I've appreciated the discussion between Lydia McGrew and Jason Dulle in the comments here regarding the effect of the follicular phase hormonal changes on endometrial thickness and potential for embryo implantation. There is an article in the latest issue of Contraception which may shed some light on these issues. I describe the study and comment in the end.
The study was done to compare two different regimens of OCs (24/4 cycle or a 21/7 cycle). They first took hormonal and ultrasound data from women not taking OCs (which are labeled "pre-treatment or cycle 1" on the graphs). They then took all of the women who ovulated and began one of the two treatment regimens (cycle 2). During the third cycle of treatment, they substituted a placebo for the first three pills in the cycle (attempting to simulate missed pills.) Lastly, they tested the women during their following cycle to see if any changes persisted. During cycle two, where the pills were taken correctly, one out of 100 women ovulated, but it was noted that she took antibiotics for scarlet fever during the study period. 4 additional women ovulated during cycle three, so the vast majority of women looked at had no luteal phase of their cycle.
Here is the data for the amount of E2 (estrogen) for the 3 groups:
As you can see, the E2 was significantly suppressed during all of cycle two. I was surprised that the E2 levels rose so quickly after 3 missed pills in cycle 3. Clearly, missing OCs greatly increases the chances of ovulation.
Here is the effect of the OCs on endometrial thickness:
First, the baseline max. endometrial thickness was at least 5mm for all women in this study. This differs from the data offered by Jason which claimed that the thickness of the endometrium was 1.1mm throughout the cycle for women on OCs. I can't claim that know which data is correct, but it seems clear they both can't be.
There was a slight increase in endometrial thickness for cycle three, where they missed the first three pills. Unfortunately, they were using mean values for the group, so the effect of those who ovulated were mixed in with the majority that did not. In other words, it is impossible to see if the luteal function of those who ovulated had significant impact on the endometrial lining. This would have been nice to see. Maybe I'll E-mail the authors and see whether they still have the information.
Lastly, the endometrial thickness rebounded to almost baseline very quickly after the pills were stopped. The post-treatment cycle was very close to the pre-treatment one - and the difference can be attributed to the fact that not all of the women ovulated in the post-treatment cycle.
Bottom Line: we still don't know for sure. This study only looked at one particular combination OC. It seems the max. endometrial lining for those on OCs is greater than 5mm and bounces back to its baseline level the following month after OCs are stopped. I believe this is indirect evidence against a post-fertilization effect, but I can't be definitive.