Oral contraceptives are drugs used to prevent pregnancy and typically contain synthetic estrogen and progesterone.
Normal Mechanism of Estrogen and Progesterone in Pregnancy
Egg Production & Fertilization: The ovaries produce an egg (ovum) and release it into the fallopian tube, where it may be fertilized by sperm.
Uterus Preparation: The uterus prepares for a fertilized egg by thickening the endometrium (uterine lining).
Role of Hormones:
Estrogen thickens the endometrium.
Progesterone thickens both the endometrium and the cervical mucus, preventing sperm from passing.
Menstruation: If fertilization doesn’t occur, levels of progesterone and estrogen decline, leading to menstruation.
Types of Oral Contraceptives
Combined Pills (Estrogen + Progesterone)
Mechanism:
Estrogen prevents FSH secretion (via negative feedback), suppressing ovulation.
Progesterone prevents LH release, further preventing ovulation and thickening the cervix.
Both hormones alter the endometrium to prevent implantation.
Effectiveness: Highly effective.
Side Effects: Weight gain, nausea, vomiting.
Progesterone-Only Pills (Mini Pills)
Mechanism:
Progesterone prevents LH release, inhibiting ovulation.
Thickens cervical mucus to prevent sperm entry.
Effectiveness: Less effective than combined pills, suitable for women who can’t use estrogen.
Summary of Oral Contraceptives
Classification | Example | Mechanism of Action | Uses | Side Effects |
Progestin-Only | Norgestrel, Levonorgestrel | Inhibit ovulation, thicken cervical mucus, thin endometrium | Contraception, emergency contraception (Levonorgestrel) | Irregular bleeding, weight gain, depression, acne |
Antiprogestin | Mifepristone | Binds to progesterone receptors, blocking action of progesterone | Emergency contraception, medical abortion | Heavy bleeding, nausea, headache, fatigue |