The interval between the first and second menstruation after delivery is still about 28 days. After delivery, as the uterus continues to recover after delivery, the time of the first menstruation is uncertain due to temporary ovulation of the ovaries under the condition of breastfeeding. After the first menstrual period, the ovarian function of ovulation will gradually recover, and if the next menstrual period can continue as scheduled after the ovarian ovulation, the second menstrual period can also come normally, so the interval is about 28 days under normal conditions. Due to the influence of breastfeeding, the ovaries may not be in a normal state and ovulation may not be fixed, so the interval between periods may not necessarily be around 28 days, depending on the situation of breastfeeding and the physiological changes in the hypothalamic-pituitary-ovarian axis.