3. If breakthrough ovulation occurs, the effects of contraceptives on the endometrium make the embryo less likely to implant.
This is the highly debated issue. Those who write in support of COCs admit that the endometrium is thinner during non-ovulatory cycles (as is typical with Pill users). For the purposes of argument, they may even grant that a thinner endometrium may be less hospitable for implantation (though this is not completely clear). However, if ovulation
takes place, a completely different hormonal milieu exists. As summarized earlier, ovulation leaves behind the corpus luteum, a rich source of estrogen and progesterone. After the six days required for the embryo to travel down the uterine tube into the uterus, these hormones have transformed the endometrium, which has now become receptive for implantation.
There is no doubt that this is true at least
some of the time. This should' be obvious
from the known "failure" rate of the Pill
cited earlier (0.1-5%). In other words, some Pill-users get pregnant. The key questions become: How often does the user of COCs ovulate and conceive, only to have such a conception fail to implant? How does this rate compare with non-Pill users?
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