• combined preparation (progestin and estrogen), the so-called Yuzpe regimen, which is pretty much outdated by now due to the mono-preparation.
  

OR

mifepristone (antiprogestin)

• high dose abortion pill, aka the "infamous" RU486© and Mifeprex©, not FDA approved
  
• low dose Emergency contraceptive Pill, not FDA approved (I've not found any brand name)

ulipristol acetate (antiprogestin)

• as used in Ella© -> a second generation antiprogestin, (or successor of mifepristone, if you will)

As for the methods of actions:

• a variety of studies over the years have shown no effect of levonorgestrel on the endometrial growth. Its method of action lies solely in the prevention of fertilization/ovulation.
  
• as for ulipristol acetate, to date, there is no backed scientific evidence that its effect is caused by interference with the nidation of a fertilized egg.
  
• mifepristone, however, does effect endometrial growth, at least when administered in high doses. Since it is not approved as an ECP in the US nor Australia (afaik), though, it does not count into this debate.

So, cliff's notes:

if the ECP used is levonorgestrel-based as in the OT, science says that no abortion (as per definition of "post-fertilization"!) occurs.

if the ECP used is ulipristol acetate -based, science says it hasn't found proof of post-fertilization effects, but can't completely disprove it yet.

if the ECP used is mifepristone-based - you are probably currently residing in China or another country where it is approved, or are participating in a clinical trial.

A most excellent scientific report on this matter can be found

here