A contraceptive implant is a thin flexible 40 mm long plastic polymer rod which is impregnated with progestin.
Birth Control - Image Credit: Michael Kraus / Shutterstock
It is inserted under the skin of the medial aspect of the upper arm. It releases progestin at a slow constant rate.
The rod contains 68mg of etonogestrel, which is released at 60-70 micrograms during the first year, slowly tapering to 25 micrograms daily in the third year.
Progesterone is a hormone synthesized in the ovaries, the female internal sex organs. It is largely produced in the second half of the menstrual cycle and during pregnancy.
It regulates the cervical mucus consistency, smooth muscle contraction, endometrial thickening and maturation, and other changes which are essential for the proper continuation of a pregnancy.
Mechanism of Action
A synthetic progestin is released from the implant rod or rods in a continuous low-dose fashion into the blood. This causes the following to happen:
Inhibition of Ovulation
The menstrual cycle depends upon the ovarian cycle, which in turn is regulated by the master glands in the brain, which make up the hypothalamopituitary axis (HPA).
The hypothalamus releases pulses of gonadotropin-releasing hormones (GnRH) into the pituitary gland.
There they stimulate the synthesis and release of the gonadotropic hormones, the follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
FSH stimulates ovarian follicle growth, and oocyte maturation. Its level rises through the first half or proliferative phase of the menstrual cycle, causing thickening of the endometrium by cell division.
A few ovarian follicles start to enlarge. One of them soon becomes the dominant follicle. Oocytes inside the developing follicles mature.
The cells surrounding the oocyte are called granulosa cells. They synthesize the hormone estrogen, which also rises to its peak near mid-cycle.
At this point the LH secretion also rises abruptly, which is called the LH surge. This precedes ovulation by 72 hours. In other words, it promotes the rupture of the dominant follicle. This allows the ovum and follicular fluid to escape into the peritoneal cavity in close proximity to the fimbriated end of the ipsilateral fallopian tube, which quickly picks it up. The ovum can now be fertilized by a sperm in the fallopian tube.
After this event, the zygote formed starts to divide, forming a blastocyst while moving towards the uterine end of the tubes. Within five days it reaches the uterine cavity and implants in the prepared endometrium, marking the start of a pregnancy.
Following ovulation, the granulosa cells of the ovary undergo luteinization. The cells become progesterone-secreting cells, and the follicle itself becomes the corpus luteum.
Progesterone stimulates thickening and vascular dilatation within the endometrium, with hypertrophy of the endometrial glands. This is the secretory phase or second half of the menstrual cycle.
If fertilization fails to happen, the corpus luteum becomes fibrous and the progesterone levels fall, causing the endometrium to start to shed - the beginning of the next menstrual cycle.
The ovarian hormones exert inhibitory feedback on the HPA. As a result of the action of the contraceptive implant, constant low levels of progesterone are maintained in the blood.
This suppresses the HPA and hence the ovarian cycle. The inhibition of ovulation, as a result of this action, is the primary mechanism of action of the implant.
Action Upon cervical mucus
The uterine cervix contains a thick blob of mucus which has different characteristics during various parts of the cycle. It is responsive to the ovarian hormones.
Progesterone causes it to become thick and opaque, as well as stubborn and sticky. This prevents free entry of sperms into the uterus, and so inhibits fertilization of the oocyte.
Action Upon Endometrium
Very low progesterone levels prevent normal endometrial maturation, causing it to remain thin and poorly vascularized. This renders it hostile to implantation, even if fertilization occurs.
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 implant acts on ovulation, fertilization, and post-fertilization events to prevent conception.
When the implant is inserted immediately after the last active pill of a pack of oral contraceptives taken properly, or within the first five days of a menstrual cycle, it has immediate contraceptive action.
If timed otherwise, other methods of contraception must be used for seven days until it becomes effective. It may be inserted at any time provided the possibility of pregnancy is ruled out.
Duration of Action
The duration of action of the implant is three years, and following this it is usually removed and a new one inserted through the same tiny incision. If contraindications develop at any time, or if the woman desires to conceive, it can be quickly and easily removed in most cases.
Return to fertility is rapid, and pregnancy has been reported to occur within one or two weeks of removal. As soon as the implant is removed, therefore, the contraceptive effect is considered to have stopped.
Reviewed by Susha Cheriyedath, MSc