The number of days of pregnancy is counted from the first day of the last menstrual period. If you are 40 days pregnant, it is already 40 days from the day of the last menstrual period, and not from the day of intercourse, because you can get pregnant only when you have intercourse during ovulation. If you have a regular period with a cycle of 28-30 days, ovulation should be counted from the day you get your period, and ovulation should occur around the 14th day. So it is about 26 days before the day of intercourse to get pregnant. However, this is only the approximate date, because the duration of ovulation is about 10 days, and there are also pregnant women with irregular periods, and at this time, the size of the gestational sac can be checked by ultrasound, and the date of intercourse can be deduced.