Which one of the following is the mechanism of action of oral contraceptive pills?

Oral contraceptive pills (OCPs) primarily function by altering the body's hormonal balance to prevent pregnancy [1]. The most common formulations are combined oral contraceptives containing both estrogen and progestin [2]. These hormones work by providing negative feedback to the hypothalamus and pituitary gland, which inhibits the release of an egg from the ovaries, a process known as ovulation [1]. By preventing ovulation, there is no egg available in the fallopian tube for fertilization by sperm [4]. Additionally, these pills thicken the cervical mucus to create a physical barrier that inhibits sperm from reaching the uterus and may also alter the uterine lining to prevent implantation [3]. Unlike spermicides, they do not kill sperm, nor do they function by killing eggs or zygotes; their primary mechanism is the suppression of the ovulatory cycle.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 7: How do Organisms Reproduce? > 7.3.3 (d) Reproductive Health > p. 125
2. [2] https://www.ncbi.nlm.nih.gov/books/NBK283/
3. [4] https://opa.hhs.gov/reproductive-health/preventing-pregnancy-contraception
4. [3] https://www.sciencedirect.com/science/article/abs/pii/S0002937899701201