To calculate the pregnancy interval for that table it is necessary to use the calendar. Essentially from the beginning of the calendar look month by month for a birth "B" or termination "T". Once one of them are found, position in the month before the start of the pregnancy for that birth/termination and count the number of months until another birth or termination is found. In the case of births assume that duration of pregnancy is 9 months. In the case of terminations look for the first character different from a pregnancy "P" after the month where the "T" was found. It is also important to take into consideration the date of the last birth/termination before the beginning of the calendar because that needs to be used to compute the duration for births/terminations that occurred in the last 5 years (59 months), but the previous birth/termination occurred before the calendar.