The last third of pregnancy involves the greatest increase in mammary gland mass and involves estrogen, progesterone, relaxin, and prolactin. A few days prior to farrowing, colostrum forms and lactogenesis begins through cellular development, genetic expression, and some milk secretion. Lactogenesis is primarily controlled by prolactin, and it signals the switch from producing colostrum to producing milk components. After farrowing, mammary growth and function is controlled by the suckling of the piglet. Each piglet will suckle from one teat, and increased removal of milk will send signals for that gland to secrete more milk. Milk ejection occurs every 45-60 minutes for 10-15 seconds. Sows that have more cycles with less milk will release more milk in a day than those that release more milk at less frequent intervals. If a mammary gland stops being stimulated through weaning, piglet death or removal, inhibitory factors will stop milk secretion. If a teat is not suckled within three days of farrowing milk secretion will not begin, and a longer period between farrowing and suckling will result in decreased milk secretion. It is important that the interplay between piglet and mammary gland is not interrupted, and that stressors which might affect milk secretion by avoided in the farrowing room.