This is an ostretic tool based on the same algorithm used by gynecologists and obstetricians to calculate the presumed date of the birth.
The algorithm is construct on the average woman population data: An average pregnancy has a duration of 38 weeks, that's 266 days.
The calculation of the weeks of pregnancy, always resulting not possible to determine the exact moment of the conception, it's calculate starting from the first day of the last menstruation, from which the most probable date of the birth (understood the suffering) is calculated: considering that the duration of the menstrual cycle - generally of 28 days - it differs in reality from woman to woman (it has gone since 22 a 45 days) it is necessary to also plan this datum to get a more truthful respect.
The system calculates the presumed day of the conception, the weeks of pregnancy and the percentage of development of the child on the total one of the 38 standard weeks.
For further close examinations we postpone to the page of wikipedia: Gravidanza