The contraceptive vaginal ring is a small, flexible, silicone ring that is inserted in the vagina. It contains progestin and estrogen, the same hormones that are used in the combined oral contraceptive pills. These are the primary female sex hormones, and have a typically cyclic pattern of secretion which is responsible for the menstrual cycle in women.
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Inhibition of Ovulation
The menstrual cycle depends upon the ovarian cycle, which in turn is regulated by the master glands in the brain, the hypothalamopituitary axis (HPA). These release gonadotropin-releasing hormones (GnRH) which are secreted in a pulsatile manner from the hypothalamus, to reach the pituitary gland. There they induce the secretion of the gonadotropic hormones - the follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
FSH stimulates ovarian follicle growth and oocyte maturation, rising in the first half of the menstrual cycle, also called the proliferative phase. The endometrium becomes thicker as cells proliferate rapidly. A few ovarian follicles start to enlarge, with one overtaking the others and becoming the dominant follicle. Within the follicles, oocytes start to undergo maturation division. The cells within the follicles, called granulosa cells, synthesize the hormone estrogen, which is therefore showing a rising trend throughout this period, to peak near mid-cycle.
As the cycle nears mid-point, the level of LH suddenly rises, called the LH surge. This occurs roughly 72 hours before, and mediates the rupture of the dominant follicle, releasing the ovum - the important process called ovulation. The ovum and the follicular fluid escape into the peritoneal cavity in close proximity to the fimbriated end of the ipsilateral fallopian tube, which quickly picks it up. Ovulation usually occurs in alternate ovaries, with one ovum being released per month.
The ovum can now be fertilized by any sperms which make it to the fallopian tube. If so, it will start to divide, forming a blastocyst. This then migrates into the uterine cavity within five days. There it implants, or burrows deep into the endometrium, to form a pregnancy sac.
Following ovulation, the granulosa cells of the ovary are transformed into lutein-containing cells, in a process called luteinization. The follicle now becomes a corpus luteum. These cells produce copious amounts of progesterone, which promptly transforms the uterine endometrium into a thick, spongy, vascular layer suitable for implantation - the secretory phase. If fertilization fails to occur, the corpus luteum becomes atretic and the progesterone levels fall. The endometrium is shed, which marks the beginning of the menstrual period and the next ovarian cycle.
The classic pattern of ovarian hormone secretion is regulated by their inhibitory feedback on the GnRH and gonadotropin-producing cells in the HPA. The presence of a constant low level of estrogen and progesterone exerts a suppressive effect on hypothalamic and pituitary secretion which results in inhibition of the ovarian cycle and of ovulation.
The vaginal ring usually releases 15 mcg ethinyl estradiol and 120 mcg etonogestrel per day, which is less than that released by oral contraceptive pills. Even if the ring is damaged, increased hormone levels are not likely because the components are mixed in the core.
These are released in a continuous low-dose manner and absorbed through the vaginal mucosa into the bloodstream.
Action Upon Cervical Mucus
The uterine cervix is normally plugged by a mucus blob. This changes its characteristics during different parts of the cycle. In the second part of the ovarian cycle, progesterone secretion predominates, which causes the cervical mucus to become thick, tenacious, and opaque. This results in its obstructing the passage of the sperms from the vagina into the uterine cavity, and thus preventing fertilization of the oocyte.
Action Upon Endometrium
Finally, the lack of adequate progesterone levels causes the normal endometrial maturation to fail, so that it remains scanty and thin. This means it cannot support implantation, even if fertilization should occur.
Action on Smooth Muscle
Progesterone slows smooth muscle contraction. This results in slowing of tubal and uterine motility, which could promote the transport of the sperms and the zygote.
In this way, the contraceptive ring affects ovulation, fertilization, and post-fertilization events to prevent conception.
Effectiveness
With perfect usage, the vaginal ring is almost 100 percent effective.
It has the advantage over oral contraceptive pills that it does not have to be taken daily at the same time, but remains in place for 21 days. It is highly effective and reversible.
The ring may be ineffective if the woman fails to put in the ring within 24 hours of the usual change day, or the eighth day after the ring-free week
References
Further Reading
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