If a woman has regular menstruation with a 28-day cycle and a 7-day period, she is most likely to get pregnant when she has intercourse around 7 days after her period is clean, but it is not absolute. A woman’s menstrual cycle varies from 21-35 days, usually around 28 days, and the period can be divided into 4 phases. Taking the 28-day menstrual cycle as an example, the menstrual period is usually 3-7 days long. The follicular phase starts on the 5th day of menstruation and ends on the 13th day, when the follicles start to enlarge under the influence of hormones. The 14th day is the ovulation phase, when the egg is expelled. From day 15 to day 28 is the luteal phase, when the endometrium begins to prepare for embryo implantation under the influence of progesterone. If implantation does not occur within 2 weeks, the concentration of progesterone in the body decreases, which in turn causes the next menstrual period. Therefore, the best time to get pregnant is about 14 days before the onset of menstruation. However, if a woman’s menstrual cycle is irregular, the day of ovulation may be prolonged or early, at which time ultrasound can be done to monitor ovulation more accurately.