More information in a followup Kos Diary at:
http://www.dailykos.com/story/2006/2/28/25814/7830The following info comes from NancyK and jiffykeen in the comments to the above - posted at:
http://www.dailykos.com/comments/2006/2/28/25814/7830/67#67=======================
Go to the PubMed search page, enter the search terms "emergency contraception implantation ovulation", and look at some of the abstracts listed. According to current research, Emergency Contraception (EC) doesn't appear to have any post-fertilization effects. (Note that this may not be true for traditional contraception, since dosing regimens are very different; I haven't looked into it.)
Some of the people opposing EC are being very consistent, by their own logic, but this research pulls the rug out from under them. Yet nobody seems to know about it!
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?CMD=Display&DB=pubmed Mechanism of Action Combined oral contraceptives (what most of us think of as "the pill"):
* thicken cervical mucus to prevent sperm penetration into upper genital tract
* Block the LH (luteinizing hormone) surge and thus inhibit ovulation. "Escape ovulation" or the amount of time ovulation is not suppressed in someone taking the pill regularly is estimated to be about 2%, perhaps higher in the lower dosed pills
* slow sperm motility, which may delay sperm transport.
Some progestin effects additional alter the environment that would be required for embryogenesis to proceed:
* disrupt transport of the fertilized ovum
* induce endometrial atrophy, (changing the womb structure)
So probably 98% of the time, the pill works by inhibiting ovulation. It would be incorrect to say that it never works by preventing implantation
Emergency Contraception:There are two major forms of emergency contraception: progestin only (Plan B) and combined oral contraception (Preven)-not currently available
In essence EC is giving a higher dose of oral contraception, at one time. Plan B involves .75 mg of Levonorgestrel at one time and is given 12 hours later after the first dose. Studies show that it can be given all at once.
The effect of EC depends on when it is in the women's cycle. Before ovulation has occured, it works by disrupting normal follicle development and maturation resulting in anovulation or delayed ovulation. In contrast, when the treatment is administered after ovulation has occured and fertilzation has taken place, it is unknow exactly the process of prevention of pregnancy. Some of the research suggests impaired endometrial receptivity to implantation, interference with corpus luteum function, alteration of cervical mucus resulting in trapping of sperm, and direct inhibition of fertilization. So it may work by inhibiting fertization or implantation.Analysis of EC effectiveness in relation to the timing of intercourse in relation to cycle day found that effectiveness was substantially higher when intercourse and treatment occurred before ovulation, so inhibition of implantation is unlikely to be the primary mechanism of action of this method.
RU-486 is not an estrogen or progestin but an anti-progestin and blocks the effects of progesterone. It is used in pregnancy termination before 7 weeks of pregnancy, however, it will also prevent pregnancy if it is administered within 5 days after unprotected intercourse.
I think that it would be inaccurate to suggest that ECs do not work by preventing fertiliztion or implantation. It seems pretty clear cut that it works by preventing ovulation when administered to a woman in the pre-ovulatory phase of her cycle. There is not enough evidence, apparently, to definitively say how it works when administered after ovulation.
Almost all combined oral contraceptives can be used as EC if given in the correct doses. If a pharmacist won't fill the script for EC, ask the health care provider to prescribe a month's worth of OC and tell you how to take it.
Example:
Ovral: Take 2 white pills then 2 white pills 12 hours later. (similar to Preven, which is off market)
Ovrette: take 20 yellow pills then 20 yellow pills 12 hours later (same as Plan B)