The pregnancy starts on that day.

Pregnancy time is calculated according to the first day of the last menstrual period. This is more accurate. According to the last menstrual period, the corresponding gestational week and expected date can be calculated. Each person's menstrual cycle is different, so the menstrual time is relatively accurate. For example, each month's menstrual cycle is 30 days. This gestational week is better. If the gestational week shows earlier or later, it is related to the menstrual cycle. In the case of early conception, the gestational sac will be larger, and in the case of late conception, the gestational sac will be smaller, so regular review is needed.