OBs and Midwives calculate your due date using the first day of your last period.. BUT they already know that it won't be 99% accurate as women have different menstruation cycles. Hindi lahat ng babae nag-28 days cycle.. and also having the same lenght of period (if period last to 5 to 7 days). There's no exact due date, they usually call it EDD or Estimated Due Date.
Pabago bago po eh. May calculations to estimate pero marami pa rin pong dahilan sa pag aadjust https://ph.theasianparent.com/due-date-ng-buntis Maganda po consistent monitoring with your OB