Emergency contraception prevents pregnancy by all of the following mechanisms, except :
Postcoital Contraception (Interceptives): interferes with postovulatory events leading to pregnancy and is therefore known as interceptive. It is also known as 'emergency contraception' method used to prevent pregnancy after an unprotected intercourse. Emergency contraception is used following rape, unprotected intercourse or accidental rupture of a condom during coitus taking place around ovulation. It is used in misplaced IUCD and missed pill. These postcoital methods should be used mainly as 'back-up' methods The preparations available include: -Two tablets of relatively high doses of combined pill (Ovran/Eugynon 50), containing 100 mcg EE2 and 1 mg norethisterone, or 500 mcg LNG, taken within 72 h of intercourse followed by two tablets taken 12 h later (Yuzpe and Lancee, 1977). Failure rate--3.2 per 100 woman years. Mode of action: The hormones may delay ovulation if taken soon after intercourse, cause corpus luteolysis, and bring about cervical mucus changes and endometrial atrophy. -Levonorgestrel (LNG). Prostinar tablet contains 0.75 mg LNG. One tablet should be taken within 72 h of unprotected intercourse and another 12 h later. Alternately, two tablets can be taken as a single dose. The failure rate is 1.1%. The tablets can be offered up to 120 h but its efficacy decreases with the longer coital-drug interval. LNG prevents ovulation and causes desynchronization of endometrium through its receptors (luteal phase deficiency). The period may come earlier or delayed. Side effects are those of progestogens. The hormone is not teratogenic in case pregnancy does occur but risk of ectopic pregnancy remains.Advantages: It has no oestrogen and its associated side effects, It can be offered to hypeensive, cardiac and diabetic woman,It can be offered to a lactating woman,It can be given as late as 120 h after the unprotected intercourse,Single-dose therapy is an advantage.Contraindicated in liver disease, contains lactate, so allergy to galactose. The drug is also contraindicated in a woman with history of thrombophlebitis and migraine. -RU 486 (Mifepristone) : RU 486 is a steroid with an affinity for progesterone receptors. It does not prevent feilization but by blocking the action of progesterone on the endometrium, it causes sloughing and shedding of decidua and prevents implantation. It is not teratogenic. A single dose of 25-50 mg is effective in preventing pregnancy in 99.1% cases (failure rate 0.9%). It causes delayed menstruation. Ectopic pregnancy is not avoided. The drug is expensive as compared to LNG. -Ulipristal. Ulipristal is a synthetic progesterone hormone receptor modular, attaches to progesterone receptor and prevents/delays ovulation and suppresses endometrium, prevents implantation. A 30 mg tablet should be taken within 5 days. Two per cent pregnancy rate is repoed. Side effects are headache and mood changes. -Centchroman. Two tablets (60 mg) taken twice in 24 h within 24 h of intercourse can prevent implantation in 99% women. -GnRH Agonists. Daily administration of GnRH agonist (buserelin) prevents ovulation. The drug is on trial for its contraceptive effect. Very expensive--has to be given by subcutaneous injection. -Prostaglandin : Self-administered vaginal suppository containing prostaglandin following an unprotected intercourse, by viue of its luteolytic effect on the ovary and its increased motility effect on fallopian tubes and the uterus, prevents implantation and brings about menstruation. Its specific role as contraceptive is however yet to be established. -Copper T IUCD. Inseed within 5 days of intercourse can prevent implantation of a feilized ovum. Advantages of Copper T as emergency contraception are: It can be inseed as late as 5 days after the unprotected intercourse. It is cheap. Failure rate is 0.1%. It can remain as ongoing contraceptive method for 3-5 years. Reference : page 280 Shaw's Textbook of Gynaecology 16th edition
Delaying / inhibiting ovulation
inhibiting feilization
preventing implantation of the feilized egg
Interrupting an early pregnancy
Contraception