Reproductive Physiology for Contraception

Conception requires ovulation of a viable oocyte, transport of motile sperm through receptive cervical mucus to the fallopian tube, fertilisation, transport of the embryo to the uterus and implantation in a receptive endometrium. Every contraceptive method interrupts one or more of these steps, and knowing the step each method targets explains its effectiveness, onset of action, emergency use and failure patterns.

Hypothalamic-pituitary-ovarian axis. Pulsatile gonadotrophin-releasing hormone from the hypothalamus drives pituitary secretion of follicle-stimulating hormone (FSH) and luteinising hormone (LH). FSH recruits and grows follicles; LH triggers ovulation and supports the corpus luteum.

Follicular phase. Rising FSH recruits a cohort of follicles; one dominant follicle emerges by about day 7 and secretes rising oestradiol. Its length varies and accounts for most variation in cycle length.

Ovulation. Sustained high oestradiol triggers positive feedback and the LH surge. Ovulation follows about 24 to 36 hours after the onset of the surge.

Luteal phase. The corpus luteum secretes progesterone and oestradiol for about 14 days. Without implantation, it regresses, hormone levels fall and menstruation follows.

Fertile window. The fertile window spans about 6 days: the 5 days before ovulation and the day of ovulation. Sperm survive up to 5 days in fertile cervical mucus; the oocyte survives about 12 to 24 hours.

Contraceptive targets and the methods that act on them.

Oestrogen. Ethinylestradiol and estradiol suppress FSH, preventing follicle selection, and stabilise the endometrium, giving cycle control.

Progestogen. Progestogens suppress LH and the surge, thicken cervical mucus within about 48 hours, and thin the endometrium. Ovulation suppression depends on dose: traditional progestin-only pills suppress ovulation in only about half of cycles, so they rely on mucus and need strict daily timing.

Copper. Copper ions are toxic to sperm and ova and cause a sterile inflammatory reaction in the uterus, preventing fertilisation. This is why the copper IUD works immediately and is the most effective emergency method.

Onset of protection follows the target.

• Methods that act mainly on ovulation need 7 days of backup if started outside the early follicular phase.

• Progestogen mucus effect takes about 48 hours, so progestin-only pills need 2 days of backup.

• The copper IUD protects from insertion.

• Vasectomy is not effective until the ejaculate is azoospermic, usually about 3 months after the procedure.

Starting in the first days of the cycle. A hormonal method started within the first 5 days of menses (7 days for injectables and implants) suppresses the cohort before a dominant follicle is selected, so no backup is needed.

Emergency contraception. Oral emergency methods only delay ovulation and cannot act once the LH surge has peaked or ovulation has occurred. The copper IUD acts after ovulation by blocking fertilisation and implantation, so it remains effective close to ovulation.

Postpartum fertility. In women who do not breastfeed, ovulation can return as early as 25 days after birth. Exclusive breastfeeding suppresses ovulation through prolactin and disrupted GnRH pulsatility, the basis of the lactational amenorrhoea method.

Hormone-free interval. Follicles begin to grow during the 7-day pill-free interval. Missing pills next to this interval, at the start or end of a pack, lengthens it and carries the highest risk of ovulation.

Enzyme induction. Rifampicin, rifabutin, carbamazepine, phenytoin, phenobarbital, primidone, oxcarbazepine, topiramate at doses over 200 mg daily, efavirenz and ritonavir-boosted protease inhibitors increase hepatic metabolism of oestrogen and progestogen, lowering levels under the threshold that suppresses ovulation.

Irregular cycles. Anovulatory cycles in adolescence, perimenopause, polycystic ovary syndrome and lactation make calendar-based fertility awareness methods unreliable.

Perimenopause. Ovulation becomes erratic but persists. Contraception continues until 12 months of amenorrhoea after age 50, or 24 months under age 50, in women not using hormones.

Referral and escalation. Refer patients with unexplained amenorrhoea, abnormal uterine bleeding or suspected endocrine disease for evaluation before attributing changes to the contraceptive method.