Since puberty, the sow estrous cycle occurs periodically every 21 days along the year, except during pregnancy and lactation period or in anoestrus pathological cases. Secretion of the GnRH (gonadotrophin releasing hormone) from the hypothalamus provokes the secretion of FSH (follicle stimulating hormone) and LH (luteinizing hormone) from the adenohipophysis that will act on the ovary. Both hormones act with synergy, being FSH responsible of the follicular growth. Along with follicular development, the oestrogen secretion increase and provokes the estrous signs in the sow (red vulva, vaginal discharges, standing reflex and mounting behaviour between them).

When the blood oestrogen reaches a level, it produces a positive feedback on the hypothalamus which provokes the secretion of the preovulatory LH surge from the hipophysis, which is responsible of the ovulation of the preovulatory follicles. After ovulation, the oestrogen levels decrease and start to increase the levels of progesterone from the corpus luteum formed in the place of the ovulated follicles. Progesterone is responsible of preparing the endometrium to the reception of the embryos. Also, by means of a negative feedback, it avoids the secretion of GnRH and consequently the secretion of FSH and LH and the follicular growth.  If pregnancy is not established, the prostaglandin F2α from the uterus reach the ovary and the luteal regression occurs and consequently a decrease in the progesterone levels, which reactivates the secretion of gonadotrophins and a new estrous cycle starts.

Factors that have influence on the development and normal timing of the estrous cycle include the health and nutritional status (body condition) of the sow and the environment conditions (temperature, light, photoperiod), housing conditions (animal density, homogeneous batches) and management practices (right stimulation, boar contact, lactation length, absence of stress). If any of these factors failed, it can produce abnormal estrous cycles. The most frequent are seasonal anoestrous, lactation anoestrous, cycles of abnormal length (shorter or longer), anovulatory cycles and silent cycles.